ACTIVE FOOD PACKAGING Edited by M.L. ROONEY Principal Research Scientist CSIRO Division of Food Science & Technology North Ryde New South Wales Australia
BLACKIE ACADEMIC & PROFESSIONAL An Imprint of Chapman & Hall
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Preface
Food packaging materials have traditionally been chosen to avoid unwanted interactions with the food. During the past two decades a wide variety of packaging materials have been devised or developed to interact with the food. These packaging materials, designed to perform some desired role other than to provide an inert barrier to outside influences, are termed active packaging. The benefits of active packaging are based on both chemical and physical effects. Active packaging concepts have often been presented to the food industry with few supporting results of background research. This manner of introduction has led to substantial uncertainty by potential users because claims have sometimes been based on extrapolation from what little proven information is available. The forms of active packaging have been chosen to respond to various food properties which are often unrelated to one another. For instance, many packaging requirements for postharvest horticultural produce are quite different from those for most processed foods. The objective of this book is to introduce and consolidate information upon which active packaging concepts are based. Scientists, technologists, students and regulators will find here the basis of those active packaging materials which are either commercial or proposed. Some types of active packaging are inevitably omitted but the book should assist the inquirer to understand how other concepts might be applied or where they should be rejected. Chapter 1 is the editor's overview of the field. Here I have sought to define active packaging and to establish its limits and the background to its development. The history of oxygen scavenging is used as a case study. Chapter 2 is contributed by Dr Devon Zagory of Devon Zagory & Associates Inc., California. Dr Zagory has been a major contributor to present knowledge of modified atmosphere packaging of horticultural produce. In this chapter he discusses the background to the use of active packaging to remove ethylene from the headspace of respiring produce. He discusses the options currently available and their limitations. Chapter 3, by Dr Kit Yam of Rutgers University, New Jersey and Dr Dong Sun Lee of Kyungnam University in South Korea, address the interface between active packaging and equilibrium modified atmosphere packaging now in use. They introduce a simple method of modelling gas atmospheres to show where additional active packaging concepts are required.
Chapter 4 is the editor's discussion of the field of active packaging based on polymers. This includes use of thermoplastics for films and more rigid containers but also provides background useful when consideration is given to other polymer-based coatings. The aim has been to unify the many alternative unrelated concepts being offered to packers of both fresh and processed foods. Chapter 5 is contributed by Dr Bernard Cuq and Dr St6phane Guilbert of CIRAD-SAR, and Dr Nathalie Gontard of ENSIA-SIARC, all of Montpellier, France. Their research into the edible coating of foods needs no introduction. They describe in this chapter how edible coatings are often already active packaging and introduce the current and potential use as delivery systems for food additives. Chapter 6 is contributed by Dr J.P. Smith of McGiIl University, Montreal, Canada, Yoshiaki Abe, President of Mitsubishi Gas Chemical Europe GmbH and Dr Jun Hoshino, Chief Microbiologist of Mitsubishi Gas Chemical Company, Inc., Tokyo. Jim Smith has published the results of many of the key investigations of the impact of modified atmosphere and active packaging on food microbiology. Mr Abe and Dr Hoshino have been responsible for the introduction of 'Ageless' oxygen scavenging sachets, particularly outside Japan. In this chapter they consolidate knowledge of sachet-based technologies. Chapter 7 is co-authored by Dr John Budny, President of PharmaCal Ltd., California and Dr Aaron Brody, who is Managing Director of Rubbright Brody Inc. Dr Brody was formerly with Schotland Business Research of Princeton and is a widely respected packaging consultant and author. These authors provide the background to and opportunities for the use of enzymes in active packaging. This is still a frontier field and requires the carefully explained background presented in this chapter. Chapter 8 is contributed by Dr Fred Teumac of ZapatA Technologies Inc. of Hazleton, Pennsylvania, USA, who was jointly responsible (with Advanced Oxygen Technologies Inc.) for developing the first commercially successful oxygen scavenging closures for bottled beer. In this chapter he presents this and other systems as case studies in active package development. Chapter 9, by Stanley Sacharow, President of The Packaging Group Inc., New Jersey, is an examination of the role played by active packaging in current commercial use. Stan Sacharow is a leading international consultant in packaging and has written several books and many articles on packaging technology. This chapter provides the necessary understanding of the status of active packaging in the USA today. Active packaging for microwaveable foods is a key topic. Chapter 10 is by Dr Jeremy Selman, Head of the Food Technology Division of the Campden Food and Drink Research Association in the UK. Dr Selman is widely respected for his work on food quality monitoring via
time-temperature indicators (TTIs). In this chapter he provides the background to TTIs and summarizes their role with much tabulated information. He provides substantial unity to the TTI field by discussing their recent introduction to thermal process validation. Chapter 11 completes this work by drawing together the implications of active packaging for food safety. Prof. Joseph Hotchkiss of Cornell University, New York, has contributed widely to research and discussion of the role of packaging in food safety. In this chapter he draws together the safety issues which arise from the use of active packaging and illustrates how active packaging can itself contribute to food safety in the use of antimicrobial films. Acknowledgements I extend my thanks to all the contributors, many of whom I have known and all of whom I have respected for several years. I appreciate their commitment in working within the tight schedule required. I also thank Lyn Keen for her tolerant preparation and reorganization of much of the manuscript, and Andrew Sennett for preparing so many versions of some of the graphics. My thanks also go to Drs Bob Holland, Brian Patterson, Bob Johnson, Mark Horsham, Alister Sharp, Candiera Albert and Michael McNaIIy for refereeing my contributions within CSIRO. I appreciate the advice and forbearance of the staff at Blackie A&P. Finally, I thank my wife Sally, and my children Helen, James and Kathy for their quiet patience and active support during the preparation of this book. M.L.R.
Contributors
Y, Abe
Mitsubishi Gas Chemical Europe GmbH, Immermannstrasse 45, Deutsch-Japannische Center, 40210 Dusseldorf, Germany
AX. Brody
Rubbright-Brody Inc., 733 Clovelly Lane, Devon, PA 19333-1808, USA
JA. Budny
PharmaCal Ltd., 31308 Via Colinas, Suite 107, Westlake Village, CA 91362, USA
B. Cuq
CIRD-SAR, 73 rue J.F. Breton, BP 5035, 34032 Montpellier, France
N. Gontard
ENSIA-SIARC, 1101 Avenue Agropolis, BP 5098, 34033 Montpellier, France
S. Guilbert
CIRD-SAR, 73 rue J.F. Breton, BP 5035, 34032 Montpellier, France
J. Hoshino
Mitsubishi Gas Chemical Company, Inc, 1-1, Nijuku 6-Chome, Katsushika-ku, Tokyo 125, Japan
J.H. Hotchkiss Institute of Food Science, Cornell University, 119 Stocking Hall, Ithaca, NY 14853-7201, USA D.S. Lee
Department of Food Engineering, Kyungnam University, 449 Wolyoung-dong, Masan City 630-701, Korea
M.L. Rooney
Principal Research Scientist, CSIRO Food Research Laboratory, PO Box 52, North Ryde, New South Wales 2113, Australia
S. Sacharow
The Packaging Group Inc, PO Box 345, Milltown, NJ 08850, USA
J.D. Selman
Director of Food Technology Division, CFDRA, Chipping Campden, Gloucestershire, GL55 6LD, UK
JLP. Smith
McGiIl University, Department of Food Science and Agricultural Chemistry, Macdonald Campus 21, 111 Lakeshore, Ste-Anne-de-Bellevue, Quebec, H9X 3V9, Canada
F.N. Teumac
Vice-President Research and Development, ZapatA
Technologies Inc., PO Box 2278, Forest Road, Humboldt Industrial Park, Hazleton, PA 18201, USA K.L. Yam
Assistant Professor, Department of Food Science, Rutgers University, PO Box 231, New Brunswick, NJ 08903-0231, USA
D. Zagory
Devon Zagory & Associates, Postharvest Technology Consultants, 759 North Campus Way, Davis, CA 95616, USA
Contents
Preface ...............................................................................
iii
Acknowledgements ............................................................
v
Contributors ........................................................................
vi
1.
1
Overview of Active Food Packaging ........................ 1.1
Active, Intelligent and Modified Atmosphere Packaging .................................................................
1
Origins of Active Packaging ......................................
3
1.2.1
Why Active Packaging ................................
3
1.2.2
Historical Development ..............................
4
1.3
Literature Review ......................................................
10
1.4
Scope for Application of Active Packaging ................
12
1.4.1
Do-it-yourself Active Packaging ..................
17
1.5
Physical and Chemical Principles Applied ................
20
1.6
Implications for Other Packaging ..............................
27
1.6.1
Whole Packages Designed to Be Active ....
29
1.7
Limitations of Current Approaches ............................
31
1.8
Future Potential .........................................................
32
1.9
Regulatory Considerations ........................................
33
References ..........................................................................
33
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viii
1.2
2.
Contents
ix
Ethylene-removing Packaging ..................................
38
2.1
The Chemistry of Ethylene ........................................
38
2.1.1
Synthesis ....................................................
38
2.1.2
Degradation ................................................
39
2.1.3
Adsorption and Absorption .........................
40
Deleterious Effects of Ethylene .................................
41
2.2.1
Respiration .................................................
42
2.2.2
Fruit Ripening and Softening ......................
42
2.2.3
Flower and Leaf Abscission .......................
42
2.2.4
Chlorophyll Breakdown ..............................
43
2.2.5
Petal Inrolling in Carnations ........................
43
2.2.6
Postharvest Disorders ................................
43
2.2.7
Susceptibility to Plant Pathogens ...............
43
Interactions of Ethylene and Other Gases ................
44
2.3.1
Oxygen .......................................................
44
2.3.2
Carbon Dioxide ...........................................
44
2.3.3
Ozone .........................................................
45
Ethylene Sources in the Environment .......................
45
2.4.1
Combustion ................................................
45
2.4.2
Plant Sources .............................................
45
2.4.3
Ripening Rooms .........................................
46
2.4.4
Fluorescent Ballasts and Rubber Materials .....................................................
46
Microorganisms ..........................................
46
Commercial Applications in Packaging .....................
46
2.2
2.3
2.4
2.4.5 2.5
2.5.1
Potassium Permanganate-based Scavengers ................................................
46
2.5.2
Activated Carbon-based Scavengers .........
47
2.5.3
Activated Earth-type Scavengers ...............
48
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x
Contents 2.5.4
3.
New and Novel Approaches to Ethyleneremoving Packaging ...................................
50
Acknowledgements .............................................................
51
References ..........................................................................
51
Design of Modified Atmosphere Packaging for Fresh Produce ............................................................
55
3.1
Introduction ...............................................................
55
3.2
Literature Review ......................................................
57
3.3
Feasibility Study ........................................................
59
3.3.1
Optimum Conditions ...................................
60
Respiration Rates ......................................................
61
3.4.1
Temperature Effect .....................................
61
Measurement of Respiration Rates ...........................
62
3.5.1
Flow-through System .................................
62
3.5.2
Closed System Method ..............................
63
Model Equations and Package Requirements ..........
64
3.6.1
Unsteady-state Equations ..........................
65
3.6.2
Steady-state Equations ..............................
66
Polymeric Films for MAP Applications ......................
67
3.7.1
Perforation and Microporous Films .............
68
3.7.2
Temperature Compensating Films .............
70
3.7.3
Ceramic-filled Films ....................................
70
Concluding Remarks .................................................
70
Nomenclature ......................................................................
71
References ..........................................................................
72
Active Packaging in Polymer Films .........................
74
4.1
74
3.4 3.5
3.6
3.7
3.8
4.
Introduction ...............................................................
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Contents
xi
Oxygen Scavenging ..................................................
74
4.2.1
Forms of Oxygen-scavenging Packaging ...
76
4.2.2
Plastics Packaging as Media for Oxygen Scavenging .................................................
77
Brief History of Oxygen-scavenging Films ...........................................................
80
4.2.4
Chemistry of Oxygen Scavenging ..............
83
4.2.5
Chemical Barrier to Oxygen Permeation ....
92
Moisture Control Films ..............................................
94
4.3.1
Liquid Water Control ...................................
95
4.3.2
Humidity Buffering ......................................
96
4.4
Removal of Taints and Food Constituents ................
99
4.5
Ingredient Release .................................................... 102
4.2
4.2.3
4.3
4.5.1
Antioxidant Release from Plastics .............. 103
4.6
Permeability Modification .......................................... 105
4.7
Current Use Commercially ........................................ 106
4.8
Regulatory and Environmental Impacts .................... 106
References .......................................................................... 107
5.
Edible Films and Coatings as Active Layers ........... 111 5.1
Introduction ............................................................... 111
5.2
Use of Edible Active Layers to Control Water Vapor Transfer ..................................................................... 114
5.3
Use of Edible Active Layers to Control Gas Exchange .................................................................. 121
5.4
Modification of Surface Conditions with Edible Active Layers ............................................................. 126
5.5
Conclusion ................................................................ 134
Acknowledgements ............................................................. 135 References .......................................................................... 135 This page has been reformatted by Knovel to provide easier navigation.
xii 6.
Contents Interactive Packaging Involving Sachet Technology ................................................................. 143 6.1
Introduction ............................................................... 143
6.2
Oxygen Absorbents ................................................... 144
6.3
6.4
6.2.1
Classification of Oxygen Absorbents .......... 145
6.2.2
Main Types of Oxygen Absorbents ............ 149
6.2.3
Factors Influencing the Choice of Oxygen Absorbents ................................................. 152
6.2.4
Application of Oxygen Absorbents for Shelf-life Extension of Food ....................... 153
6.2.5
Advantages and Disadvantages of Oxygen Absorbents .................................... 161
6.2.6
Effect of Oxygen Absorbents on Aflatoxigenic Mold Species ........................ 164
Ethanol Vapor ........................................................... 164 6.3.1
Ethanol Vapor Generators .......................... 166
6.3.2
Uses of Ethicap for Shelf-life Extension of Food ........................................................... 168
6.3.3
Effect of Ethanol Vapor on Food Spoilage/food Poisoning Bacteria ............... 171
6.3.4
Advantages and Disadvantages of Ethanol Vapor Generators .......................... 171
Conclusion ................................................................ 172
References .......................................................................... 172
7.
Enzymes as Active Packaging Agents .................... 174 7.1
Enzymes ................................................................... 174
7.2
Potential Roles of Enzymes in Active Packaging ...... 176
7.3
History ....................................................................... 178
7.4
Oxygen Removal ....................................................... 179
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xiii
7.5
Antimicrobial Effects .................................................. 186
7.6
Time-temperature Integrator-indicators ..................... 188
7.7
Lactose Removal ...................................................... 189
7.8
Cholesterol Removal ................................................. 190
References .......................................................................... 191
8.
The History of Oxygen Scavenger Bottle Closures ..................................................................... 193 8.1
Background ............................................................... 193
8.2
Oxygen Measurements ............................................. 193
8.3
8.2.1
Techniques for Measuring the Oxygen Content of Bottles ....................................... 193
8.2.2
Results of Measurements ........................... 194
8.2.3
Oxygen Ingress .......................................... 195
8.2.4
Combining the Effect of Initial and Ingress Oxygen ....................................................... 196
Oxygen Scavenger Liners ......................................... 197 8.3.1
Theoretical .................................................. 197
8.3.2
Commercial Activity .................................... 197
8.3.3
Health and Environmental Concerns .......... 199
8.4
The Effect of Scavenging Closures on Beer Flavor .. 199
8.5
The Advantages of Oxygen Control Bottles .............. 200
8.6
The Future of Oxygen Scavenging Closures ............ 200
References .......................................................................... 201
9.
Commercial Applications in North America ............ 203 9.1
Packaging Overview ................................................. 203
9.2
Marketplace Susceptors ............................................ 203 9.2.1
Susceptor Types ........................................ 204
9.2.2
Field Intensification Devices ....................... 206
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xiv
Contents 9.2.3
Susceptor Applications ............................... 208
9.3
Application of Temperature Indicator to Microwaveable Packaging ........................................ 209
9.4
Active Packaging – Produce ..................................... 209
9.5
9.4.1
Oya Produce Bags ..................................... 209
9.4.2
Oya Test Results ........................................ 210
9.4.3
Modified Atmosphere Produce ................... 211
Oxygen Absorber Food Applications ......................... 211 9.5.1
9.6
Bottle Closures – Oxygen Scavengers ....... 213
Other Applications ..................................................... 213
References .......................................................................... 214
10. Time-temperature Indicators .................................... 215 10.1 Introduction ............................................................... 215 10.2 Indicator Systems ...................................................... 217 10.3 Indicator Application Issues and Consumer Interests .................................................................... 227 10.4 Chemical Indicators for Thermal Process Validation .................................................................. 230 10.5 Conclusions ............................................................... 234 References .......................................................................... 234
11. Safety Considerations in Active Packaging ............ 238 11.1 Introduction ............................................................... 238 11.2 Packaging and Food Safety ...................................... 238 11.3 Passive Safety Interactions ....................................... 240 11.3.1
Barriers to Contamination ........................... 240
11.3.2
Prevention of Migration .............................. 241
11.4 Active Safety Interactions .......................................... 242 11.4.1
Emitters and Sorbers .................................. 243
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xv
11.4.2
Active Packaging and Migration ................. 243
11.4.3
Barrier to Contamination ............................ 244
11.4.4
Indirect Effects on Safety ........................... 244
11.4.5
Indicators of Safety/spoilage ...................... 245
11.4.6
Direct Inhibition of Microbial Growth ........... 246
11.4.7
Modified Atmosphere Packaging ................ 246
11.4.8
Antimicrobial Films ..................................... 248
11.4.9
Rational Functional Barriers ....................... 250
11.4.10 Combined Systems .................................... 252 11.5 Conclusions ............................................................... 252 References .......................................................................... 253
Index .................................................................................. 256
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1
Overview of active food packaging M.L. ROONEY
1.1 Active, intelligent and modified atmosphere packaging Packaging may be termed active when it performs some role other than providing an inert barrier to external conditions. Hotchkiss (1994) includes the term 'desired' when describing the role, and this is important in that it differentiates clearly between unwanted interactions and desired effects. This definition reflects the element of choice in how active packaging performs and the fact that it may play some single intended role and otherwise be similar to other packaging in the remainder of its properties. These latter two aspects also reflect that active packaging is something that is designed to correct deficiencies which exist in passive packaging. A simple example of this situation is when a plastics package has adequate moisture barrier but an inadequate oxygen barrier. Active packaging solutions could be the inclusion of an oxygen scavenger, or an antimicrobial agent if microbial growth is the quality-limiting variable. Active packaging has developed as a series of responses to unrelated problems in maintenance of the quality and safety of foods. Accordingly a range of types of active packaging has been developed. Each of these has a range of descriptive terms. Horticultural produce has for some years now been packaged in 'smart films', and oxygen has been removed from package headspaces by oxygen scavengers, free-oxygen absorbers (FOAs) and deoxidisers. Carbon dioxide can be released by emitters or can be absorbed by getters, and microwaves can be controlled in packages by susceptors or shields (Sacharow and Schiffman, 1992). Regional differences in terminology are also seen. The terms 'freshness preservative' and 'functional' and 'avant garde' are also used to describe active packaging materials (Katsura, 1989; Louis and de Leiris, 1991). There has been a range of trade names for those packages where a generic form has not been coined, with the result that we have SmartCap (ZapatA - Advanced Oxygen Technologies) for closures for beer bottles and Oxyguard for Toyo Seikan Kaisha Ltd. for caps for similar use. Smart packages have been defined by Wagner (1989) as 'doing more than just offer protection. They interact with the product, and in some cases, actually respond to changes'. In this sense, most packaging media are active to some degree. However, there are forms of packaging which are clearly distinct subclasses. The term equilibrium modified atmosphere (EMA)
packaging is used to distinguish the situation where the choice of the permeability ratio of oxygen/carbon dioxide determines whether respiring horticultural produce generates a viable gas atmosphere or not (Gill, 1990). Thus where modified atmosphere packaging (MAP) is used with processed foods and involves merely flushing with an initial gas mixture the packaging is not active. EMA packaging is one of the borders between active and passive packaging. Some aspects of EMA packaging are discussed in this book because the limitations of conventional packaging films are being increasingly addressed using active materials. If the physical interactions of a package with the food are removed we are left only with the chemical (and increasingly, biochemical) effects. Such a restriction is probably unduly strict and in time we should expect to see further subdivision of active packaging to take account of whether 'activity' is a property of the packaging material itself or of inserts within the package. We are beginning to see reference to the benefits of active packaging in the popular press with reference to 'packaging that is niftier and cooks your food' and 'Hi-Tech' packaging (Sprout, 1994). The active packaging so described includes susceptors and reflectors in microwaveable packs as well as horticultural smart films that absorb ethylene. These are described together with temperature sensitive labels that help determine when food is cooked, i.e. 'doneness indicators'. There are other areas of packaging developing concurrently and there are areas of overlap with active packaging as noted with MAP above. Probably the closest area is Intelligent Packaging, a grouping of technologies defined in the CEST publication by Summers (1992) as 'an integral component or inherent property of a pack, product or pack/product configuration which confers intelligence appropriate to the function and use of the product itself. This grouping covers the area of product identity, authenticity, traceability, tamper evidence, theft protection, and quality as indicated by timetemperature indicators. The latter was originally included by Labuza (1987) in his seminal review of active packaging. Time-temperature indicators also fit the definition of active packaging given above; they play a role in defining the steps that need to be taken to ensure the quality and safety of the packaged food. A somewhat related field of packaging which so far has fallen between the two definitions is that of gas composition indicators. To date they have been used in the form of tablets to indicate when oxygenscavenging sachets have achieved their purpose (Anon, undated). There have been steady efforts made for several years to produce oxygen-indicating printing inks but thus far, like the pellets, these indicators largely change colour at oxygen levels below 0.1%. The description of this field as interactive packaging is also seen. There is some benefit in such a description as it links desired and undesired interactions of foods and their packaging, such as flavour scalping (Hirose et
al, 1989). However, this lack of distinction is probably a disadvantage overall, hence the naming of this book. 1.2 Origins of active packaging 1.2.1 Why active packaging Inherent in the definition is the point that active packaging now plays a role in the protection of the food which is additional to the classic purposes of any packaging, viz. containment, protection, convenience and communication (Robertson, 1993). This role can be addressing a single aspect of the packaging requirements of a food, such as making up for an inadequacy of a packaging material - which is already a compromise. Thus, for semiaseptic or retort trays of steamed rice, oxygen-scavenging sachets are bonded to the lid to consume oxygen passing through the trays, especially when retorted. Ethanol-releasing sachets are used with bakery products of high aw to suppress mould growth because low oxygen barrier packaging is used. These examples show that there are opportunities to reduce the cost of packaging materials or packaging processes by use of active packaging with cheaper passive packaging. This has been demonstrated by Alarcon and Hotchkiss (1993), who showed that crusty bread rolls packaged in a medium barrier plastics laminate had a shelf-life similar to that provided by a more expensive foil laminate (see Chapter 6). The key to using chemical forms of active packaging for economic benefit is that the benefit must be achievable before the chemical is exhausted. This is particularly important in the area of fresh and extended shelf-life foods as originally described by Labuza and Breene (1989) but not only in that area. For example, the suppression of enhanced oxygen permeation of retortable trays (caused by retorting) by application of a desiccant layer is important while the water is being absorbed rapidly during retorting, in order to impart multi-year shelf-life at ambient temperatures. The key focus of active packaging therefore is the match of the package properties to those of the food, a target that normally has been considered to require compromise. The result of this matching is therefore optimisation of shelf-life and permitting processes, formulations and presentation which were previously impossible. It is possible to add hurdles to enhance the safety of food packaging processes. It is in this area that antimicrobial packaging may make a substantial contribution as it develops from its current early stage. It should be emphasised that active packaging is not a generally applied concept like, for instance, water vapour barrier packaging plastics. It is rather the application of specific packaging properties to specific situations. In this way, we see a large number of niche markets, some niches being very
large indeed as with oxygen scavenging crown closures for bottled beer. In some instances we see the need for application of two forms of active packaging to achieve a goal. This has been demonstrated by Naito et al (1991) who showed that use of an oxygen absorber alone suppressed growth of bacteria and of yeast (Hansenula anomala) in packages of sponge cake but that the inclusion of an ethanol or carbon dioxide emitter was even more effective. The use of combined-effect sachets, for instance, is now commonplace - see Chapter 6. 1.2.2 Historical development Because it applies to a collection of niche markets, active packaging has a very uncoordinated history. This aspect can best be examined by seeking to subdivide it, albeit somewhat artificially, into processed food packaging and fresh food packaging. Probably the earliest form of active packaging was the wine skin which, while causing a variety of unwanted interactions, was intended to collapse as the wine was consumed without necessarily increasing the quantity of oxygen within. Just how good a barrier the skin is to oxygen permeation is not known, so this is a far from perfect example. The most obvious commencement of active packaging was the use of tinplate for construction of cans. The tin is sacrificially corroded, protecting the iron base can, while concurrently saving the food from contamination with large amounts of iron. The latter, with its two readily accessible oxidation states, can function as an autoxidation catalyst when residual oxygen is present. The tin also acts as a reducing agent for food constituents such as pigments. It also contributes to development of the flavour associated with die 'traditional' canned orange juice. This became economically significant with the introduction of aluminium cans when it was observed that the 'traditional' flavour was not generated in the absence of tin. A subsequent development was the introduction of sulfur-staining resistant lacquers (or enamels). The sulfur staining is due to the decomposition of sulfur-containing amino acids in foods to produce compounds which react with the tinplate, so staining the metal surface. The introduction of zinc oxide results in a reaction which forms products not observable in the otherwise white lacquer. 1.2.2.1 Iron-based oxygen scavengers. Since tinplate cans were the basic packaging of processed foods for most of the past century, it is not surprising that the next development was also targeted at canned food. Tallgren (1938), in a patent, described the use of iron, zinc or manganese powders to remove oxygen from the headspace of cans. This invention paved the way for the subsequent development of the iron-based oxygen scavengers of commerce today.
Table 1.1 History of iron-based oxygen scavengers Additional reagents
Form
Water absorbents Iron compounds Sodium carbonate Carbon, water Alkali metal halides Ammonium chloride Sodium chloride Un-named
powder powder powder powder powder powder powder film
Reference
Country
Tallgren, 1938 Isherwood, 1943 Buchner, 1968 Nawata el al., 1977 Mitsubishi, 1977 Kureha, 1982 Fujishima, 1985 Koyama/Oda, 1992
Finland UK Germany Japan Japan Japan Japan Japan
The development of iron-based oxygen scavengers is summarised in Table 1.1 which shows movement from Europe to Japan as development of commercially useable systems occurred. Concurrent with the development of iron-based scavengers was the development of other inorganic and organic systems, both based on the use of sachets. The reaction of sodium dithionite with oxygen can be triggered by the presence of water from a food headspace. The steps involved in the scavenging of oxygen by such agents are shown in the following equations (Nakamura and Hoshino, 1983). Na2S2O4 + Ca(OH) 2 + O
2
-*
Na 2 SO 4 + CaSO 3 + H2O
(1.1)
Na 2 S 2 O 4 + O 2
->
Na 2 SO 4 + SO 2
(1.2)
Ca(OH)2 + SO2
->
CaSO 3 + H2O
(1.3)
The reactions involved in the scavenging of oxygen by iron-based compositions are discussed in Chapter 6. Additional processes involving organic chemicals have also been developed, commencing with catechols and related substances and resulting in the widespread claims of use of ascorbic acid in patents in recent years. The recent growth in the number of patent applications, found via the Derwent Index, for systems which do not involve iron can be seen in Figure 1.1. The Derwent Index catalogues patent applications by the earliest national application. Any applications for coverage of the same matter in other countries are indexed therewith. These results were generated by taking the numbers of earliest applications without consideration of the number of additional countries covered. Not all applications are necessarily granted but patenting activity is an indication of interest in developing new ideas. The results in Figure 1.1 show the distribution of such early applications for composition claims over the 20-year period from 1975 to the first half of 1994. The incremental unit is 2 years and the applications were distinguished by their involvement with or frequent inclusion of zinc, copper or nickel since non-food applications also constitute a very large potential market. The interest in systems not involving elemental metals was initially the strongest although claims for metallic systems peaked around 1980 after
Other
Applications
Metal
rears Figure 1.1 Substrates described in patent applications for oxygen scavengers.
Mitsubishi Gas and Chemical Co. released Ageless sachets in 1978. The fall in the number of patent applications in the mid-1980s is reflected in both the number of metallic compositions and that of other compositions. The subsequent, increasingly strong activity in both areas of chemistry reflects the need for processes which overcome deficiencies in existing products. If the preparation of an active packaging material or process is viewed as a product development topic in its own right then there are two facets which should be evident. The first is the composition and the second is the design of the remainder of the product, including its packaging. Some active packaging technologies have been based on the introduction of a sachet into the package of food and thus have the characteristics of the protective and functional packaging of any other sensitive product. The results in Figure 1.2 show that, initially, innovation was directed towards establishing claims for novel compositions and that the level of activity between 1977 and 1982 was almost the same as that between 1989 and 1994. What has changed are the chemical reactions involved in these compositions. Initially, greatest interest was shown in the reactions undergone by various sulfites in the presence of alkali powders which absorb the sulfur dioxide formed. Subsequent developments were based on oxidation of iron powder or ferrous compounds and this has continued with the growth in the number of compositions based on organic reactions. Prominent among these is oxidation of ascorbic acid or of ethylenically unsaturated compounds such as fatty acids, squalene and rubbers.
Applications
Composition
Design
Years Figure 1.2 Patent applications for oxygen scavengers based on composition or design.
Evidence of the maturity of this field of active packaging is shown by the trend in patent applications for novel designs aimed at overcoming deficiencies in either handling or effectiveness found in the early versions. The ease of triggering an active state in oxygen-scavenging compositions is more important than with most other forms of active packaging. Design patents were scarcely considered for the first 8 years of this period until the products of companies such as Mitsubishi Gas and Chemical Co. and Toppan Printing Co. made an impact on food packaging in Japan. Since that time the number of applications for new designs has increased sharply, with the numbers of design applications over the last 10 years exceeding those for new compositions. This maturity of the market is seen in the progressive expansion of use of such products in the USA, Europe and Australasia. The current production of oxygen-scavenging sachets alone is understood to exceed 7 billion per annum in Japan, several hundred million in the USA and some tens of millions in Europe. Another of the aspects of oxygen-scavenger development is the respective positions of sachet and film-based technologies. These aspects are considered in detail in Chapter 4. A recent trend is the incorporation of scavengers into the packaging material itself, rather than being used in a sachet which hitherto had been the main commercial application. Chapter 4 deals with development of scavenging plastics for packages in general, and the specific commercial application to bottle closures, especially as liners for crown seals for beer bottles, is discussed in Chapter 8. Besides the main lines of research and development of oxygen scavengers referred to above there have been other, less popular, lines of research and
commercial development. The catalytic conversion of hydrogen to water, initially in tinplate cans then in laminate pouches, was first described by King (1955) who saw the need for removal of oxygen from spray-dried canned milk powder. There has also been continuous research into ways of stabilising enzymic catalysts for oxygen removal. The oxidation of glucose, catalysed by glucose oxidase, has been studied intermittently following the work with OxyBan by Scott and Hammer (1961). This subject is discussed in detail in Chapter 7. 1.2.2.2 Composite systems. An early observation was that in-pack removal of oxygen from package headspaces creates a corresponding decrease in pressure which results in either package collapse or development of a partial vacuum. The latter may be tolerable in rigid packaging where the seal integrity is good, but in flexible packs pressure decreases of as little as a few kPa result in package collapse where the headspace is small. The simultaneous release of carbon dioxide by sachets which consume oxygen was devised by several companies (see Chapter 6). This concurrent release of carbon dioxide can be additionally useful in the suppression of microbial growth (Naito et al9 1991). The concentration of this gas needs to reach 20%, preferably more, and is potentially useful where packages already have some carbon dioxide in the atmosphere. Such systems are based on either ferrous carbonate or a mixture of ascorbic acid with sodium bicarbonate. More effective use of dual-function active packaging inserts has been foreseen by several companies in this field. The potential for generation of conditions suitable for growth of anaerobic pathogenic microorganisms exists if oxygen scavengers are used inappropriately. Accordingly, the release of ethanol or carbon dioxide concurrent with oxygen removal has been cited for this purpose by both Toppan Printing KK (1985) and Leon et ah (1987). This performance of commercial dual-function scavenging sachets is discussed in Chapter 6. The historical development of other forms of emitters of ingredients such as flavours, antioxidants and antimicrobial agents is dealt with in other chapters. This applies also to packaging for microwaveable foods where there is the potential for one form of active packaging to interact with another. Thus there is the need to ensure that oxygen-scavenging sachets do not interfere with microwave heating of packages of ready-to-eat foods such as retorted steamed rice. This product is marketed with an organic reagent, ascorbic acid, in a sachet bonded to the inside of the lid of the tray with a hot melt adhesive. An additional approach to overcome the same problem, also by Mitsubishi Gas and Chemical Co., is the patenting of scavengers based on boron and its compounds.
1.2.2.3 Horticultural active packaging. Active packaging for horticultural produce has evolved via the concept of 'smart films'. It has long been known that respiring produce will generate its own modified atmosphere, but in so doing it may reach a stage of ripeness which is not desired. As long ago as the 1970s choices were made between packaging films in an effort to achieve a modified atmosphere which suppressed respiration and lengthened shelf-life (Prince, 1989). Since there was difficulty in achieving satisfactory performance with many produce-film combinations, highly permeable porous patches were introduced. This enabled packers to use film of any commodity while ensuring that there was adequate exchange of oxygen and carbon dioxide. This is unsuitable for developing equilibrium modified atmospheres close to the optimum for all produce but is useful both in preventing injury due to excessively high concentrations of CO2 and in stopping anaerobic respiration from being initiated. This approach has been developed progressively in the USA with the patches of microporous film developed by Hercules Freshold Corp. and with subsequent patches, the gas permeability of which changes with the degree of hydration. Concurrently, other workers have approached the same problem by seeking to make the entire packaging film more permeable or by seeking to develop some enhancement of selectivity. This field has developed either by use of patents or merely by use of advertising claims with little or no scientifically developed results supporting such claims. Thus films have been made porous by inclusion of porous powders such as zeolites or volcanic rock, or by inclusion of crushed rocks. More recently, porosity has been increased by burning or etching controlled diameter holes to prevent both condensation and oxygen depletion. Recently, again, some considerable scientific modelling effort has been focused on the use of a single pore in a package to achieve the necessary rate of gas exchange (Mannapperuma and Singh, 1994). During the 1980s, when marketing of films containing inorganic powders was commenced, widely ranging claims were made for effects such as the emission of infra-red radiation from ceramics. Claimed benefits include preservation of the produce and absorption or scavenging of ethylene. Evidence for removal of ethylene from package headspaces other than by enhanced permeation through porous solids or porous film does not appear to be substantial. Some films containing inorganic powders are claimed to offer multiple benefits for packaging of horticultural produce. Katsura (1989) reports that FH film containing Oya stone dispersed on the film surface absorbs ethylene, is a freshness preservative, and controls levels of water vapour, oxygen and carbon dioxide in the pack. Chemical scavenging of ethylene, however, has been the subject of considerable research since the work of Scott et al. (1970). Their work with potassium permanganate in porous slabs of vermiculite in bags of bananas
demonstrates the early success of this approach. The detail of the historical development of research and commercial development of such processes is described in Chapter 2. The use of edible or otherwise non-toxic coatings on fruit skins as vehicles for active chemicals has been known for several years. The modification of the atmosphere close to the skins of fruit has been achieved by shrink wrapping. Both these techniques assist in eliminating the build-up of surface water as a cause of microbial growth on fruit surfaces. The recent introduction of minimally processed fruits and vegetables in packaged form has heightened awareness in this area. The importance of hygiene in the preparation of sliced and diced produce has been discussed by Varoquaux and Wiley (1994). The potential for antimicrobial edible films is reviewed in Chapter 5. 1.3 Literature review Active packaging has developed as a series of topics which are related only because they involve the package influencing the environment of the food. The literature in this field consists very largely of patent applications, technical leaflets and reviews. The latter have often been presented at conferences where specialised audiences have been able to take up the ideas presented. Reports of academic scientific investigation have been limited largely to occasional assessments of the appropriateness of some of these technologies. The literature in this field is therefore discussed in terms of the reviews. Sneller (1986) reported on the impact of smart films on controlled atmosphere packaging although the first broadly based reviews were presented in Iceland and Australia in 1987 (Labuza, 1987; Rooney, 1987). The first use of the term active packaging was proposed at that time by Labuza, who defined active packaging as a range of technologies, some of which now represent the borderlines between active, 'intelligent', and modified-atmosphere packaging (Labuza and Breene, 1989). The essential features of these 'freshness enhancers' have been summarised in a short review by Sacharow (1988). Katsura (1989) reviewed the range of functional packaging materials which had been commercialised with particular reference to Japan. He demonstrated the attention that had been paid to freshness preservative packaging. Wagner (1989) summarised the range of smart packages and emphasised the role of microwaveable-food packaging. Rooney (1989a,b; 1990) concentrated on chemical effects, particularly oxygen scavenging. The role of oxygen scavengers in maintaining the benefits of MAP for processed foods was reviewed by Smith et al. (1990) following their own research into suppression of microbial growth (see Chapter 6).
The International Conference on Modified Atmosphere Packaging at Stratford-upon-Avon (UK) in 1990 organised by the Campden Food and Drink Research Association included several reviews relating to active packaging. Louis described several innovative active packages which generated modified atmospheres. Abe gave the first comprehensive quantitative assessment of the impact of active packaging. He estimated the market size for each of the broad classes of such packaging systems. His review reveals that around 6.7 billion oxygen-scavenging sachets and 70 million ethanol-generating sachets were manufactured in Japan in both 1989 and 1990. The estimated market for films containing mineral powders was only 1000 tonnes in 1989 with 40% of consumption as home use. The review by Robertson (1991) emphasised the application of active packaging to processed foods. The emphasis was placed on crown seals for bottled beer, oxygen-scavenging plastics films and microwave susceptors. The use of the term active packaging rather than smart films was noted by that reviewer and by Sacharow (1991) who also noted the use of sachets of potassium permanganate in silica gel for ethylene removal in produce packs. By this time the claimed benefits of freshness preservation technologies for horticultural products were being examined critically, especially in Japan. Ishitani (1993a,b) surveyed the number of patent applications for this purpose from 1984 to 1989. Over the first two years the annual rate was around 35 applications. This increased to a peak rate of 220 per annum in the second half of 1987 before dropping to around 60 per annum in 1989. It was noted that initial developments were directed at the needs for lowtemperature maintenance and moisture control. The boom in 1987 was the consequence of the attention being paid to gas composition control and ethylene removal. By 1989 gas composition was the main object of developments but moisture control and coating methods were also important. Ishitani (1993a) observed two factors that led to much rethinking. These were the lack of data on the requirements of produce and doubts about the capacity of powder-filled plastics to remove enough ethylene. More recent developments have been focused on ethylene removal at high humidities and on matching gas composition and temperature to the requirements of enzymic systems of plants. Several recent books on MAP have included discussion of the gas-packaging requirements for horticultural produce as well those for some processed foods (Ooraikul, 1993; Parry, 1993). The environmental aspects of active packaging have not been considered to any great extent in reviews to date. Rooney (1991) addressed some issues drawing attention to the need to consider the nature of the packaging which can be replaced by these new technologies. The current state of development and commercial application of active packaging has been reviewed in three papers at the symposium Interaction: Foods - Food Packaging Material held in Sweden in June 1994. Miltz et al (1994) reviewed the field in general, Ishitani (1994) concentrated on
Japanese developments, especially antimicrobial films, Day (1994) concentrated on fresh produce and Guilbert and Gontard (1994) focused on edible and biodegradable packaging. Several posters described original research and that of Paik described photoprocessing of a film surface to generate antimicrobial properties. Perdue (1993) has briefly reviewed antimicrobial packaging from the viewpoint of the Cryovac Division of W.R. Grace Company and presents a somewhat pessimistic picture. 1.4 Scope for application of active packaging Active packaging is still developing as a collection of niche markets so it is not surprising that a diverse range of packages active in the physical and chemical sense are either proposed or commercially available. Early among these was the use of the reaction of lime with water to generate heat for selfheating cans of sake (Katsura, 1989). The Verifrais process for meat
Respiring Produce
Delayed ripening Temperature abuse Fungal growth
Aseptic Liquids
Oxidation
active packaging Prepared Meals
Microwave cooking
Oxidation Hydration
Colour Retention
Dry Foods
Chilled Meats
Mould Growth
Bakery Products REQUIREMENT
FOODCLASS
Figure 1,3 Properties of foods amenable to active packaging.
packaging uses the reaction of organic acid with bicarbonate to produce carbon dioxide in response to meat drip in foam trays. The carbon dioxide released helps to suppress microbial growth. Some properties of foods which can be addressed by active packaging are summarised in Figure 1.3. These properties are grouped depending upon whether they are designed to sustain living foods, suppress insect or microbial life in any foods, prevent oxidative attack on food constituents, retain flavour, or facilitate serving of the food for consumption. The ways in which such packaging performs these roles are elaborated in the remainder of this section. Active packaging can been seen in one sense as a means of maintaining the optimum conditions to which a food was exposed at the immediately preceding step in its handling or processing. Passive packaging has been used in an effort to minimise the deleterious effects of a limited number of external variables such as oxygen, water, light, dust microorganisms, rodents and to some extent, heat. Hence, active packaging has the potential to continue some aspects of the processing operation or to maintain chosen variables at particular levels. This aspect of active packaging is a unifying theme and crosses the border between foods such as plant produce, and processed foods, including those thermally processed. A second aspect of active packaging is that it can be involved in the preparation of the food for consumption. This includes aspects of temperature modification either for organoleptic or food safety purposes. These properties therefore include heating, cooling and foaming. Non-processed, respiring food such as agricultural and horticultural produce, fish, crustaceans and other seafood can be stored and/or shipped over long distances provided the respiration requirements are satisfied under controlled temperature conditions. Thus if the packaging can regulate the supply of oxygen to the animal or produce such that a minimum respiration rate can be sustained, an enhanced period of prime-quality life can often be achieved. In plant products the optimum oxygen concentration of the environment varies with the species, and levels down to 1% may be possible without inducing anaerobic respiration (Labuza and Breene, 1989). The generation of elevated levels of carbon dioxide to suppress ethylene synthesis and to suppress microbial action can be achieved by selection of plastics films of appropriate permeabilities. However, achievement of the optimum balance of oxygen and carbon dioxide concentrations by use of plastics films alone is frequently impossible, particularly as allowance must be made for temperature abuse. It is possible to predict the potential packaging requirements for horticultural produce by modelling the properties of the food and the packaging film. There have been several reports published on approaches to modelling such systems, and they have been compared by Solomos (1994), who has tabulated the characteristics provided for in each of the models.
Some of these are quite complicated and they are set out in Chapter 3 to provide an easy-to-use model. This form of packaging is commonly termed modified atmosphere packaging (MAP) or more appropriately equilibrium modified atmosphere (EMA) packaging. EMA packaging involving selection of polymer films is, as mentioned previously, the borderline between active and passive packaging. Chapter 3 shows how modelling can indicate quickly whether available packaging plastics are going to be suitable for maintaining EMA over the temperature range chosen. Several approaches to overcoming the limitations of these films have been reported. One which is still in its infancy is the use of liquidcrystal polymers which undergo a phase change at a characteristic temperature. The permeability of the polymer to oxygen sharply increases as this temperature is exceeded, thus providing the oxygen necessary to prevent packaged horticultural produce from switching from aerobic to anaerobic respiration. The present state of the art is not sufficiently advanced to cover EMA films which match produce over a wide temperature range, but research has opened up this possibility. An alternative involves pores in portions of a package which open when the temperature exceeds a precisely set value. This has been achieved by filling pores in a patch on a package with a wax which melts appropriately (Cameron and Patterson, 1992). This wax, when liquid, is drawn away by a wick such as a microporous film to leave the pore open to gas exchange. This type of high-temperature emergency valve is applicable to packages over a wide range of sizes. Microporous patches are already used commercially on retail trays of some fruit. The use of pores in packaging materials to actually balance the atmosphere in packages of respiring fruit has been the subject of some research and a large amount of marketing. Several forms of crushed rock, coral and synthetic zeolites have been incorporated into extruded film but there has been very little disinterested scientific evaluation done. Such films extend the range of gas permeability values of the commodity films in current use. Some results for P-Plus, a porous film currently manufactured by Sidlaw Packaging, Bristol (UK), have been presented (Gill, 1990). Predictive research and some experimental verification of the effects of single pores in produce packages have been reported (Mannapperuma and Singh, 1994). The effects of changes in temperature on gas composition need to be evaluated. Extension of the post-harvest life of fruits and vegetables requires more than EMA packaging. The water relations between the horticultural foodstuff and its atmosphere need to be balanced both to prevent dehydration and to avoid condensation induced by temperature abuse. Since the RH of such packages exceeds 95%, a temperature drop from 12°C to 110C at the pack surface can cause condensation. The visually unpleasant appearance in retail packs is frequently overcome by antifogs in the plastic
and innovative forms of active packaging are described in Chapter 4. Microporous pads containing inorganic salts have been shown to buffer the water vapour pressure (Shirazi and Cameron, 1992). Some of these are used commercially in the USA and Japan but others are close to commercial development. There have been some patents directed towards use of combination effects in active packaging for horticultural produce. Thus there have been patents of combination CO2 emitter/water vapour absorbers and otherwise similar compositions but including an oxygen scavenger as well. Yet another is Neupalon, which is described in Chapter 2. This would bring the advantages of reducing the time the packaged horticultural product is subjected to high oxygen levels and inhibiting the onset of ripening, particularly with climacteric fruit. The rapid oxygen scavenger films of Rooney (1982) and Maloba (1994) could be suitable for this purpose if they met with regulatory requirements. Other approaches to enhancing the storage life of horticultural produce have been directed towards removing ethylene produced by ripening fruits and vegetables. Since ethylene is both produced by ripening fruits and triggers their ripening it is essential to prevent those fruits which are further along the ripening process from triggering ripening of others in the same enclosed space. Injured fruits are a particular problem in this regard and this emphasises the need for strict quality control in EMA packaging. The isolation of packages containing fruit rapidly generating ethylene may be the appropriate target of technologies for ethylene removal. Chapter 2 includes the approaches taken both commercially and in research reports. The challenge appears to be to provide independently verifiable chemical processes which function satisfactorily to remove ethylene at physiologically significant concentrations in packages under conditions of high humidity and possibly in the presence of condensation. Since the quantities of ethylene are tiny, the cost should not be the major obstacle to commercial development. Produce packages normally have large headspaces so both sachet and packaging film scavengers should prove acceptable. Several other 'freshness enhancing' properties have been claimed for some commercial films but the processes occurring therein have not been clarified. This matter is dealt with in more detail in Chapter 2. Besides horticultural produce and living seafood which are meant to be kept alive during transportation, there is the very important field of chilled meats which retain muscular respiration for some hours or days postslaughter. While beef, for instance, is capable of oxygen scavenging by muscle respiration for a few days at meatworks chiller temperatures of-1°C to 1°C, it is no longer capable of doing so for the remainder of the desired storage period, usually 4-8 weeks. Lactic acid bacteria lower the pH and suppress the growth of Bronchothrix thermosphacta and Pseudomonas spp. and other species. There is scope for oxygen scavenging films in bag
construction to prevent oxygen permeation and for lactic-acid-releasing films to enhance this effect in some cases. The removal of residual oxygen from MAP meat packs by oxygen scavengers would increase security and decrease the need for slow, sophisticated packaging processes in this case. The carbon dioxide levels are normally very high (> 99%), as in the Captech process (Gill, 1989), so oxygen scavengers would need to operate wet in this environment. An additional definition of active packaging specific to horticultural produce distinguishes between passive and active modified atmosphere packaging (Zagory and Kader, 1988). The passive form which we are considering as EMA involves choice of the packaging material for its ratio of permeabilities to O2 and CO2 as well as for their absolute values. Active MAP has been defined as gas atmosphere replacement by flushing or evacuation-back flushing, although the option of adding other active agents has also been considered (Kader, Zagory and Kerbel, 1989). Modified atmosphere packaging of non-living foods is now a mature area of research and has resulted in filling significant niche markets, particularly in the bakery, cheese and fresh pasta areas. Fresh pasta, which has been a recent success internationally, is dependent on MAP (Castelvetri, 1990). The growth of moulds, while suppressed by elevated carbon dioxide concentrations, is not uniformly affected across the range of species. Low levels of oxygen can in some cases support some species of mould, particularly as carbon dioxide is lost by permeation of packaging films. There is a need to remove most residual oxygen which may reach more than 1% when flushing is used without prior evacuation. Oxygen concentrations below 0.1% are desirable especially when cut surfaces are exposed, as in pizza-type cheeses and some MAP meats. Besides mould growth, chemical effects such as oxidative attack on colours in preserved meats (Andersen and Rasmussen, 1992), nutrient degradation such as vitamin C loss which can result in browning products (Waletzko and Labuza, 1976), and rancidity generation in fats and oils (Nakamura and Hoshino, 1983) can be prevented or minimised by use of oxygen scavengers. The substantial development work aimed at overcoming these problems is demonstrated by the results shown in the Historical Development section of this chapter. One benefit to researchers of oxygenabsorbers is allowing ultimate effects of near-zero oxygen content atmospheres to be evaluated so that prediction of shelf-lives under other less perfect conditions can be more firmly based. Although initial development, and current commercial practice, is based on sachets of scavengers inserted into packs, much recent research and development has been directed towards scavenging polymers which can address problems with oxidisable liquids such as beer, wines, fruit juices and other beverages. Polymers, because of their ease of melt formation, can take the scavenging capacity to localised
areas such as closures and to areas of close contact of product and package as found with meats, cheeses and wet foods generally. The ability of polymers to act where there is close contact opens the way to provide a variety of food additives via a diffusive mechanism. This includes antimicrobial action (Halek and Garg, 1988) or antioxidant (Han et aL, 1987) effects. To date, the use of such packaging has been restricted to controlled release of antioxidant into cereal products (Miltz et aL, 1989). The benefit of slow release of antimicrobial agents and antioxidants is the potential for maintenance of the requisite high concentration at wet food surfaces. This applies especially to high-water-content foods in which diffusion from the surface into the bulk can deplete surface concentrations (Torres et aL, 1985). This effect has been noted by Smith et aL (1990) who investigated the effectiveness of ethanol-emitting sachets on the growth of Sacchawmyces cerevisiae on apple turnovers. For active packaging to fulfil a useful role in this field it will be necessary for it to provide controllable, slow release matched to the needs of the food. Water-triggered sachets of silica containing ethanol are very much a first generation approach to this form of packaging. The role of edible food coatings in release of food additives is discussed in Chapter 5. Besides antimicrobials and antioxidants there is a wide variety of other agents that can be added to foods or which can act on them. Thus flavours can be added to offset degradation on storage, enzymes can remove oxygen or other undesirable food components, and insecticides can repel insects or kill them with permitted fumigants. There is a potential ethical dilemma which may arise from the application of such approaches to food packaging. There is also the potential for foods to be self-promoting via the aroma of their packaging. Thus a desirable flavour might be generated by an outer layer of a package to attract customers rather than being released from an inner layer to offset scalping or processing losses. In an extreme case, supermarkets might become a confusing garden of unbalanced aromas competing for the organoleptic senses of the customer in much the same way as package print attracts the customer visually. At this point the packaging ceases to be active in the sense of the present definition and can be described as intelligent in the definition of the CEST report (Summers, 1992). Introduction of many of these forms of active or related packaging technologies will necessitate serious consideration of explanatory labelling. In some cases this may require regulation, as with oxygen-scavenging sachets in Japan, the USA and Australia where the "Do not eat label" is required. In Australia at least, minimum sizes are specified to reduce risk of ingestion. 1.4.1
Do-it-yourself active packaging
Shelf-life extension of foods in the home, particularly following opening of the original package, can be seen as a natural extension of the systems used
in previous centuries. For instance the modern bag-in-box systems such as Intasept (Southcorp Flexible Packaging, Australia) is an extension of the Spanish wine-skin concept. Various evaporative coolers for bottled wine or butter etc. were the predecessors of self-cooling cans frequently invented but less frequently manufactured (Katsura, 1989). Following the successful introduction of cling-wraps as short-term moisture barriers in the home, there has been a steady progression of developments in active packaging for in-home use. Typical applications are the short-term packaging of unused food portions such as chicken or fish pieces or freshness retention in vegetables. Most of these forms of packaging are directed towards refrigerated items. An exception to this would be a desiccant pack for cookies or biscuits replacing the early canisters with regenerable silica gel desiccant in the twist cap insert in the lid. Such desiccant could be regenerated in the oven with gentle heating. It is surprising that such a packaging adjunct is not widely available since biscuit packs made from oriented polypropylene readily tear and generally are not resealable. Examples of do-it-yourself active packaging already available are listed in Table 1.2. The potential for the reduction of the surface aw of food pieces by Pichit multilayer film is described in more detail in Chapter 4. The sale of such film in domestic packs of 10 sheets, 19 cm x 27 cm, or on a perforated roll (as with polyethylene bags for plant produce in supermarkets) is particularly effective. Different approaches to modification of water vapour transfer from foods such as produce are found in the various perforated bags available in many countries. Perforations in the Ziploc recloseable bags of Dowbrands (USA), are a clear example. Less quantifiable are the effects of mineral-loaded bags such as are supplied as Natural Radiation Bag (Mitsubishi, Japan). The latter contains zeolite particles with large pores and is available for meat packaging or produce packaging in retail sizes. Such bags as are made by Mitsubishi and Asahi are claimed to have 'natural radiation' or 'infra-red radiation' effects, which are discussed in as much detail as is possible in Chapters 2 and 3. ANICO bags have a different type of inclusion in the polymer, consisting of iron and ascorbic acid (Anico, undated). Freshness may be enhanced by release of superoxide, although the mechanism has not been demonstrated in detail. Evidence is presented for reduced microbial growth on four foods Table 1.2 'Do-it-yourself active packaging Process
Example
RH Modification Freshness Freshness Natural radiation Oxygen scavenger
Pichit - humectants Asahi - zeolite Anico - iron sulfate + ascorbate Ceramic particles Mitsubishi - iron powder
Oxygen (%)
when wrapped in paper impregnated with ANICO. Results presented demonstrate the removal of ammonia, hydrogen sulfide and methyl mereaptan by the reactive ingredients in solution. The quantity of powdered inclusions is insufficient to cause substantial change in the water vapour transmission rate. An entirely different approach has been developed by Mitsubishi in the form of bags with a 'roll-over and clamp' style of removable pressure seal in kit form for use with cookies. The cookies are placed in the oxygenbarrier laminate bag and an oxygen-scavenger sachet is removed from a protective metallised film sachet and inserted before the pressure seal is applied. This is particularly effective in removing the oxygen from the headspace within 10 h as shown in Figure 1.4. This type of domestic active packaging is very suitable for foods unlikely to be affected by anaerobic pathogens. The regulatory dilemma arises in assessing the likelihood of domestic purchasers using such a scavenger system with raw fish or vegetables prone to pathogen infestation. A partially regenerable system involving use of haemoglobin analogues to buffer the oxygen concentration at low levels might be useful in some instances. This might take advantage of the high equilibrium constant for reversible binding of oxygen to a cobalt complex. Some such complexes reversibly bind oxygen until its partial pressure becomes so low that the rate of dissociation equals that of association. The result is buffer action. By
Time (hours) Figure 1.4 Oxygen scavenging from loosely packed biscuits packs (•) compared to tightly packed packets (A) (simulating domestic use).
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careful choice of the equilibrium constant for binding, an oxygen concentration which is a compromise between protection against oxidation and suppression of anaerobic organisms could be achieved. It should be noted that oxygen levels needed for anaerobe suppression are higher than has often been claimed (Smith et al., 1990). A metal complex would require a high binding constant for oxygen at 4°C and a much lower one at 1000C. Accordingly, the oxygen can be desorbed by immersion of the device in boiling water before reuse. This process could, in principle, be repeated until the accumulation of products of side reactions resulting in cobalt (3) formation reduced oxygen uptake unacceptably. Aquanautics Inc. (USA) investigated the use of such complexes for extraction of oxygen from seawater for submarine use. Other military applications were provision of oxygen for welding on warships and for provision of life-support oxygen in high-altitude aircraft. Retail sales of microwave susceptor films have been limited, presumably as a result of the US FDA concerns about the effect of high temperatures on the adhesives which bind the polyester layer to any backing. However, susceptor films consisting of paper/adhesive/metallised polyester have been marketed in Australia since the mid 1980s. 1.5 Physical and chemical principles applied The fact that active packaging has developed as a series of responses to the needs of niche markets should not hide the small number of underlying principles being applied (successfully) thus far. Table 1.3 summarises the use to which these principles have been applied either commercially or in patent applications or other publications. 1.5.1.1 Porosity control. Researchers in modified atmosphere packaging of respiring horticultural produce have long sought to generate equilibrium modified atmospheres (EMAs) by use of the permselectivity towards carbon dioxide over oxygen of plastics films. Although the ratio of permeability to carbon dioxide to that of oxygen commonly varies from 3.3 to 8.3 (see Table 4.4), this range is insufficient when considered in conjunction with the absolute values of these properties. Hence research became directed more at methods of controlling the size and distribution of pores in packaging materials. This research has ranged from the modelling of the size of single pores in a package (Mannapperuma and Singh, 1994) to the incorporation of coral sand (Abe, 1990). The temperature-controlled opening of pores in polymer films is a substantially more difficult concept to apply in practice. Cameron and Patterson (1992) devised a system that allows sufficient oxygen to prevent anaerobic respiration to enter a package if the temperature is raised too high
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careful choice of the equilibrium constant for binding, an oxygen concentration which is a compromise between protection against oxidation and suppression of anaerobic organisms could be achieved. It should be noted that oxygen levels needed for anaerobe suppression are higher than has often been claimed (Smith et al., 1990). A metal complex would require a high binding constant for oxygen at 4°C and a much lower one at 1000C. Accordingly, the oxygen can be desorbed by immersion of the device in boiling water before reuse. This process could, in principle, be repeated until the accumulation of products of side reactions resulting in cobalt (3) formation reduced oxygen uptake unacceptably. Aquanautics Inc. (USA) investigated the use of such complexes for extraction of oxygen from seawater for submarine use. Other military applications were provision of oxygen for welding on warships and for provision of life-support oxygen in high-altitude aircraft. Retail sales of microwave susceptor films have been limited, presumably as a result of the US FDA concerns about the effect of high temperatures on the adhesives which bind the polyester layer to any backing. However, susceptor films consisting of paper/adhesive/metallised polyester have been marketed in Australia since the mid 1980s. 1.5 Physical and chemical principles applied The fact that active packaging has developed as a series of responses to the needs of niche markets should not hide the small number of underlying principles being applied (successfully) thus far. Table 1.3 summarises the use to which these principles have been applied either commercially or in patent applications or other publications. 1.5.1.1 Porosity control. Researchers in modified atmosphere packaging of respiring horticultural produce have long sought to generate equilibrium modified atmospheres (EMAs) by use of the permselectivity towards carbon dioxide over oxygen of plastics films. Although the ratio of permeability to carbon dioxide to that of oxygen commonly varies from 3.3 to 8.3 (see Table 4.4), this range is insufficient when considered in conjunction with the absolute values of these properties. Hence research became directed more at methods of controlling the size and distribution of pores in packaging materials. This research has ranged from the modelling of the size of single pores in a package (Mannapperuma and Singh, 1994) to the incorporation of coral sand (Abe, 1990). The temperature-controlled opening of pores in polymer films is a substantially more difficult concept to apply in practice. Cameron and Patterson (1992) devised a system that allows sufficient oxygen to prevent anaerobic respiration to enter a package if the temperature is raised too high
Table 1.3 Physical and chemical principles applied in active packaging Principle
Application
Porosity Control
Gas pressure release Gas composition balance
Polymer Permeability
Gas composition balance Temperature compensation
Melting of Waxes
Time-Temperature indicators Temperature compensation
Thermal Expansion
Doneness indicators
Energy Shielding
Microwave shielding Thermal insulation Shock absorption
Energy Transfer
Microwave crisping UV absorption
Inorganic-Organic Oxidation
Oxygen scavenging Oxygen permeation barrier Oxygen indicator Carbon dioxide generation Ethylene scavenging Taint removal
Enzyme Catalysis
Oxygen scavenging Time-Temperature indication Lactose removal Cholesterol removal
Acid-Base Reaction
CO2 absorption CO2 generation Odour absorption
Adsorption
Taint removal Oxygen scavenging Ethylene scavenging Water removal
Absorption
Humidity buffering Condensation control Drip collection
Hydrolysis
Sulfur dioxide release Benomyl release
Desorption
Ethanol release Hinokitiol release Water release
Organic Reactions
Ethylene removal Oxveen barrier
during distribution. Their approach was to block a pore in a package with a wax which melted at the chosen upper temperature limit. When molten, the wax was drawn away by an absorbent wicking material. The use of sophisticated pore control is not limited to respiring produce packaging. DRG Flexible Packaging pic introduced the Ventflex® tray in which long capillaries formed horizontally in the lid to regulate gas
exchange. Yet another approach is to weld a plastic pressure-release valve into the wall of packs for freshly roasted coffee beans to release the carbon dioxide held in the beans under pressure for some days. This valve has gone through at least two designs with the early protrusion from the package surface being replaced by a design with the valve flush with the outer surface. 1. 5.1.2 Polymer permeability. Equilibrium modified atmosphere generation is still the major goal of polymer permeability modification. Modelling produce requirements with film properties is becoming increasingly popular (Chapter 3). A review of this area is also found in Solomos (1994). However, with temperature abuse in produce distribution, the temperature dependence of film permeability to oxygen and carbon dioxide is quite different from that of the respiration rate. Hence a film which allows generation of a satisfactory EMA at one temperature may cause anaerobic respiration at another. Use of liquid crystal polymers is an approach to solving this problem but those currently available do not offer large enough changes in permeability to be very significant at present. Polymer blending has been used to develop sufficiently high permeability to water vapour and smoke flavours at 400C while retaining a substantial oxygen barrier at 23°C. The accrued knowledge of polymer permeability properties is already very great and other applications in active packaging may be expected. 1.5.1.3 Melting of waxes. Use of wax melting for porosity controlled temperature compensation has been mentioned above. This property was used for indicating temperature abuse, especially in chilled and frozen foods (see Chapter 10). 1.5.1.4 Thermal expansion. As the package is being used more extensively as the vessel in which foods are reheated or cooked, the opportunities for indicating completion of the heating of the food are expanding. To date the so-called 'doneness' indicators for turkeys are an initial example. These plastic inserts in the food expand when the temperature at a predetermined depth reaches a chosen value. Other forms of time-temperature indication may replace these devices but a considerable amount of modelling of thermal processing may need to be done first. It is perhaps surprising that thermal expansion has not yet been used in temperature compensating devices for balancing gas compositions in EMA packs of horticultural produce. 1.5.1.5 Energy shielding. The insulation of foods by corrugated fibreboard cartons or polystyrene foam cartons scarcely satisfies the definition of active packaging. However, insulation has been brought into retail units in
such novel ways that they deserve attention. Labuza and Breene (1989) reviewed novel two-layer packages both with and without an intermediate layer such as a chillable gel which imparts an increased thermal load. The use of labels which expand to give an insulating foam when plastics lunchcups are retorted is a novel example. A higher level of sophistication is found in the inclusion of selective shielding of portions of microwaveable meals by appropriate placement of metallised strips in the top dome of packs. This makes possible the engineering of food products with mixtures of components with very different heating requirements (see Chapter 9). The concept of shielding the food from energy applied externally can also be applied to shock absorption by layers of foam. One such laminate is described by Yoshizaki (1976). This approach is not substantially different from the application of shrinkable foamed sleeves to glass soft-drink bottles. 1.5.1.6 Energy conversion. The ease with which energy is converted from one form to another has led to the development of microwave susceptors (Robertson, 1993). Although the regulation of susceptors is complicated by concerns about migration, their potential in the crisping of pastries and other foods has been widely accepted (Sacharow and Schiffman, 1992). Energy transfer has long been used in protection of plastics for use in exposed conditions (Brydson, 1982). Absorption of this energy has normally been followed by energy conversion to heat, either by the process of internal conversion or by energising chemical reactions. Such processes have been proposed increasingly for food packaging, in the patent literature. This follows the demonstration of the effects of light, both in the near UV and visible regions, on foods (Andersen and Rasmussen, 1992). 1.5.1.7 Adsorption. The introduction of adsorbents into food packages, both in sachets and dispersed in plastics, has been one of the main approaches to active packaging. The major advantage such materials have over chemical reagents is their absence of migration into the food, or their inertness. Adsorbents used or described in the literature include activated carbon, zeolites, silica gel, wood fibres and other forms of cellulose as well as various clays and crushed rocks (Labuza and Breene, 1989). A disadvantage of adsorbents is the often-reversible nature of the bonding and the limited capacity. They have been used, or proposed for use, for intercepting taints permeating plastics packages (Kiru Kogyo KK, 1994), adsorbing water vapour (Wagner, 1990), releasing water as a reagent in oxygen scavenging (see Chapter 6), adsorbing ethylene to some extent (Louis and de Leiris, 1991), and binding odorous products of some oxygen-
scavenging reactions (Inoue and Komatsu, 1988; Toppan Printing Co., 1992). 1.5.1.8 Absorption. Absorption processes in active packaging have largely involved control of the availability of water in packages. The simplest form of absorption has been removal of weep from meats orfishby microporous pads containing superabsorbent polymers. This absorption process has been coupled with carbon dioxide release in the case of the Verifrais process (Sacharow, 1988) and in oxygen or ethylene scavenging in the TM Corrugated case (Louis and de Leiris, 1991). More complicated, but meeting a complex need, are the humidity-buffering and condensationcontrol systems for cartoned horticultural produce described by Patterson, Jobling and Moradi (1993). The humidity-buffering action can also be seen in the carbohydrate and glycol mixture used in Pichit sheet for food-surface de watering in the home (Labuza and Breene, 1989). 1.5.1.9 Desorption. The use of paniculate solids as vehicles for delivery of active ingredients has been a commonly used process commercially. The use of a variety of porous solids such as vermiculite to hold water for oxygen scavenging has already been mentioned and is described in Chapter 6. The desorption of ethanol from dry silica gel is the basis of Ethicap sachets (Freund Co.) which consist of 55% silica gel and 35% ethanol. When these are exposed to water vapour in the headspace of a food pack, they desorb ethanol and a masking flavour. An extension of this is the concurrent ethanol release and oxygen scavenging by Negamold (Freund Co.). These processes rely on the stronger binding of water to silica gel displacing the more weakly held ethanol. Carbohydrates such as dextrins have been the subject of substantial research as vehicles for carrying ingredients in the pharmaceutical and flavour industries because of the range of binding forces which can be found. Cyclodextrins in particular are attractive because of their cyclic structure and owing to their favourable regulatory status in some countries. Cyclodextrins exist in three forms, a, (3 and 7, which consist of ring-shaped molecules with 6, 7 and 8 anhydroglucose units joined, respectively. To date they have been used to release hinokithiol antimicrobial agent from non-woven fabrics and plastics films under the Hosenshi trade name of Seiwa Chemical (Katsura, 1989). 1.5.1.10 Hydrolysis. To date, very little use has been made of hydrolytic reactions. The desorption of ethanol mentioned above may involve some hydrolysis. Use has been made of the release of sulfur dioxide from sodium metabisulfite in quilted pads of microporous material which are placed in cartons on top of table grapes. Water vapour from the grapes is absorbed and releases the gas which is required to act as a fungicide to prevent attack at
the stem junction of the fruit. Thus far, there has been no satisfactory method of controlling the rate of hydrolysis, with the result that excessive rates of release can cause partial bleaching of the grapes. An unintended hydrolytic reaction appears to have been found by Halek and Garg (1988) who set out to bind Benomyl fungicide to a carboxylicacid-containing film, Surlyn (Dupont). The fungal inhibition results obtained when the film was placed in contact with inoculated agar indicated detachment of the fungicide from the polymer and diffusion into the agar. 1.5.1.11 Acid/base reactions. The abundance of food constituents and permitted additives which are mildly acidic or basic has facilitated the introduction of acid/base reactions for altering packaged food environments. The reaction of ascorbic acid with sodium bicarbonate is commonplace in oxygen scavengers which replace oxygen consumption with carbon dioxide generation. Conversely, the removal of unwanted carbon dioxide from packs by reaction with lime in coffee packs has also been introduced commercially (Russo, 1986). The use of food acids dispersed in polymer films for removal of malodorous amines from fish has been patented by Hoshino and Osanai (1986). The interest in deodorising food packaging in Japan should result in more research and development in this area. Katsura (1989) noted that Nippon Unicar has introduced a flavonoid-based compound in low-density polyethylene for this purpose also. Combinations of many of these principles are found in commercial packages and in proposed systems. The Verifrais system and related systems which have followed involve a superabsorbent polymer to absorb water, the latter dissolving an acid and bicarbonate to release carbon dioxide in a controlled manner. 1.5.1.12 Inorganic/organic oxidation. Since oxygen scavenging was one of the earliest forms of active packaging to be introduced the use of oxidation reactions for several other purposes readily followed. The oxidation of metals had been limited to headspace oxygen scavenging but recent developments in bringing water, oxygen and iron together in plastics films has made an oxygen permeation barrier possible by this method (anon., 1994). Oxidation of organic substrates such as polyamides, rubbers, fatty acids and ascorbic acid has been developed for oxygen permeation barriers in plastics films (Chapter 4). Dyes which can be reversibly oxidised have been developed as indicators of oxygen concentration by formulation with reducing agents (Nippon Soda KK, 1993). Carbon dioxide generation is possible as ferrous carbonate is oxidised by molecular oxygen. This has been patented for applications where package collapse on oxygen scavenging cannot be accepted.
Following the early work of Scott et al (1970) on oxidation of ethylene by potassium permanganate, a variety of these rather non-specific oxidant sachets have become available. Their regulatory status should be examined before they are used. Whereas such non-specific oxidising systems have the possibility of generating taints, the reaction of ferrous iron with organic acids has been developed in plastics films for oxidative removal of taints (Anico, undated). 7.5.7.75 Organic reactions. Otherwise unclassified organic reactions have also been proposed in several patent applications. Ethylene removal by reaction with tetrazines has been demonstrated to occur efficiently at physiologically important concentrations (Holland, 1992). The Maillard reaction seems to be the basis of a patent in which reducing sugars and amino acids are brought together under moist conditions in a plastic film (Goyo Shiko KK, 1993). The products are claimed to reduce oxygen permeation but may also migrate into packaged liquid food to act as antioxidants. 1.5. L14 Enzymic catalysis. One of the earliest attempts at producing a sachet-based oxygen-scavenging system was based on the reaction of glucose with oxygen, catalysed by glucose oxidase in the presence of catalase (Scott and Hammer, 1961). Several systems involving both sachet and in-film chemistry have been developed but have not yet been proven in commercial practice. Kuhn and Kuhn (1991) have claimed spreading either the enzyme or glucose on the packaging material and adding the other component immediately before sealing. Ernst and Vonraffay (1991) and Ernst and Ernst (1992) described compositions of silica gel, glucose oxidase, glucose and water. The water can be supplied absorbed in microcrystalline cellulose. Carbon dioxide can be formed as oxygen is removed if an alkaline earth carbonate is included in the composition. The compositions can be pellets, in sachets or between two layers of an oxygen-barrier laminate. Glucose oxidase has been given GRAS status in the USA (Labuza and Breene, 1989) so there is considerable interest in making such systems function efficiently. A range of other applications of enzymes in active packaging are discussed in Chapter 7. The enzymatic hydrolysis of a lipid has been introduced as a timetemperature indicator by I-Point Biotechnology in the USA. The resulting change in pH was detected by means of a dye following mixing of the substrate and enzyme. This listing of principles and applications is not intended to be exhaustive but rather is aimed at indicating the basis and diversity of active packaging approaches.
1.6 Implications for other packaging The decision to address at least one attribute of a food by means of active packaging can have a significant impact on the choice of the total package. The impact can vary from the choice of resin into which to blend zeolites or ceramics for permeability modification to the temperature stability of outer packaging of microwave-reheatable packs for pastries. Impacts may be such as to allow the use of cheaper packaging, as with the use of oxygen scavengers in packaging relatively short shelf-life foods. Alternately it might be necessary to use more expensive materials in order to gain the benefits of the active agents. Several forms of active packaging require direct contact between the food and the active components of the package. When multilayer structures are used this implies that the heat-seal layer contains the active components, as with Zeopac antimicrobial trays offered commercially by Mitsubishi in Japan. In this laminate the cast polypropylene layer containing Zeomic zeolite is closest to the food. Zeomic is a zeolite in which silver ion is bonded in the surface layer of the pores. It is manufactured by Shinanen New Ceramic Company in association with Mitsubishi (Abe, 1990). The zeolite particles have a larger diameter than the thickness of this foodcontact layer and are thus exposed to any liquid at the food-package interface. Other silver zeolites are available, such as Bactekiller manufactured by Kanebo Zeolite Co. (Louis and de Leiris, 1991). Hirata (1992) has found that silver zeolite, used at 1% concentration in the heat-seal polyethylene layer of a laminate, can reduce the surface bacterial count on the plastic from 105-106 cells/ml to 10 cells/ml in 24 hours. The cells are spread on the surface in aqueous suspension. While there is still discussion as to whether the microorganisms need to come in contact with the silver surface, it seems far more probable that the silver ion is dissolved in the liquid by leaching (Ishitani, 1994). This form of packaging has been used for fresh oysters (Abe, 1990) and oolong tea (Ishitani, 1994). Halek and Garg (1988) demonstrated the antifungal effect of Benomyl bound to the ionomer film Surlyn (Dupont) by coupling with dicyclohexyl carbodiimide. The aim of this work was to determine whether a surfacebound antifungal agent would be effective, thus reducing the chance of the residues remaining in the food. It was therefore essential that the heat-seal layer should be the Surlyn. In fact, it was demonstrated that some of the Benomyl probably broke free of the ionomer and diffused into the agar test medium. Where it is intended that the active agent is to be released into, or onto, the food there may be less restriction on the location of the film layer. Thus there are opportunities to use slow-release binding agents such as cyclodextrins (Katsura, 1989) or microencapsulation agents either within the heatseal or other layers. The only requirement therefore is that the layer(s)
between the active one and the food is sufficiently permeable. This is the basis of the ethanol-releasing sachets Ethicap and Negamold (both from Freund Ltd., Tokyo) and of the CO2-releasing sachets manufactured by Mitsubishi Gas and Chemical Co., Toppan Printing Co. (both of Tokyo) and Multiform Desiccants Inc. of Buffalo, NY. The permeability of the sachet material to water vapour or ethanol is of critical importance in the application of these sachets. Most frequently, microporosity of a hydrophobic film material has been made use of to achieve this. Similar considerations to the above apply when active packaging is chosen to remove a food/headspace component such as oxygen, carbon dioxide or odours. The need to separate the scavengers from the food by a layer impermeable to the scavenger has thus far been relatively simple, as inorganic substances such as iron powder have been used both commercially (Koyama and Oda, 1992) and in proposed systems (Teijin, 1981; Toyobo, 1981). The choice of film materials permeable to oxygen, carbon dioxide or organic off-flavours has been relatively simple as polyolefins serve this purpose well. Where concurrent permeation of the barrier layer to water is also required, problems are encountered with the polyolefins which are highly permeable to non-polar gases and vapours but are not sufficiently permeable to water vapour. This problem is overcome in sachet manufacture by use of microporous films which are hydrophobic and resist liquid water penetration (Mitsubishi, 1983). Substantial numbers of patents describe methods of achieving the requisite permeabilities to water and oxygen using this approach. The careful choice of food contact layer plastics for retortable packages by Toyo Seikan (Koyama and Oda, 1992) and American Can Company has resulted in the development of heat-triggered oxygen scavengers which function when the water vapour transmission rate of the laminate reaches the necessary value. The premature oxidation of the reduced iron scavenger in the Toyo Seikan Oxyguard process at extrusion temperatures of around 220 0 C is minimised by keeping the plastics dry. When the container fabricated from these plastics is retorted at 12O0C for 30 min, water vapour permeating the container wall is absorbed and takes part in the rusting of the iron. Retention of water by the composition allows the Oxyguard to continue to act as a chemical oxygen barrier when the package returns to room temperature. This is the opposite situation to that with conventional passive retort package material based on EVOH where retention of water in the EVOH lowers the oxygen barrier and increases oxygen availability to the packaged food for many weeks after retorting (Tsai and Wachtel, 1990). The Oxyguard and other oxygen-scavenging plastics compositions can reduce the cost of the barrier layer by reducing the need for inclusion of mica platelets in the EVOH or for desiccants in the polypropylene layer of retortable trays. Another form of active packaging which can also reduce complexity of oxygen-barrier packaging is ethanol release. Hirata (1992) has
shown that common commodity polymers such as oriented polypropylene form a sufficient barrier to ethanol permeation to retain this sterilant in the package for extended periods of time. Thus in those applications where suppression of mould by ethanol is possible, simple packaging materials can be considered since oxygen control may not be necessary. Currently ethanol use in this way is not widely permitted, with the result that present opportunities are limited. Smith et ah (1987) have suggested that the use of ethanol could reach approval status in North America for 'brown and serve' products such as pastries. 1.6.1 Whole packages designed to be active Some forms of active packaging do not require changes in design of the package in order to achieve their effects. Examples of these include the insertion of the various sachets which modify the gas atmosphere, as described in Chapter 6. Other examples are the various forms of porous or mineral-filled plastics which can be used with respiring horticultural produce. In some instances the consumer may not be aware that the package is different from its passive counterpart as with the oxygen-scavenging closures for bottled beverages, described in Chapter 8. Redesign of packages is necessary for some effects to be achieved, particularly when some of the physical principles listed in Table 1.3 are involved. This is particularly important where shielding of components in microwaveable meal packs is desired, because the location of the shields is critical (Sacharow and Schiffman, 1992; Robertson, 1993). The use of susceptors to achieve surface crisping of foods means that the distance between the food surface and the susceptor must be carefully controlled. Amcor Fibre Packaging in Australia has developed a susceptor-coated corrugated inner layer of packs for pastries. The exposed corrugating medium provides a heating surface claimed to be ideal for such crisping. A corrugated fibreboard base for a susceptor film is also used to drain fat coining from foods like fish on heating. Wagner (1989) reported that in 1988 Japan exported over 30 million selfheating cans for sake alone. Aluminium cans for sake are heated when lime and water in the base are mixed. Steel cans for coffee are also heated using the same chemistry. This process also has been used to heat lunch boxes (Katsura, 1989). Self-cooling cans have also been marketed in Japan for raw sake. The endothermic dissolution of ammonium nitrate and chloride in water is used to cool the product. It is necessary to shake the can to ensure the mixing of the salts with water. Packages for chilled foods have also been redesigned to allow for the presence of an active component. Control of carbon dioxide concentration in packs for meat or fish under modified atmospheres is the aim of a French
package designed to be active (Louis and de Leiris, 1991). The Verifrais package manufactured by Codimer consists of a heat sealable lid with a tray with a false bottom which is perforated to allow juice from the product to drain into the base of the tray. A porous sachet containing sodium bicarbonate and ascorbic acid is in the base. The delay in access of the liquid to the sachet is controlled by an absorbent paper pad located between the sachet and the porous tray. The commencement of dissolution of the acid and bicarbonate can be delayed for up to 24 hours, by which time the carbon dioxide concentration can be decreasing due to dissolution in the meat and permeation of the tray and lid. Release of carbon dioxide from the sachet serves to offset this loss. Meats can have a shelf-life of up to 21 days at 2-4°C in this pack compared with the 10-15 days under normal MAP (Louis, 1990). The shelf-life of fish can be extended similarly to 10 days versus 4-5 days under MAP. A more recent variant on this design has the carbon-dioxide-generating components concealed in a foam tray with channels for juice drainage (Leon et ai, 1987). The preferred chemicals are sodium chloride (50%), citric acid (25%), and sodium carbonate (25%). The chemicals can be in aggregate form rather than held in sachets as with Verifrais. These chemicals help to reduce package collapse owing to outward permeation of carbon dioxide by generation on a continuous basis. The aim is to prevent growth of anaerobic pathogens on meats and seafood packed in low-oxygen-content modified atmospheres. An alternative to these active packaging processes is the Valle Spluga process which involves insertion of a pellet of solid carbon dioxide into the pack before sealing to provide sufficient additional gas to retain package shape (Louis, 1990). The carbon dioxide is pelletised in-line to a predetermined dosage. The packaging material needs to have a strong heat-seal if leakage is to be avoided within the first day of packing. Although the mineral-filled and other porous horticultural-produce carton liners differ very little from conventional liners, not all horticultural active packaging is so simple. The Ventflex laminate pack introduced by DRG Packaging in the UK relies on channels between the layers lidding the film to allow some regulation of evolution of carbon dioxide by the packaged produce. Oxygen can enter through the same channels or more slowly through the polyester barrier film forming the lid. The condensation control carton design of Patterson et al (1993) reduces substantially the chance of produce damage by water condensation caused by temperature abuse. The carton is lined with three additional layers which provide in-pack buffering of the water vapour in the carton headspace. Louis and de Leiris (1991) describe the TM corrugated case of the Mantsune Co. which, while superficially similar to the above, has no microporous food contact polymer but does contain a ceramic claimed to absorb ethylene. This carton lining is designed to act as a humidity buffer. Redesign of packages has also been used to address the problems of
packaging of freshly roasted coffee. Taylors of Harrogate, Tea and Coffee Ltd. (UK) pack the beans in foil laminate pouches which resist the pressure of carbon dioxide evolved by the beans. This gas evolution usually occurs for at least the first day after roasting. The Taylors' bag contains a one-way plastic release valve welded into the face of the bag and this prevents seal rupture. The original valve design had a protruding component and so was welded into the side gusset. An alternative approach was adopted by General Foods in the USA. A Mitsubishi Ageless sachet which absorbs carbon dioxide and oxygen was included with the coffee in their packs (Russo, 1986). The General Foods pack did not require redesign to accommodate the sachet. Both active packaging approaches were designed to retain the easily oxidised fresh coffee flavours normally lost when coffee is allowed to stand for 24 hours to desorb carbon dioxide. The forms of active packaging which involve minimal changes to the preexisting package design have been most readily adopted commercially so far. The widespread use of sachets for headspace atmosphere modification demonstrates this point. The number of oxygen-scavenging sachets manufactured in Japan in 1989 was estimated at 6.7 billion (Abe, 1990). The expansion of this market appears to have been limited by the preference of food manufacturers not to include sachets in the packages with the foods. Recent introduction of sachets bonded to the package walls should facilitate greater market penetration (see Chapter 6). 1.7 Limitations of current approaches The ideal active package would sense the requirements of the food and adjust its properties in order to meet them. At the same time this would be cost-effective and have minimal environmental impacts. Current commercial active packages do not meet these criteria although there are some which are second-generation concepts. The latter include meat packs with inserts which release carbon dioxide in response to absorption of weep or juice (Sacharow, 1988). Horticultural active packaging is not yet based on films with a sufficiently wide range of CO2-O2 permeability ratios. This ratio would need to be temperature responsive in the same manner as the enclosed fruit or vegetable. The Intellimer side-chain-crystallisable polymers from Landec Corporation (Menlo Park, CA, USA) offer some answers to the temperature problem (Stewart et al, 1994). There is a lack of convincing evidence that several of the films designed to scavenge ethylene do in fact work satisfactorily under actual conditions of use. Industry confidence would be well served by self-indicating films which change colour on reacting with ethylene at physiologically important concentrations. The control of humidity in liner bags of fruit and vegetables is still in its early stages of
development. Inserts or films capable of regulating humidity levels are required, especially where temperature abuse is likely. The use of sealed plastic bags for equilibrium modified atmosphere generation makes this requirement more critical. Current commercial oxygen scavenging sachets are supplied in a wide variety of forms to suit particular temperature and relative humidity conditions. These designs place limitations on the shelf-life of the sachets once the master-packs are opened and before insertion into the food package. An ideal sachet would be useable under a wide variety of conditions and activated for a particular application by the food packer. Some early steps towards overcoming the shelf-life limitation have been reported in the patent literature. The use of a peelable barrier on a sachet with a self-adhesive backing has been proposed. Attempts to include oxygen scavengers in plastics packaging materials have not yet been successful commercially except in closure liners for beverage bottles. Compositions which are able to withstand extrusion and yet be activatable are required. 1.8 Future potential The future of any innovation in packaging depends upon the extent to which it can satisfy the requirements of the product packaged. Commercial development therefore will be driven by needs as perceived in the food industry or in other industries with related problems. It is clear from patent searches that inventors of active packaging frequently see potential applications for their concepts in several industries. This is evident in claims for use of oxygen scavengers in the packaging of clothes, pharmaceuticals, fine chemicals such as amines, printing inks, electronic components, metals and many more areas. Some iron-based oxygen scavengers have been suggested for use in hand-warmers for skiers. If the potential of active packaging technologies is to be realised there will need to be a recognition that changes in packaging can open up new methods of presenting foods. The use of oxygen-scavenging plastics as chemical barriers to permeation should allow retortable plastics to provide product shelf-lives closer to those found using metal cans. Horticultural produce, such as flowers, should be transportable internationally with reduced losses. Acceptance of active packaging solutions to food industry problems will continue to depend upon evidence of effectiveness demonstrated by independent investigators. The lack of hard evidence supporting many claimed benefits of some early horticultural produce packages has inhibited commercial usage. If the majority of patent claims already made prove useful and economically viable, active packaging has a bright future.
1.9 Regulatory considerations At least three types of regulation have an impact on the use of active packaging in foods. First, any need for food-contact approval should be established before any form of active packaging is used. Second, environmental regulations of packaging material usage can be expected to increase in the coming years. Third, there may be a need for labelling in cases where active packaging can give rise to consumer confusion. Food-contact approval will often be required because active packaging may affect foods in two ways. A substance may migrate into the food or may be removed from it. Migrants may be intended or unintended. The intended migrants include food additives which would require regulatory approval in terms of their identity and concentration. Unintended additives include active substances which achieve their purpose inside the packaging material and do not need to enter the food. Food additive regulations require identification and quantitation of any such migration. The likely introduction of the 'Threshold of Regulation' concept by the US FDA early in 1995, and the chance of the European Union adopting similar concepts later, may facilitate the approval procedures for active packaging. Removal of oxygen from packages may lead to the growth of anaerobic pathogens in some cases. The manner in which oxygen scavengers are used may be subject to regulation. Developments in active packaging discussed in this book are not necessarily commercial and care should be taken in using any of them. Environmental regulations for packaging materials often refer to recyclability or identification to assist in recycling. The effect of active packaging materials on recycling may need to be determined on a case-by-case basis. Active packaging is often used currently to allow foods to be packaged with simpler materials than would otherwise be possible. The environmental impact of the food-package combination should be considered. Labelling is currently required to reduce the risk of ingestion of sachets of oxygen scavengers, ethanol emitters, etc. Some form of external labelling may be required when various forms of indicator come into use. Such indicators would show gas composition, thermal history, or 'done-ness' in the case of microwaved foods. Some active packages may be expected to look different from their passive counterparts. It may be advisable to use labelling to explain this even in the absence of regulation. References Abe, Y. (1990) Active Packaging - a Japanese Perspective. Proceedings International Conference on Modified Atmosphere Packaging, Part 1, 15-17 October, Stratford-uponAvon, UK. Alarcon, B. and Hotchkiss, J.H. (1993) The effect of FreshPax oxygen-absorbing packets on
the shelf-life of foods. Technical Report, Dept. of Food Science, Cornell Univ., NY, pp. 1-7.
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2
Ethylene-removing packaging D. ZAGORY
Ethylene is a chemically simple, ubiquitous chemical that has diverse and profound effects on the physiology of plants. Ethylene has so many different effects on plants, is effective in such low concentrations, and its effects are so dose-dependent, that it has been identified as a plant hormone. Though many of the effects of ethylene on plants are economically positive, such as induction of flowering in pineapples, de-greening of citrus and ripening of tomatoes, often ethylene has been seen to be detrimental to the quality and longevity of many horticultural products. For this reason, there has long been interest in removing ethylene from the horticultural environment and in suppressing its effects. Some of the diverse ways in which to absorb, adsorb, counteract or chemically alter ethylene have led to products designed to reduce its deleterious effects. This chapter will briefly review the chemistry, physiology and agricultural effects of ethylene preparatory to describing the research and commercial effort undertaken to incorporate ethylene control agents in packages for horticultural products. Some of this effort has met with commercial success, but much has not. However, with the rapid growth of packaging of fresh fruits and vegetables, particularly fresh cut salads and fruits, opportunities for such products are bound to increase. Therefore, it is timely to review the basis and activities relating to these products to better elucidate the possible forms that they can and will take and to point out some of the advantages and disadvantages of the various approaches likely to emerge. 2.1 The chemistry of ethylene The ethylene molecule is of the alkene type, being simply two carbons linked by a double bond with two hydrogen atoms on each carbon. Such a simple molecule can be synthesized through several different pathways and is subject to many kinds of chemical reaction. 2.1.1 Synthesis Ethylene can be synthesized both biologically and non-biologically. It is a common component of smoke and can be found as a product of aerobic
combustion of almost any hydrocarbon. It is thus a common air pollutant, its chief source being automobile engines. Biological sources of ethylene include higher plant tissues, several species of bacteria and fungi, some algae, and some liverworts and mosses. The biosynthetic pathways for ethylene are diverse among these different organisms. The pathway of synthesis from methionine has been described in detail for higher plants (Yang and Hoffman, 1984). The pathways for synthesis in bacteria appear to be diverse since any of several carbon sources other than methionine will serve as precursors (Sato et al, 1987). Nitrogenfixing bacteria can reduce acetylene to ethylene (Dillworth, 1966). Approximately 25-30% of fungal species tested produce ethylene on appropriate media (Fukuda et al, 1984; Hag and Curtis, 1968). The pathways of plant and fungal ethylene synthesis appear to be distinct, as the inhibitor rhizobitoxin blocks synthesis in plants but not in the fungus Penicillium digitatum (Owens et al, 1971). The pathway of ethylene synthesis in nonvascular plants may be different from that in vascular plants (Osborne, 1989a). Because this chapter is primarily concerned with methods of eliminating ethylene, not producing it, it is not necessary to go into the details of production by different organisms. This has been reviewed in detail elsewhere (Abeles et al, 1992). The important point is that environmental ethylene can be biologically produced by a wide range of organisms, both visible and invisible, and such sources ought to be considered when devising strategies to reduce ambient ethylene. 2.1.2 Degradation Ethylene undergoes several types of degradation reactions. Because of its double bond, ethylene absorbs ultraviolet (UV) radiation at 161, 166 and 175 nm (Roberts and Caserio, 1967). Ultraviolet photodecomposition of atmospheric ethylene is an important environmental ethylene sink (Scott and Wills, 1973) and yields primarily hydrogen, acetylene, n-butane and ethane (Noyes et al, 1964). Soil microorganisms can degrade ethylene and at least one species, Mycobacterium paraffinicum, is thought to be an efficient oxidizer of ethylene (Abeles et al, 1992). Ethylene reacts with ozone to yield water, carbon dioxide (CO2), carbon monoxide (CO), and formaldehyde (Scott et al, 1957). Ultraviolet light will interact with oxygen (O2) in air to form ozone which breaks down ethylene, but UV light will directly degrade ethylene as well. Thus, UV light will effectively eliminate ethylene even in low O2 atmospheres (Shorter and Scott, 1986). However, the reaction is inefficient at very low ethylene concentrations such as those found in fresh produce environments so the commercial potential of ozone as an ethylene scrubber is limited. Atomic oxygen will also react with ethylene and can form an array of compounds
including ethylene oxide, ethane, CO, propylene, acetaldehyde, propanol, butanol, hydrogen and dioxyketone (Leighton, 1961). The double bond of ethylene makes it very reactive through a number of reaction pathways. The double bond will undergo hydrogenation, in the presence of any of several metal catalysts, to yield ethane (Morrison and Boyd, 1966). Ethylene will react with halogens (chiefly chlorine and bromine) through halogenation and hydrohalogenation reactions to form dihaloalkanes. Thus, ethylene can be eliminated from air by passing it over brominated activated charcoal to form dibromoethane (Talib, 1983). Brominated charcoal filters are relatively efficient removers of ethylene. Up to 90% of the bromine will react with ethylene. However, bromine also reacts with water to form HBr and Br2 gas is released from the carbon filter. These compounds are injurious to plant tissues and corrosive to stainless steel. In addition, brominated activated charcoal is hygroscopic and will become wet in humid conditions. Alternatively, ethylene will react with hydrogen halides to form ethyl halides (Morrison and Boyd, 1966). Ethylene reacts with concentrated sulfuric acid to form ethyl hydrogen sulfate or with water in the presence of acids to yield ethanol (Morrison and Boyd, 1966). Certain oxidizing agents react with ethylene to form glycols. The most common of these oxidizing agents is potassium permanganate (KMnO4) which oxidizes ethylene to ethylene glycol and thence to CO2 and water (Morrison and Boyd, 1966). Potassium permanganate is often adsorbed onto Celite, vermiculite, silica gel or alumina pellets. Permanganate scrubbers are also effective in adsorbing air pollutants such as O3, H2S, SO2, NO and NH3. It is clear that ethylene is a very reactive compound that can be altered or degraded in many ways. This creates a diversity of opportunities for commercial methodologies for the removal of ethylene and, in fact, many different methods have been used. However, many of the common reactions undergone by ethylene require high concentrations of ethylene and/or high temperatures and pressures. Therefore, many of the processes most commonly used to modify ethylene in the petrochemical industry are not appropriate for the conditions generally found in a food package environment. 2.13
Adsorption and absorption
In addition to chemical cleavage and modification, ethylene can be absorbed or adsorbed by a number of substances including activated charcoal, molecular sieves of crystalline aluminosilicates, Kieselguhr, bentonite, Fuller's earth, brick dust, silica gel (Kays and Beaudry, 1987) and aluminium oxide (Goodburn and Halligan, 1987). A number of clay materials have been reported to have ethylene adsorbing capacity. Examples include cristobalite (> 87% SO2, > 5% AlO 2 , > 1% Fe2O3) (Kader et ai,
1989), Ohya-ishi (Oya stone) and zeolite (Urushizaki, 1986a). Oya stone is mined from the Oya cave in Tochigi Prefecture in Japan. The cave has been used to store fresh produce and is reputed to confer added storage life. The salutary properties of the cave are thought to reside in the largely zeolitic stone interior. To improve its ethylene adsorptive capacity, the Oya stone is first finely ground with a small amount of metal oxide. The mixture is then kneaded and heated to 200-9000C, then oxidized with ozone or electromagnetic radiation (Urushizaki, 1986b). Some regenerable adsorbents have been shown to have ethylene adsorbing capacity and have the benefit of being reusable after purging. Examples of such adsorbents include propylene glycol, hexylene glycol (Rizzolo et al, 1987a), squalene, Apiezon M, phenylmethylsilicone, polyethylene and polystyrene (Rizzolo et a/., 1987b). Some adsorbents have been combined with catalysts or chemical agents that modify or destroy the ethylene after adsorption. For example, activated charcoal has been used to adsorb ethylene. In some cases, the activated charcoal has been impregnated with bromine or with 15% KBrO3 and 0.5M H2SO4 to eliminate the activity of the ethylene (Osajima et a/., 1983). A number of catalytic oxidizers have been combined with adsorbents to remove ethylene from air. Examples include potassium dichromate, KMnO4, iodine pentoxide, and silver nitrate, each respectively on silica gel (Eastwell et al, 1978). Electron-deficient dienes or trienes, such as benzenes, pyridines, diazines, triazines and tetrazines, having electron-withdrawing substituents such as fluorinated alkyl groups, sulphones and esters (especially dicarboxyoctyl, dicarboxydecyl and dicarboxymethyl ester groups), will react rapidly and irreversibly with ethylene at room temperature and remove ethylene from the atmosphere. Such compounds can be embedded in permeable plastic bags or printing inks to remove ethylene from packages of plant produce (Holland, 1992). Metal catalysts immobilized on absorbents, such as platinized asbestos, cupric oxide-ferric oxide pellets and powdered cupric oxide, will effectively oxidize ethylene, but in many cases the reactions require high temperatures ( > 1800C). Clearly such systems would be inappropriate for food packaging applications. 2.2 Deleterious effects of ethylene Ethylene has long been recognized as a problem in postharvest handling of horticultural products. Since the discovery in 1924 that ethylene can accelerate ripening in fruits (Denny, 1924) it has become clear that ethylene can be the cause of undesirable ripening of fruits and vegetables. It is now recognized that ethylene, in very low amounts, can be responsible for a wide
array of undesirable effects in plants and plant parts. The physiological effects of ethylene are so important, so diverse, and are induced by such small amounts of ethylene that it is considered a plant hormone. The diverse physiological effects of ethylene have been extensively reviewed elsewhere (Abeles et ai, 1992) so only those effects that are deleterious to packaged plant produce will be discussed here. 2.2.1 Respiration Perishability of produce generally is well correlated with respiration rate. Commodities such as asparagus, broccoli and mushrooms that have very high respiration rates are very perishable, having postharvest lives measured in days. Those commodities such as nuts, dates, dried fruits, potatoes and onions that have very low respiration rates have postharvest lives measured in months (Kader, 1985). Reduction of respiration rate increases postharvest life and elevation of respiration rate generally decreases it. This is one of the reasons why low temperature is so important in postharvest management. Reducing the temperature rapidly reduces the respiration rate of the product. Ethylene accelerates the respiration of fruits, vegetables and ornamental plants. In the case of climacteric fruit, ethylene can induce a rapid and irreversible elevation in respiration leading directly to maturity and senescence. In non-climacteric plant organs, ethylene induces a reversible increase in respiration. In most cases, exposure to a few parts per million (ppm) of ethylene leads to increased respiration and increased perishability. 2.2.2 Fruit ripening and softening Ethylene has been referred to as a 'ripening' hormone because it can accelerate softening and ripening of many kinds of fruit. Exposure of mature fruit to ethylene leads to increased respiration, increased production of endogenous ethylene, and softening of fruit tissues (Abeles et aiy 1992). This is achieved through the direct or indirect stimulation of synthesis and activity of many ripening enzymes by ethylene. Some fruits, such as bananas and tomatoes, are often deliberately exposed to high concentrations of ethylene (~ 100 ppm) to induce rapid ripening. In most cases, for packaged fruits it would be desirable to prevent exposure to ethylene and thereby prevent rapid ripening. 2.2.3 Flower and leaf abscission Cell wall hydrolysis of specific cells at the base of leaves, petioles, petals, pedicels and fruit leads to abscission of the distal organ (Abeles et al., 1992).
Ethylene has been shown to accelerate abscission for many, though not all, plants and plant parts (Jankiewicz, 1985; Osborne 1989b; Reid, 1985a). Ethylene causes flower and leaf abscission of many potted ornamental plants (Cameron and Reid, 1983). 2.2.4 Chlorophyll breakdown Ethylene increases the rate of chlorophyll degradation in leaf, fruit and flower tissues (Aharoni, 1989; Knee, 1990; Kusunose and Sawamura, 1980; Makhlouf et al, 1989). This can be of particular concern in the case of leafy green vegetables such as spinach, immature fruits such as cucumbers and squash, and flowers such as broccoli (Reid, 1985b). The presence of low levels of ethylene can cause yellowing and reduced quality. 2.2.5 Petal inrolling in carnations Low concentrations of ethylene (< 1 ppm) cause inrolling (or sleepiness) of the flower petals of sensitive carnation varieties accompanied by a loss of turgor in the petal tissues (Halevy, 1986). Some carnations are so sensitive to ethylene that they have been used as ethylene bioassays. Such sensitive varieties are often subjected to a pulse treatment with silver thiosulphate to render them insensitive to the effects of ethylene (Cameron and Reid, 1983). 2.2.6 Postharvest disorders Ethylene can be responsible for a number of specific postharvest disorders of fruits and vegetables. Examples include russet spot (small oval brown spots, primarily on the midrib) of crisphead lettuce, formation of bitter-tasting isocoumarins in carrots, sprouting of potatoes, and toughening of asparagus (Reid, 1985b). 2.2.7 Susceptibility to plant pathogens Many postharvest plant pathogens are opportunistic microorganisms that thrive on injured or senescent tissues. To the degree that ethylene accelerates senescence and causes specific physiological disorders, it also enhances the opportunities for pathogenesis. The growth of a number of postharvest pathogens is directly stimulated by ethylene (Barkai-Golan, 1990; BarkaiGolan and Lavy-Meir, 1989; Kepczynska, 1993). In addition, several postharvest plant pathogens produce ethylene (Barkai-Golan, 1990) and this ethylene may compromise the natural defences of the plant tissues.
2.3 Interactions of ethylene and other gases The activity and reactivity of ethylene depends, in part, on the presence of other atmospheric gases. The user of packaging materials for the removal or inactivation of ethylene should consider the presence and concentrations of oxygen, carbon dioxide, ozone and ethylene and their interactions with each other and with plant tissues. 2.3.1 Oxygen Ethylene production, biosynthesis and explosiveness are all related to ambient oxygen concentration. Most pathways of ethylene synthesis, whether biological or chemical, are oxidative conversions or cleavages. Although rice and some other aquatic plants have been reported to synthesize ethylene in the absence of O2 (Ku et al, 1970), most plants require O2 for ethylene synthesis. However, the oxygen affinity of ethyleneforming enzyme (EFE) is much less than that for respiratory enzymes. The K1n for conversion of 1-aminocyclopropane-l-carboxylic acid (ACC) to ethylene in apple is about 1.4% O2 (Banks et al., 1984; Bufler and Streif, 1986) but K1n values for other plants organs are generally 3-10% (Burg and Thimann, 1959; Lieberman et al, 1966). In some cases, reduced O2 in a package may more effectively reduce ambient ethylene through reduced ethylene synthesis than ethylene-adsorbing capacity built into the package. However, reduced O2 apparently slows the conversion of ACC to ethylene, resulting in accumulation of ACC (Burg and Thimann, 1959; Hansen, 1942; Imaseki et al, 1975; Jackson et al, 1978). Upon exposure to higher O2 concentrations, the accumulated ACC will be rapidly converted into ethylene so low O2 must be maintained continuously to maintain low ethylene concentrations. The combustion of organic materials requires O2 and results in ethylene as one of the combustion products. Ethylene at concentrations between 3.1 and 32% by volume, is explosive in air (Reid, 1985b). Neither of these conditions occurs in packages. 232
Carbon dioxide
Carbon dioxide may stimulate, inhibit or have no effect on ethylene synthesis, depending on the plant tissue (Abeles et al, 1992) and the concentration of CO2. More importantly, CO2 renders normally sensitive plant tissues insensitive to the effects of ethylene, thereby preventing abscission (Wittenbach and Bukovac, 1973), floral senescence (Nichols, 1968), chlorophyll loss (Aharoni and Lieberman, 1979) and growth (Chadwick and Burg, 1967).
2.3.3 Ozone As was mentioned above, ozone oxidizes ethylene to simple breakdown products and has been used experimentally to remove ethylene from produce storage areas. However, ozone would not normally be found or introduced into a food package. 2.4 Ethylene sources in the environment Ethylene is ubiquitous at low levels in the environment. It is a common pollutant that can be detected with sensitive instruments. As most methods of adsorbing or decomposing ethylene have finite capacities for activity, it seems prudent to reduce environmental ethylene to avoid saturating the environment of the package with ethylene. Ethylene can come from many sources both within and outside the package. Although there are no national standards for environmental ethylene, California, USA standards recommend human exposures to no more than 0.5 ppm for 1 h or 0.1 ppm for 8 h (Anon., 1962). Such levels are below damage thresholds for all but the most sensitive horticultural commodities. 2.4.1 Combustion Ethylene is a common breakdown product of virtually all aerobic combustion processes. Burning agricultural wastes, wildfires, diesel- or propanepowered forklifts, cigarette smoke, truck and auto exhaust, and industrial stack emissions are all common sources of ethylene. In addition, the heat generated by combustion (from forklifts, for example) can raise the temperature of the product sufficiently to stimulate production of productgenerated ethylene. Ambient atmospheric levels of ethylene are normally in the range of 0.001-0.005 ppm (Abeles et al., 1992), however, urban air levels as high as 0.5 ppm have been measured (Scott et al, 1957). Such high levels are sufficient to have physiological effects on some fresh produce. Removing agricultural sources of ethylene and insulating storage rooms from ethylene air pollution can significantly reduce ambient ethylene. 2.4.2 Plant sources Growing plants do not normally produce enough ethylene to alter ambient atmospheric levels of the chemical. In closed areas, such as storage rooms, packing houses, shipping containers, greenhouses and warehouses, plantgenerated ethylene can be significant (Abeles et al., 1992). Sensitive products should not be held or stored in proximity to ethylene-generating products or product-ripening rooms.
2.4.3 Ripening rooms Bananas and tomatoes are routinely ripened by exposure to 50-100 ppm ethylene in large sealed rooms. When such rooms are vented, the dispersal of ethylene can be significant. When ripening rooms are built into produce storage or distribution warehouses, the ethylene can come in contact with other products being held in the warehouse. If that produce were packed in ethylene-adsorbing packaging, the ethylene at such levels might saturate the packaging and render it ineffective. 2.4.4 Fluorescent ballasts and rubber materials The ballasts that hold fluorescent lights are sources of ethylene. In addition, rubber materials exposed to heat or UV light can release ethylene (Reid, 1985b). 2.4.5 Microorganisms Although several soilborne microorganisms produce ethylene, others degrade it. The net effect appears to be that the soil serves primarily as a sink for ethylene. Postharvest plant pathogens growing on stored products in enclosed holding areas can be important sources of ethylene. AU infested foodstuffs should be immediately discarded. 2.5
Commercial applications in packaging
Several of the technologies described above have been incorporated into packaging materials that are either commercially available or are likely to become available in the near future. As is common in the commercial sector, some of the claims for ethylene ad-/absorbing capacity for these packaging materials have been poorly documented and thus the efficacy of the materials is difficult to substantiate. Most substances designed to remove ethylene from packages are delivered either as sachets that go inside the package or are integrated into the packaging material, usually a plastic polymer film or the ink used to print on the package. 2.5.1 Potassium permanganate-based scavengers Many vendors offer ethylene adsorbers based on KMnO4 immobilized on any of several minerals. These products are available in sachets for packages and on blankets that can be placed in produce-holding rooms. Potassium permanganate is not integrated into food-contact packaging because of its
toxicity. However, sachets could be used inside produce packages and have been shown to effectively scavenge ethylene from packages of bananas, persimmons, kiwifruit, avocados (Ben-Arie and Sonego, 1985; Fuchs and Temkin-Gorodeiski, 1971; Hatton and Reeder, 1972; Krishnamurthy and Kushalappa, 1985; Liu, 1970; Maotani et al.9 1982; Scott et al9 1970). Typically, such products contain ~ 4-6% KMnO4 on an inert substrate such as perlite, alumina, silica gel, vermiculite, activated carbon or celite (Abeles et al, 1992). The performance and useful lives of these scavengers depends on the substrate surface area and the content of reagent (KMnO4). Formulations differ in density and surface area of substrate and the loading of reagent. Some suppliers of KMnO4-based ethylene scavengers are listed in Table 2.1. This table is not a complete listing of all companies supplying such products but only those known to the author at the time of writing. 2.5.2 Activated carbon-based scavengers Various metal catalysts on activated carbon will effectively remove ethylene from air passing over the bed of carbon. Commercial units, known as swingtherm ethylene converters, are based on such a system. However, they require heat and movement of gases and so are not applicable to packaged produce. Activated charcoal impregnated with a palladium catalyst placed in paper sachets effectively removed ethylene in an experiment on maintaining quality of lightly processed kiwifruit, banana, broccoli and spinach (Abe and Watada, 1991). The Japanese company Sekisui Jushi has developed a product, Neupalon, that is a sachet containing activated carbon and a water absorbent capable of Table 2.1 Suppliers (USA) of potassium permanganate ethylene scavengers Air Repair Products, Inc. PO Box 1006 Stafford, TX 77477 Cams Chemical Company, Inc. 1001 Boyce Memorial Drive Ottawa, IL 61350 Complete Control PO Box 1006 Stafford, TX 77477 DeltaTrak, Inc. PO Box 398 Pleasonton, CA 94566 Ethylene Control, Inc. PO Box 571 Selma, CA 93662
ExtendaLife Systems PO Box 55044 Hayward, CA 94545-0044 Loomix, Inc. 405 E. Branch Street PO Box 490 Arroyo Grande, CA 93420 Purafil, Inc. PO Box 80434 Chamblee, Georgia 30366 Purity Corporation 9539 Town Park Houston, TX 77036
Note: Nippon Greener Co. is reported to use potassium permanganate by Abe (1990) in his listing of ethylene absorbers.
absorbing up to 500-1000 times its weight of water. The company provides data showing that Neupalon adsorbs 40 ml ethylene per m2. Honshu Paper, also in Japan, has a product called the Hatofresh System that is based on activated carbon impregnated with bromine-type inorganic chemicals. They do not specify which bromine compounds are used. The carbon-bromine substance is embedded within a paper bag or corrugated box, which is used to hold fresh produce. They claim that the bag will adsorb 20 ml ethylene per g of adsorbent. It is unlikely such bags could be used in most developed countries due to the reaction of bromine compounds with water, which can release toxic bromine gas. Mitsubishi Chemical Company of Japan produces a product called SendoMate which is based on palladium catalyst on activated carbon which adsorbs ethylene and then catalytically breaks it down. The product comes in woven sachets that can be placed in packages of produce. 2.5.3 Activated earth-type scavengers In the past several years a number of packaging products have appeared based on the putative ability of certain finely dispersed minerals to adsorb ethylene. Typically these minerals are local kinds of clay that are embedded in polyethylene bags which are then used to package fresh produce. Many, though not all, of the bags are marketed by Japanese or Korean companies, though some are also sold in the United States and Australia. The Cho Yang Heung San Co. Ltd. of Korea markets a film bag called the Orega bag, based on the US patent of Dr Mitsuo Matsui (Matsui, 1989). Fine porous material derived from pumice, zeolite, active carbon, cristobalite or clinoptilolite is sintered together with a small amount of metal oxide before being dispersed in a plastic film. Neither plastics containing chlorine such as polyvinyl chloride or polyvinylidene chloride, nor plasticizers, are apparently suitable for these applications (Choi, 1991). The inorganic materials have pores ranging from 2000 to 2800A and the resulting film is reported to have the capacity to adsorb at least 0.005 ppm ethylene per hour per m2 (Choi, 1991). Adsorption of this small amount of ethylene may not be helpful for some situations. Another such film is described in a US Patent assigned to Nissho and Co. Ltd. of Japan (Someya, 1992). This film incorporates finely ground coral (primarily calcium carbonate), having pore sizes in the range of 100,000-500,000A. After incorporation in a polyethylene film, the coral is claimed to absorb ethylene. However, no data have been presented to support this claim. A product called Ethad® has been developed by the Rubber Research Institute of Malaysia; it releases ethylene in order to stimulate the production of latex by rubber trees. The product is based on powdered zeolite in viscous oil or grease. The zeolite is reported to adsorb 8% ethylene by weight
(Abeles et ah, 1992). Apparently Ethad® has not been used to adsorb ethylene in packages. Evert-Fresh Corporation markets Evert-Fresh bags in the USA. The bags are, presumably, polyethylene with Japanese Oya stone dispersed within the film matrix. Oya stone has putative ethylene-adsorbing capacity. Evert-Fresh Corp. offers shelf-life data for several fresh commodities to demonstrate the benefits of their bags. A product called BO Film is marketed by the Odja Shoji Co. Ltd. of Japan. It is a low-density polyethylene film extruded with finely divided crysburite ceramic which is claimed to confer ethylene-adsorbing capacity (Joyce, 1988). There are many other similar bags being sold throughout the world offering improved postharvest life of fresh commodities due to the adsorption of ethylene by the minerals dispersed within the film. The evidence offered in support of this claim is generally based on shelf-life experiments comparing common polyethylene bags with mineralized bags. Such evidence generally shows an extension of shelf-life and/or a reduction of headspace ethylene. Such data are unconvincing. Although the finely divided minerals may adsorb ethylene, they will also open pores within the plastic bag and alter the gas-exchange properties of the bag. Because ethylene will diffuse much more rapidly through open pore spaces within the plastic than through the plastic itself, one would expect ethylene to diffuse out of these bags faster than through pure polyethylene bags. In addition, CO2 will leave these bags more readily and O2 enter more readily than is the case for a comparable polyethylene bag. These effects can improve shelf-life and reduce headspace ethylene concentrations independently of any ethylene adsorption. In fact, almost any powdered mineral can confer such effects without relying on expensive Oya stone or other speciality minerals. Hercules Chemical Company relied on this effect while using calcium carbonate to improve the gas-transmission properties of their Fresh Hold breathable bags without making any claims regarding ethylene adsorption (Anderson, 1989). Although the minerals in question may have ethylene-adsorbing capacity, the data supporting the commercial products incorporating these minerals fail to demonstrate such capacity. Even if they do have ethylene-adsorbing capacity, it is possible that they will lack significant capacity while embedded in plastic films. The ethylene would have to diffuse through the plastic matrix before contact with the dispersed mineral, thus greatly slowing any processes of adsorption. Once the ethylene has diffused part-way through the plastic film, venting to the outside may be nearly as fast and effective as adsorption on embedded minerals. In a study performed in Australia with BO film, the mineral in the bag took up little ethylene (Joyce, 1988). Furthermore, in studies with pure mineral granules of Cera-sutora A, the author found that the ethylene
sorption capacity of the material was only - 170 nmol/g after 15 h at 200C (Joyce, 1988). This amount of ethylene sorption is insignificant. In studies in the USA, the author tested four proprietary bags from Japan, all containing dispersed minerals and all claiming ethylene-adsorbing capacity. Weighed samples of each bag were placed in sealed jars with sampling ports attached. A second set of jars were left empty. We injected a known quantity of ethylene into each jar. Each day for seven days we sampled the ethylene concentrations in each jar. We could detect no differences in ethylene concentrations between the jars with film and those without film. Our conclusion was that none of the four films adsorbed measurable amounts of ethylene (Zagory et al.9 1988). In the future, it would be useful if companies claiming ethylene-adsorbing capacity for their products presented direct evidence for these claims. Shelflife studies and headspace analysis of ethylene concentrations do not support claims of ethylene-adsorbing capacity. Direct measurement of ethylene depletion in closed systems containing samples of the bags without any produce to confound the results would be much more instructive. Furthermore, such studies should be done at low temperature and high relative humidity to mimic the conditions under which they will be expected to perform. 2.5 A
New and novel approaches to ethylene-removing packaging
There are some new and unusual approaches to developing ethyleneremoving packaging that deserve mention. Perhaps the most promising new development in ethylene-removing packaging is the use of electron-deficient dienes or trienes incorporated in ethylene-permeable packaging. The preferred diene or triene is a tetrazine. However, since tetrazine is unstable in the presence of water, it must be embedded in a hydrophobic, ethylene-permeable plastic film that does not contain hydroxyl groups (Holland, 1992). Appropriate films would include silicone polycarbonates, polystyrenes, polyethylenes and polypropylenes. Approximately 0.01-1.0 M dicarboxyoctyl ester of tetrazine incorporated in such a film was able to effect a ten-fold reduction in ethylene in sealed jars within 24 h and a 100-fold reduction within 48 h (Holland, 1992). The tetrazine film has a characteristic pink color when it is new and turns brown when it becomes saturated with ethylene so it is possible to know when it needs replacing. A new product called Frisspack has been developed in Hungary for use in storage of fresh fruits and vegetables. The product consists of a chemisorbent of small particle size dispersed among the fibers in the early phase of paper production. The result is a paper sheet with putative ethyleneadsorbing capacity. The nature of the chemisorbent and data supporting the
claim of ethylene adsorption are not available. No response was received from the vendor following the author's request for information. Although there are many packaging products claiming ethylene-removing capabilities, few of the claims are backed up with reliable data. Standardized procedures for demonstrating efficacy would aid the development of this growing industry. In addition, a thorough understanding of the physiological effects of ethylene and its importance in sealed permeable packages should precede any use of these products. In many cases, the elevated carbon dioxide levels common in modified atmosphere packages may obviate the need for ethylene removal. In other cases, with very sensitive commodities such as kiwifruit and carnations, ethylene-adsorbing capability may be crucial in the maintenance of shelf-life and commercial quality. Acknowledgements Thanks for literature and helpful discussion are owed to: Linda Dodge, Cheryl Reeves, Michael Reid, Michael Rooney, Mikal Saltveit and Kit Yam. References Abe, Y. (1990) Active packaging - a Japanese perspective. Proceedings of the International Conference on Modified Atmosphere Packaging, October 15-17, 1990. Campden Food and Drink Research Assoc. Abe, K. and Watada, A.E. (1991) Ethylene absorbent to maintain quality of lightly processed fruits and vegetables. J. Food ScL, 56(6), 1589-92. Abeles, F.B., Morgan, P.W. and Saltveit, M.E. (1992) Ethylene in Plant Biology. Academic Press, Inc., 414 pp. Aharoni, N. (1989) Interrelationship between ethylene and growth regulators in the senescence of lettuce leaf discs. J. Plant Growth ReguL, 8, 309-17. Aharoni, N. and Lieberman, M. (1979) Ethylene as a regulator of senescence in tobacco leaf discs. Plant Physiol, 64, 801-4. Anderson, H.S. (1989) Controlled atmosphere package. US Patent No. 4842875. Anon. (1964) California Standards for Ambient Air Quality and Motor Vehicle Exhaust. Technical Report, Supplement No. 2. Additional Ambient Air Quality Standards. State of
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Press, Boca Raton. Barkai-Golan, R. and Lavy-Meir, G. (1989) Effects of ethylene on the susceptibility to Botrytis cinerea infection of different tomato genotypes. Ann. Appl. BioL, 114, 391-6. Ben-Arie, R. and Sonego, L. (1985) Modified-atmosphere storage of kiwifruit (Actinidia chinensis Planch) with ethylene removal. Scientia Hortic, 27, 263-72. Bufler, G. and Strief, J. (1986) Ethylene biosynthesis of Golden Delicious apples stored in different mixtures of carbon dioxide and oxygen. Scientia Hortic, 30, 177-85. Burg, S.P. and Thimann, K.V. (1959) The physiology of ethylene formation in apples. Proc. Natl Acad. ScL, 45, 335-44.
Cameron, A.C. and Reid, M.S. (1983) Use of silver thiosulphate to prevent flower abscission from potted plants. Scientia Hortic, 19, 373-8. Chadwick, A. V. and Burg, S.P. (1967) An explanation of the inhibition of root growth caused by indole-3-acetic acid. Plant PhysioL, 42, 415-20. Choi, S.O. (1991) Orega ultra-high gas permeabilityfilledfilmfor fresh produce packaging. CAP'91, Sixth Int. Conf. CA/MA/Vacuum Packaging. Schotland Business Res., Inc. pp. 197-208. Denny, F.E. (1924) Hastening the coloration of lemons. / Agr. Res., 27, 757-69. Dillworth, MJ. (1966) Acetylene reduction by nitrogen-fixing preparations from Clostridium pasteurianum. Biochem. Biophys. Acta, 127, 285-94. Eastwell, K.C., Bassi, P.K. and Spencer, M.E. (1978) Comparison and evaluation of methods for the removal of ethylene and other hydrocarbons from air for biological studies. Plant PhysioL, 62, 723-6. Fuchs, Y. and Temkin-Gorodeiski, N. (1971) The course of ripening of banana fruits stored in sealed polyethylene bags. J. Amen Soc. Hort. ScL, 96, 401-3. Fukuda, H., Fujii, T. and Ogawa, T. (1984) Microbial production of C2-hydrocarbons, ethane, ethylene and acetylene. Agric. Biol. Chem., 48, 1363-5. Goodburn, K.E. and Halligan, A.C. (1987) Modified atmosphere packaging - a technology guide. Publication of the British Food Manufacturing Association, Leatherhead, UK. pp. 1-44. Halevy, A.H. (1986) Pollination-induced corolla senescence. Acta Hort., 181, 25-32. Hansen, E. (1942) Quantitative study of ethylene production in relation to respiration of pears. Bot. Gaz., 103, 543-58. Hatton, T.T. and Reeder, W.F. (1972) Quality of LuIa avocados stored in controlled atmospheres with or without ethylene. / Amer. Soc. Hort. ScL, 97, 339-41. Holland, R. V. (1992) Absorbent material and uses thereof. Australian Patent Application No. PJ6333. Hag, L.L. and Curtis, R.W. (1968) Production of ethylene by fungi. Science, 159, 1357-8. Imaseki, H., Kondo, K. and Watanabe, A. (1975) Mechanism of cytokinin action on auxininduced ethylene production. Plant Cell PhysioL, 11, 827-9. Jackson, M.B., Gales, K. and Campbell, DJ. (1978) Effect of waterlogged soil conditions on the production of ethylene and on water relationships in tomato plants. J. Exp. Bot., 29, 183-93. Jankiewicz, L.S. (1985) Mechanism of abscission of leaves and reproductive parts of plants. A model. Acta Soc. Bot. PoL, 54, 285-322. Joyce, D.C. (1988) Evaluation of a ceramic-impregnated plastic film as a postharvest wrap. HortScience, 23, 1088. Kader, A.A. (1985) Postharvest biology and technology: An overview. In: A.A. Kader, R.F. Kasmire, F.G. Mitchell, M.S. Reid, N.F. Sommer and J.F. Thompson (eds), Postharvest Technology of Horticultural Crops. University of California, Cooperative Extension Special Publication 3311, 192 pp. Kader, A.A., Zagory, D. and Kerbel, E.L. (1989) Modified atmosphere packaging of fruits and vegetables. Crit. Rev. Food ScL Nut., 28(1), 1-30. Kays, SJ. and Beaudry, R.M. (1987) Techniques for inducing ethylene effects. Acta Hort., 201, 77-116. Kepczynska, E. (1993) Involvement of ethylene in the regulation of growth and development of the fungus Botrytis cinerea. Plant Growth Regul., 13, 65-9. Knee, M. (1990) Ethylene effects in controlled atmosphere storage of horticultural crops. In: M. Calderon and R. Barkai-Golan (eds.), Food Preservation by Modified Atmospheres. CRC Press, Boca Raton, pp. 225-35. Krishnamurthy, S. and Kushalappa, CG. (1985) Studies on the shelf-life and quality of robusta bananas as affected by post-harvest treatments. /. Hortic. ScL, 60, 549-56. Ku, H.S., Suge, H., Rappaport, L. and Pratt, H.K. (1970) Stimulation of rice coleoptile growth by ethylene. Planta, 90, 333-9. Kusunose, H. and Sawamura, M. (1980) Ethylene production and respiration of postharvest acid citrus fruits and Wase Satsuma Mandarin fruit. Agr. Biol. Chem., 44, 1917-22. Leighton, P.A. (1961) Photochemistry of air pollution, Academic Press, NY.
Lieberman, M., Kunishi, A., Mapson, L.W. and Wardale, D.A. (1966) Stimulation of ethylene production in apple tissue slices by methionine. Plant Physiol, 41, 376-82. Liu, F.W. (1970) Storage of bananas in polyethylene bags with an ethylene absorbent. HortScience, 5, 25-7. Makhlouf, J., Willemot, C , Ami, J., Castaigne, F. and Emond, J.-P. (1989) Regulation of ethylene biosynthesis in broccoli flower buds in controlled atmospheres. J. Amer. Soc. Hort. ScL, 114, 955-8. Maotani, T., Yamada, M. and Kurihara, A. (1982) Storage of Japanese persimmon of pollination constant non-astringent type in polyethylene bags with ethylene absorbent. J. Jap. Soc. Hort. ScL, 5, 195-202. Matsui, M. (1989) Film for keeping freshness of vegetables and fruit. US Patent No. 4847145. Morrison, R.T. and Boyd, R.N. (1966) Organic Chemistry, 2nd edition. Allyn & Bacon, Inc., Boston, USA 1204 pp. Nichols, R. (1968) The response of carnations (Dianthus caryophyllus) to ethylene. J. Hort. ScL, 43, 335-49. Noyes, W.A., Jr., Hammond, G.S. and Pitts, J.N., Jr. (1964) Vacuum ultraviolet photochemistry. Adv. Photochem., 3, 226-9. Osajima, Y., Sonoda, T., Yamamoto, F., Nakashima, M., Shimoda, M. and Matsumoto, K. (1983) Development of ethylene absorbent and its utilization. Nippon Nogeikagaku Kaishi, 57, 1127-33. Osborne, DJ. (1989a) The control role of ethylene in plant growth and development. In: Clijsters, H. et al. (eds.). Biochemical and Physiological Aspects of Ethylene Production in Lower and Higher Plants. Kluwer Academic Publishers, pp. 1-11. Osborne, DJ. (1989b) Abscission. CHt. Rev. Plant ScL, 8, 103-29. Owens, L.D., Lieberman, M. and Kunishi, M. (1971) Inhibition of ethylene production by rhizobitoxin. Plant Physiol, 48, 1-4. Reid, M.S. (1985a) Ethylene and abscission. HortScience, 20, 45-50. Reid, M.S. (1985b) Ethylene in postharvest technology. In: A.A. Kader, R.F. Kasmire, F.G. Mitchell, M.S. Reid, N.F. Sommer and J.F. Thompson (eds) Postharvest Technology of Horticultural Crops. University of California, Cooperative Extension Special Publication 3311, 192 pp. Reid, M.S., Paul, J.L., Farhoomand, M.B., Kofranek, A.K. and Staby, G.L. (1980) Pulse treatment with the silver thiosulphate complex extends the vase-life of cut carnations. /. Amer. Soc. Hort. ScL, 105, 25-7. Rizzolo, A., Polesello, A. and Gorini, F. (1987a) Laboratory screening tests of some suitable regenerable adsorbents to remove ethylene from cold room atmospheres. 1. Glycols and polyglycols. In: Third Subproject: Conservation and Processing of Foods - A Research Report (1982-1986), National Research Council of Italy, Milano. pp. 99-100. Rizzolo, A., Polesello, A. and Gorini, F. (1987b) Laboratory screening tests of some suitable regenerable adsorbents to remove ethylene from cold room atmospheres. 1. Apolar phases. In: Third Subproject: Conservation and Processing of Foods - A Research Report (1982-1986), National Research Council of Italy, Milano. pp. 101-2. Roberts, J.D. and Caserio, M.C. (1967) Modern Organic Chemistry, Benjamin, NY. Sato, M., Urushizaki, S., Nishiyama, K., Sakai, F. and Ota, Y. (1987) Efficient production of ethylene by Pseudomonas syringae pv. glycinea which causes halo blight of soybeans. Agric. Biol Chem., 51, 1177-8. Scott, W.E., Stephens, E.R., Hanst, P.C. and Doerr, R.C. (1957) Further developments in the chemistry of the atmosphere. Proc. Amer. Petrol. Inst., 37, 171-83. Scott, KJ., McGlasson, W.B. and Roberts, E.A. (1970) Potassium permanganate as an ethylene absorbent in polyethylene bags to delay ripening of bananas during storage. Aust. J. Expt. Agric. Anim. Hush, 10, 237-40. Scott, KJ. and Wills, R.B.H. (1973) Atmospheric pollutants destroyed in an ultraviolet scrubber. Lab. Pract., 22, 103-6. Shorter, AJ. and Scott, KJ. (1986) Removal of ethylene from air and low oxygen atmospheres with ultraviolet radiation. Lebensm. Wiss. U. Technol., 19, 176-9. Someya, N. (1992) Packaging sheet for perishable goods. US Patent No. 5084337.
Talib, Z. (1983) Ethylene in the storage of fresh produce. In: Developments in Food Preservation, Vol. 2, S. Thorne, (ed), Applied Science Publishers, London and New York. pp. 149-77. Urushizaki, S. (1987a) On the Effects of Functional Films. Autumn Meeting of Japan Society Horticultural Science. Symposium: Postharvest Ethylene and Quality of Horticultural Crops. University of Kyushu, October 8, 1987. Urushizaki, S. (1987b) Development of Ethylene Absorbable Film and its Application to Vegetable and Fruit Packaging. Autumn Meeting of Japan Society Horticultural Science. Symposium: Postharvest Ethylene and Quality of Horticultural Crops. University of Kyushu, October 8, 1987. Wittenbach, V.A. and Bukovac, MJ. (1973) Cherry fruit abscission: Effect of growth substances, metabolic inhibitors and environmental factors. /. Amer. Soc. Hort. ScL, 98, 348-51. Yang, S.F. and Hoffman, N.E. (1984) Ethylene biosynthesis and its regulation in higher plants. Ann. Rev. Plant Physiol, 35, 155-89. Zagory, D., Brecht, B. and Kader, A.A. (1988) Unpublished data.
3
Design of modified atmosphere packaging for fresh produce K. L. YAM and D. S. LEE
3.1 Introduction Controlled atmosphere (CA) storage and modified atmosphere packaging (MAP) are two useful technologies to extend the shelf-life of fresh agricultural and horticultural produce. Simply stated, these technologies involve storing a fruit or vegetable in a modified atmosphere usually consisting of reduced O2 and elevated CO2 concentrations compared to air. The modified atmosphere reduces the rates of respiration and ethylene production, which are often associated with the benefits of retardation of physiological, pathological, and physical deteriorative processes occurring in the product. Aerobic respiration is a complicated process that involves a series of enzymatic reactions taking place through the metabolic pathways of glycolysis, the tricarboxylic acid (TCA) cycle, and the associated electron transport system (Kader, 1987). However, the overall reaction describing the respiration process may be simply expressed as C6H12O6+ 6 O2 -> 6 CO2 + 6 H2O + heat (3.1) that involves the oxidation of organic substrates (such as starch, sugars, and organic acids) to CO2 and H2O along with heat generation. Kinetic theory and Equation (3.1) suggest that the respiration rate may be reduced by decreasing the O2 and/or by increasing the CO2 concentration. There are differences between the ways CA storage and MAP create and maintain a modified atmosphere. In CA storage, a gas generator is usually used to create and control the modified atmosphere in a cold warehouse where the product is kept. In MAP, the product is kept in a carefully designed permeable package, and the modified atmosphere is created and maintained through an intricate interplay between the respiration of the product and the gas permeation of the package. MAP is a more economical technology because an expensive gas generator is not needed; however, it is also a more difficult technology to implement because of the rather complicated interactions between the product and the package. This chapter is focused on the design of MAP for fresh produce. The modified atmosphere in MAP can be created by either active or passive modification. In active modification, the modified atmosphere is created rapidly by flushing the headspace of the package with a desired gas mixture. In passive modification, the modified atmosphere is created by
allowing the produce to respire inside the package so that an equilibrium is slowly attained. In both cases, once the modified atmosphere is established, it is maintained through a dynamic equilibrium of respiration and permeation. Designing MAP for fresh produce is a complicated task, requiring good understanding of the dynamic interactions among the product, the environment, and the package. The food technologist who is asked to design MAP for fresh produce faces many difficult but practical questions, such as whether the MAP technology is applicable to the product, what is the optimum gas composition, what kind of packaging material is needed, and
Conduct literature review
Conduct feasibility study
Could CA storage provide" benefits?
Determine optimum conditions and tolerant limits
Neither CA storage nor MAP is suitable for the product
Determine respiration rates
Use mathematical model to determine package requirments
I Verify model predictions with experiments
Is suitable permeable film available?
Design of MAP possible
Only CA storage is suitable for the product
Need to develop new permeable films for MAP
Figure 3.1 Flow chart for designing MAP for fresh produce.
how to protect the product from the potential hazards of a modified atmosphere. Prince (1989) has reported that the majority of modified atmosphere packages are designed by trial-and-error methods, which often lead to poor designs that are either ineffective or injurious to the product. Although numerous articles have already been written on the various aspects of MAP, almost none of them provides an overview of the design process. To fill this gap, a simple process for designing MAP of fresh produce is presented in this chapter. The process is necessarily somewhat simplified because many biological aspects underlying the effects of modified atmosphere on the shelf-life of plant tissues are still not understood (Solomos, 1994). The steps involved in the design process are outlined in the flow diagram of Figure 3.1. 3.2 Literature review Before designing MAP for a product, the first step is to determine whether CA storage can indeed provide benefits for the product. Reviewing the Table 3.1 Recommended optimal modified atmosphere conditions for produce Commodity
Temperature range (0C)
Relative humidity (%)
0-5 0-5 0-5 0-5 0-5 0-5 8-12 0-5 0-5 8-12 0-5 8-12
0-5 0-5 5-13 12-15 0-5 0-5 10-15 0-5 0-5 0-5 0-5
Modified atmosphere O2
CO2
95 95 95 90-95 95 95 90-95 95 90 90-95 95 85-90
air 1-2 1-2 3-5 2-5 2-4 3-5 2-5 air 3-5 air 3-5
5-10 5-10 5-7 5-7 2-5 10-20 0 0 10-15 2-8 10-20 0
90 90 85-90 85-95 90-95 90-95 85-90 90 90-95 90-95 90-95
2-3 2-3 2-5 2-5 0-10 3-10 3-10 1-2 2-3 2 10
1-2 2-3 3-10 2-5 11-20 10-12 5-10 5 0-1 8 15-20
Vegetables Asparagus Broccoli Brussels sprouts Cabbage Cauliflower Corn, sweet Cucumber Lettuce Mushroom Pepper Spinach Tomato, partly ripe Fruits Apple Apricot Avocado Banana Blueberry Cherry, sweet Grapefruit Peach Pear Persimmon Strawberry
From Labuza and Breene (1989), Powrie and Skura (1991), and Katzyoshi (1992)
literature data is a good start to gather preliminary information about the product or similar products. Helpful information that may be available in the literature is assessment of the potential benefit of CA storage and MAP, optimum storage conditions (such as gas concentration, temperature, and relative humidity), O2 and CO2 tolerance limits, respiration rate, temperature below which chilling injury of the product occurs, whether the product is climacteric or nonclimacteric, and so on. The recommended O2 and CO2 concentrations for some fruits and vegetables are listed in Table 3.1. More data are available elsewhere (Prince, 1989; Labuza and Breene, 1989; Singh and Oliveira, 1994). The data may also be represented in the form of CO2 versus O2 plots (such as in Figures 3.2 and 3.3), in which the windows represent the boundary of recommended gas concentrations. The size of a window has a practical implication in that the smaller it is, the more rigid is the design requirement. However, literature data should be used only as a reference because discrepancies sometimes exist among data from different sources (due to possible reasons
CO2 Concentration (%)
Blackberry, Blueberry, Fig, Raspberry, Strawberry
Cherry
Mango, Papaya, Pineapple Avocado Persimmon Banana
Grapefruit Air P = 0.8
Kiwi. Ne< arine, Peach
Orange
Apricot
Grape Cranberry Plum
LDPEp = Cy
O2 Concentration (%) Figure 3.2 Recommended gas concentrations for CA storage of fruits. (Redrawn from Singh and Oliveira, 1994, with permission.)
such as maturity and cultivar of the product) and the criteria used in making recommendations are seldom reported. Therefore conducting a feasibility study is often required, particularly if no literature data are found for the product of interest. 3.3 Feasibility study A simple feasibility study consists in conducting experiments to monitor the quality of the product as a function of time under various modified atmospheres. To define quality, a set of instrumental and sensory quality attributes must be selected. Although the selection procedure is different for each product, it generally includes assessment of texture, flavor, odor, color, nutritional quality, and microbial growth. The effects of CA storage on the sensory and nutritional quality of fruits and vegetables have been reviewed by Weichmann (1986).
CO2 Concentration (%)
Air P = 0.8
Mushrooms Asparagus Leeks Broccoli Brussels ! >pr< >uts' Beans Cabbage
Parsley
Okra
Spinach
Cauliflower
Lettuce
Tomato Pepper Artichokes Radish
LDPE B = 0.8
O2 Concentration (%) Figure 3.3 Recommended gas concentrations for CA storage of vegetables. (Redrawn from Singh and Oliveira, 1994, with permission.)
(a) Gas In
Gas Out
Gas Sampling Port
Product Samples
Figure 3.4 Flow-through system (a) and closed system (b).
The modified atmosphere can be created using the flow-through system (Figure 3.4a) that involves storing the product in a glass jar that has an inlet port and an outlet port through which a pre-mixed gas (consisting of lowered O2 and elevated CO2) passes. For this feasibility study, the authors suggest the use of a 3-level factorial experimental design with O2 and CO2 concentrations as independent variables, while keeping temperature and relative humidity constant. The response or dependable variables are two or three relevant quality attributes for the product. The O2 and CO2 ranges are to be selected between 2 and 10% and O and 20%, respectively. The suggested temperature and relative humidity are 5°C and 90%, respectively. Air (21% O2 and 0% CO2) should always be used as a control. The purpose of the feasibility study is to determine if CA storage can provide better storage quality than air storage. If the results are not favorable, it is likely that MAP is not a suitable technology, and the food technologist should avoid spending more time on designing MAP for this particular product. 33.1
Optimum conditions
Further work is justified if the feasibility study confirms the benefit of CA storage for the product. The question then is whether the same benefit can be achieved by MAP without the use of an expensive gas generator. Since the feasibility study provides only preliminary data, more experiments are needed to more closely define the optimum conditions. This should be done by extending the experimental design to include additional O2 and CO2 concentrations that are expected to give good results. The effects of additional temperatures (O and 10°C) and relative humidities (85 and 95%) on storage quality should also be examined. There are three major design constraints: O2 tolerance limit, CO2 tolerance
limit, and temperature below which chilling injury occurs. Keeping O2 concentration above the O2 tolerance limit is necessary for maintaining aerobic respiration; otherwise, anaerobic respiration will lead to the formation of off-flavor and off-odor inducing compounds such as alcohols and aldehydes. Keeping CO2 concentration below the CO2 tolerance limit is necessary for protection of the product from unfavorable physiological disorder such as breakdown of internal tissues. Keeping the product above a certain temperature is necessary for avoiding cell damage leading to loss of flavor and invasion of spoilage organism. Usually the O2 tolerance limit varies between 1 and 3%, the CO2 tolerance limit varies between 10 and 20%, and the chilling temperature varies between O and 15°C, depending on the product - the actual values can be determined experimentally using the flow-through system illustrated in Figure 3.4a. The O2 tolerance limit may be determined by monitoring the increase in ethanol content of the tissue. 3.4 Respiration rates Respiration rate values are required for mathematical modeling and for defining the package requirements. Respiration is often a good index for the storage life of fresh produce: the lower the respiration rate, the longer the storage life (Powrie and Skura, 1991; Lebermann et aL> 1968). As Equation (3.1) shows, respiration involves the rate of O2 consumption (R0) and the rate of CO2 evolution (RCo2)- The respiratory quotient (RQ) is a convenient term, which is defined as the ratio of CO2 evolution to O2 consumption. RQs are reported to range from 0.7 to 1.3, depending upon the metabolic substrate (Kader, 1987; Kader et aly 1989). The respiration rates are known to be affected by several internal and external factors (Robertson, 1992). Internal factors include the type of product and cultivar, maturity, resistance of plant tissue to gas diffusion, and whether the product is climacteric or nonclimacteric. The external factors include temperature, C2H4 concentration, O2 and CO2 concentrations and stress due to physical damage or excessive water loss. 34.1
Temperature effect
Temperature is the most important factor because it affects both the respiration rate and the permeability of the package. In practice, most products experience some temperature fluctuations during storage and distribution. The Arrhenius model is often used to describe the temperature dependence of respiration, and the equations for rate of O2 consumption and rate of CO2 evolution are /?O2 = R°2exp(-£O2/R7^
(3.2)
#co2 = Rco2 exp(-£CO2/R 7^
(3.3)
Another common way to express the temperature dependence is Q10, defined as 10
Respiration rate at (T + 10)°C "~ Respiration rate at rC
which is applicable to either O2 consumption rate or CO2 evolution rate. Typical Q10 values for vegetables are 2.5-4.0 at 0-1O0C, 2.0-2.5 at 10-200C, 1.5-2.0 at 20-300C, and 1.5-2.0 at 30-400C (Robertson, 1992). Mathematically, the activation energy is approximately linearly proportional to Q10 if the temperature range of interest is small (less than 400C difference), a condition satisfied by most practical circumstances. Similarly, Arrhenius-type equations can also be used to describe the temperature dependence of gas permeabilities. (3.5) (3.6)
3.5 Measurement of respiration rates Because respiration rates under modified atmospheres for most fruits and vegetables are not available in the literature, they must be determined by experiment. There are three methods for measuring respiration rates: the flow-through system, the closed system, and the permeable system (Lee, 1987). The flow-through system and the closed system are illustrated in Figure 3.4. 5.5.7
Flow-through system
The experimental setup of the flow-through system is shown in Figure 3.4(a). It is important to position the inlet and the outlet tubes sufficiently far apart to ensure thorough mixing of the gas in the jar. The steady-state inlet and outlet concentrations are measured with an instrument such as a gas chromatograph. The equations for calculating the respiration rates are (3.7)
(3.8) where the subscripts in and out denote the inlet and the outlet concentrations, respectively. The flow-through system has an advantage of being able to
provide more accurate data than the closed system. However, the usefulness of the flow-through system is limited by the precision of the gas chromatography measurements, because the differences between the inlet and outlet concentrations are usually rather small. There are three ways to increase the concentration differences, as suggested by Equations (3.7) and (3.8): work only with produce of high respiration; reduce the gas flow rate; increase the sample weight. Another drawback of the flow-through system is that each experiment measures only the respiration rate at a single gas concentration, and thus much time and labor are required if respiration rates at many gas concentrations are to be measured.
3.5.2
Closed system method
The closed system method (Figure 4b) is more efficient for measuring respiration rates as a function of gas concentrations. This method involves monitoring the O 2 and CO 2 concentrations inside a closed jar containing the product as a function of time (Haggar et al., 1992). The initial gas concentrations inside the jar are usually those of air, but other gas concentrations may also be used. As the product respires, the gas concentrations in the jar change with time - from high O2/low CO 2 concentrations at the beginning to low O2/high CO 2 concentrations toward the end. The respiration rates at these O 2 and CO 2 concentrations may be calculated using the equations (3.9)
(3.10) The negative sign in Equation (3.9) signifies that the O 2 concentration in the jar decreases with time. In order to evaluate the first derivatives, the data of gas concentration versus time should first be curved fitted. The recommended functions for fitting the data are (3.11) (3.12) and their first derivatives are
(3.13) (3.14) The validity of Equations (3.11) and (3.12) should be confirmed by comparing the fitted values with the experiment data. If the comparison is poor, other forms of functions should be attempted. For convenience, the conversion factor a is omitted in Equations (3.13) and (3.14). The respiration rates calculated from Equations (3.9) and (3.10) are at O2 and CO2 concentrations unique to a particular closed system experiment. It is usually difficult to design a priori a closed system experiment to generate certain desired gas concentrations. The question is how the respiration rates obtained from closed system experiments can be useful to estimate the respiration rates at other gas concentrations. An answer is to first fit the respiration rate data with a model and then use the model to estimate the respiration rates at the desired gas concentrations. The best model presently available for this purpose is the enzyme-kinetic type respiration model proposed by Lee et al. (1991). (3.15) The model requires two different sets of adjustable coefficients (Vm, Km, and Kj): one for ROi and the other for Rcc>2. The model has been verified quite extensively using experimental data for a wide variety of products. Since the model is based on the principle of enzyme kinetics, it requires less adjustable coefficients and is likely to be more predictive than those purely empirical models (Cameron et a/., 1989; Yang and Chinnan, 1988) used in the literature. However, the applicability of the model to any new set of data should always be confirmed by comparing the predicted values with the experimental data. Overextending the model to predict respiration rates at concentrations very different from those generated from the closed system experiments should be avoided. Table 3.2 lists the model parameter values and respiration activation energies for some fruits and vegetables. The activation energies are not strong functions of O2 and CO2 concentrations (Haggar et aL, 1992). 3.6 Model equations and package requirements Mathematical models are useful for defining the package requirements for MAP. Several models (Jurin and Karel, 1963; Veeraju and Karel, 1966;
Table 3.2 Respiration model parameter values and respiration activation engergies for some products Respiration model parameters Commodity Blueberry "Coville"3 Broccolib Cauliflower0 Green pepperd
vm
Activation energy (kJ/mol)
Temp. (0C)
Respiration expression O2 consumption CO2 evolution
(mg/kgh) 68.0 51.0
(% O2)
15
0.4 0.2
2.9 4.9
147.3 163.3
7
O2 consumption CO2 evolution
210.3 235.2
0.6 1.7
2.3 1.93
62.7 66.1
13
O2 consumption CO2 evolution
133.7 134.4
1.7 1.4
3.0 3.1
21.2-48.2 21.2-48.2
10
O2 consumption CO2 evolution
54.3 31.8
6.0 2.4
1.3 4.3
48.7-57.3 48.7-57.3
(% CO2)
a
Song et al (1992); bHaggar et al. (1992); cYam et al. (1993) and Exama et al. (1993); dExama et al. (1993).
Hayakawa et al., 1975; Deily and Rizvi, 1981) are available in the literature, and some of them have been reviewed by Zagory and Kader (1988). Basically those models use the principles of O 2 and CO 2 mass balances to describe the interactions among the respiration of product, the permeability of the package, and the environment. 3.6.1
Unsteady-state equations
A simple model based on the principle of mass balance requires that Rate of O2 or CO2 accumulated in package
_
Rate of O2 or CO2
Rate of O2 or CO2
permeated into package
generated by respiration
and the mass balance equations for O 2 and CO 2 are (3.16)
(3.17) where the subscripts i and o denote the inside and outside of the package, respectively. Equations (3.16) and (3.17) are first-order linear differential equations that can be solved quite easily using a computer. They are useful for describing the unsteady-state behaviour of the MAP system, such as during the process of passive modification and during temperature fluctuations. The equations can be tailored to fit a particular physical situation through the application of initial boundary conditions. For example, the initial conditions for passive modification are [O2]; = 21 and [CO2]; = O at
t = O. Note that the respiration rates R02 and RCOi are functions of O2 and CO2 concentrations, which can be expressed using the enzyme-kinetic model of Equation (3.15). 3.6.2 State-state equations When the accumulated terms are zero, Equations (3.16) and (3.17) are reduced to the steady-state equations (3.18)
(3.19) where the subscript s denotes steady-state condition. Equations (3.18) and (3.19) describe the dynamic equilibrium behaviour of the MAP system, when the CO2 evolution rate equals the efflux rate of CO2 through the package and the O2 consumption rate equals the influx rate of O2 through the package. In most situations, steady-state or dynamic equilibrium is approached within two days. For long storage of the product, the dynamic equilibrium behavior is more important than the unsteady-state behavior. To use Equations (3.18) and (3.19) as design equations, it is necessary to keep track of how many independent or design variables are available. There are a total of 11 variables: R02, RCOi, and W are associated with the product; P02, PCQ2, S, and L are associated with the package; [O2]o, [O2J1 s, [CO2]O, and [CO2I1 s are associated with the environment. (Although temperature is not explicitly shown, it is an implicit variable that affects both the respiration rates and the permeabilities to O2 and CO2.) Once the product and the temperature are selected, six out of the 11 variables are already decided: R02 and RCOi are determined by the flow-through system or the closed system experiments; [O2J1 s and [CO2], s are assumed to be the optimum O2 and CO2 concentrations; [O2]o and [CO2J0 are 21 and 0%, respectively. With six variables fixed and two equations to satisfy, there are only (11-6-2) = 3 design variables. That is, only three out of the remaining five variables (W, 5, L, P 02 and PCO2) can be specified arbitrarily. For example, if the food technologist chooses to specify the dimensions of W, S, and L (within practical limits), the permeabilities P 02 and PCO2 must then be determined by Equations (3.18) and (3.19). The equations also provide a convenient means to reject films not suitable for a particular application. Dividing Equation (3.19) by Equation (3.18) yields
(3.20)
where [O2]0 and [CO2I0 are assumed to be 21 and 0%, respectively. Further, if RQ is assumed to be 1 and PCQJPOi is defined as (3, Equation (3.20) may be rewritten as (3.19) Equation (3.19) may be represented as a straight line with slope P on a plot of [CO2I1 s versus (21 - [O2]; s). Two such lines (P = 0.8 and (3 = 5) are shown in Figures 3.3 and 3.4. As an example of application, cauliflower requires a P = 5 (Figure 3.3), and thus a film with (3 varying considerably from 5 (such as 2) should be rejected for packaging cauliflower. However, there is no guarantee that a film with (3 = 5 will work well for cauliflower because, in addition to (3, the individual F 02 and PCOi must be also determined by solving Equations (3.17) and (3.18) simultaneously. Satisfying Equation (3.19) is a necessary but non-sufficient requirement for selecting a suitable polymeric film. There are on-going research efforts being made to develop more sophisticated models for more accurate prediction, since none of the existing models considers every factor of the MAP system. A more complete model should include the generation of H2O and heat and the effects of N2 and C2H4 in addition to balancing the O2 consumption rate and CO2 evolution rate. In the meantime, the simple model described above can be used to provide helpful information for preliminary design of MAP. 3.7 Polymeric films for MAP applications Since there are many varieties of produce, a wide range of permeabilities is required. High permeabilities are needed for rapidly respiring produce, low permeabilities for slowly respiring produce. Table 3.3 lists the permeabilities, (3 values, and permeability activation energies of some common food packaging polymeric films. Among them, low-density polyethylene and polyvinyl chloride are most widely used for packaging fruits and vegetables (Zagory and Kader, 1988). A fortunate situation occurs when the desired P 02 and PCOl requirements are met by one or more existing commercial films. If this is the case, a good chance exists for a successful design. Unfortunately, this is not often the case because the choices of suitable commercial polymeric films are rather limited. The problem can be appreciated by examining Table 3.3, which reveals that the (3 values for most films fall within a rather narrow range
al (1993). Table 3.3 Permeabilities at 100C and permeability activation energies for polymeric film Permeabilities (ml mil/m2h atm) Polymeric films Polybutadiene Low-density polyethylene Ceramic-filled LDPE Linear low-density polyethylene High-density polyethylene Cast polypropylene Oriented polypropylene Polyethylene terephthalate Nylon laminated multilayer film Ethylene vinyl acetate Ceramic-filled polystyrene Silicone rubber Perforation (air) Microporous film
PCO2
Activation energies (kJ/mol) E
PQ2 1118 110
PQ2 9892 366
PQ2 8.8 3.3
Ep.o2 29.7 30.2
P,co; 21.8 31.1
199 257
882 1002
4.4 3.9
36.8
28.4
2.1
9.8
4.6
35.1
30.1
53 34 1.8
151 105 6.1
2.9 3.1 3.3
26.8
25.9
1.7
6.0
3.5
52.6
50.0
166 116
985 630
5.9 5.4
48.4 34.5
37.0 26.2
11170 2.44 X 109 3.81 X 107
71300 1.89 X 109 3.81 X 107
6.4 0.8 1.0
8.4 3.6 13.0
0.0 3.6 3.7
From Exama et al (1993); Lee et al. (1992); Lee et al. (1994); Ohta et al (1991); Mannapperuma and Singh (1990); Anderson (1989); and Shelekshin et al (1992).
between 3 and 6; however, Figures 3.3 and 3.4 show that many fruits and vegetables require P values outside this narrow range. This problem has also been recently investigated by Exama et al. (1933), who conclude most films do not satisfy both the gas flow and selectivity requirements for many fruits and vegetables packaged in typical MAP configurations. There are at least two possible solutions for this problem. The first solution is to compensate the inadequacy of the films with techniques such as placing oxygen absorbers in the package or using two different films to selectively control the permeability. The second solution is to look for new and better films - some recent advances in the development of polymeric films suitable for fresh produce are discussed below. 3.7.1 Perforation and microporous films A major challenge is to develop films that have greater permeability and have a wider range of p values than existing types. Films of enhanced permeability are necessary for packaging high respiration rate products and for preventing the development of anaerobiosis. A wider range of (3 values, especially those below 3, is necessary to better match the respiration behavior of many products. The use of either perforation systems or microporous films is a possible solution to meet these two requirements. These systems and films have
permeabilities many orders of magnitude higher than those of non-perforated polymeric films, as well as (3 values between 0.8 and 1 (Anderson, 1989). The uses of perforation systems or microporous films in MAP are currently being studied in several laboratories. Emond et al (1991) have studied gas exchange through perforation systems. They developed empirical equations to predict the effective permeabilities to O 2 and CO 2 for various diameters, thicknesses, and temperatures. Their computer simulations showed that neither a silicone membrane alone nor a perforation system alone could provide a satisfactory gas concentration for broccoli. However, a combined system, consisting of silicone membrane with area of 0.0061 m2 and perforations of 0.006 m diameter and 0.0127 m thickness, could provide favorable conditions for broccoli. Their other computer simulations also showed that while no polymeric film or silicone membrane could provide satisfactory conditions for strawberries, a perforation system (with perforations 0.008 m in diameter and 0.00159 m thick) could provide an effective solution. However, experiments are required to confirm these computer predictions. Meyers (1985) described the use of perforations in MAP of fruits. The technique involved placing the product in a bag (or on a tray) constructed of a high-barrier film such as polyvinylidene chloride. The bag was flushed with N 2 or CO 2 as a preservative gas before sealing. After sealing, the film was perforated to assure gas outflow from the bag, to prevent distortion and to provide a gas pressure within the bag sufficient to inhibit air inflow into the container. The packaging of strawberries and nectarines using this technique was described. Mizutani et al (1993) reported that microporous polypropylene sheets could be prepared by biaxially stretching filler-containing polypropylene sheets. Examples of fillers were CaCO 3 and SiO2. The gas permeabilities of those sheets were controllable by adjusting filler content, particle size of filler, and degree of stretching. The average pore size ranged between 0.14 and 1.4 juum. Anderson (1989) has described the use of microporous films for MAP of fruits and vegetables. The package was constructed of a gas-impermeable material having a microporous membrane panel to provide controlled flows of O 2 and CO 2 through its walls. The microporous membrane was a biaxially oriented film composed of a blend of propylene homopolymer and a propylene-ethylene copoylmer having an ethylene-moiety concentration of 2-5% by weight. The film was filled with 40-60% CaCO 3 based on the total weight of the film. Depending on the loading of CaCO 3 , the permeance (defined as permeability per unit thickness) of the film ranged between 77,500 and 465,000,000 ml/m2 day atm. Good results were reported for strawberries, mushrooms, and broccoli florets with the proper selection of permeance.
3.7.2 Temperature compensating films Another challenge is to develop films that can tolerate temperature fluctuation during storage and distribution. The problem of developing such films is the mismatch of the activation energies for respiration and permeation: respiration rates of produce are strongly affected by temperature, but the permeabilities of existing packaging films are only slightly affected by temperature. In some cases, even a small temperature increase will cause rapid accumulation of CO2 and depletion of O2 in the package, a situation that may damage the product. Presently, research is being done on developing a new class of polymeric films with permeation activation energies more closely matching the respiration active energies of fresh produce. This class of polymeric films exhibits dramatic changes in permeability by transforming the polymer matrix reversibly from a crystalline state to an amorphous state as temperature is increased above a switch temperature. This switch temperature can be controlled within ± 2°C by changing the polymer side-chains. 3.7.3 Ceramic-filled films In recent years, commercial ceramic-filled polymer films have been introduced in Japan and Korea for packaging fruits. The films usually contain about 5% of very fine ceramic powder, and the manufacturers claim that these films emit far-infrared radiation or absorb C2H4 that can help to extend the shelf-life of the fruits. Although some workers (Isaka, 1988; Joyce, 1988) have reported that these films seem to improve the storage quality (especially color) of fresh produce, the benefit of using such films has not been reported in other laboratories. Lee et al (1992) have reported that the O2, CO2, and C2H4 permeabilities of ceramic-filled LDPE films are higher than those of plain LDPE film, and that the temperature dependence of the permeabilities follows the Arrhenius relationship. The higher permeabilities make these films more suitable for packaging high respiration rate products. Since ceramic is a filler, it is expected that higher loadings of ceramic filler should yield higher permeabilities. 3.8 Concluding remarks This chapter provides some practical suggestions for designing MAP of fresh produce. The model equations are a time-saving tool to reduce the number of experiments and to answer many 'what-if questions. As mentioned before, the model equations are oversimplified because they do not include many factors such as transpiration of the product, diffusivity of skin and flesh to O2 and CO2, effect of C2H4, etc. Thus the model predictions should be used with an understanding of their limitations, and must always be verified with experimental data.
Nomenclature
[CO 2 ] [CO2J1 [CO2J1 s [CO 2 ] in [CO 2 ] out [CO 2 ] O [O 2 ] [O2J1 [O 2 ] in [C^lout [Cy^s [O 2 ] o a (S a t , a2 bj, b 2 , C1, c 2 JE1CO2, E02 EpCO2, EpOi F K1n K1 L MCO2 M02 Patm ^co2> ^ 2 PCO2 P02 P R ^Co2' R o 2 RCo2 RQ2 S t T V Vm W
% CO 2 concentration % CO 2 concentration inside the package at any time % CO 2 concentration inside the package at steady state Inlet CO 2 concentration in flow-through system (%) Outlet % CO 2 concentration in flow-through system CO 2 concentration outside the package (%); 0% for air O 2 concentration (%) O 2 concentration inside the package at any time (%) Inlet O 2 concentration in flow-through system (%) Outlet O 2 concentration in flow-through system (%) Steady-state O 2 concentration inside the package (%) O 2 concentration outside the package (%); 2 1 % for air Conversion factor (1 hr"1) Permeability ratio, PCQJPO2 (dimensionless) Coefficients (Ir 1 ) Coefficients (dimensionless) Activation energies for respiration (J/mole) Activation energies for permeability (J/mole) G a s flow rate (ml/h) M i c h a e l i s - M e n t e n constant (% O 2 ) Inhibition constant (%• C O 2 ) Thickness of film ( m m ) Molecular weight of C O 2 (0.044 kg/mole) Molecular weight of O 2 (0.032 kg/mole) Pressure of 1 atmosphere (atm) Pre-exponential factors for permeability ( m g mil/m 2 h atm) Permeability to CO 2 (mg mil/m 2 h atm) Permeability to O 2 (mg mil/m 2 h atm) Pressure in the package or the jar (Pa) Gas constant (8.314 J/mol K) Pre-exponential factors for respiration (mg/kg h) Rate of CO 2 evolution (mg/kg h) Rate of O 2 consumption (mg/kg h) Package surface areas (m 2 ) Time (h) Absolute temperature ( 0 K) Free volume in package or in jar (ml) Maximum respiration rate (mg/kg h) Product weight (kg)
References Anderson, H.S. (1989) Controlled atmosphere package. US Patent 4842875. Cameron, A.C., Boylan-Pett, W. and Lee, J. (1989) Design of modified atmosphere packaging systems: modeling oxygen concentrations within sealed packages of tomato fruits. J. Food ScL, 54, 1413-16, 1421. Deily, K.R. and Rizvi, S.S.H. (1981) Optimization of parameters for packaging of fresh peaches in polymeric films. /. Food Processing, 5(1), 23-41. Emond, J.P., Castaigne, F., Toupin, CJ. and Desilets, D. (1991) Mathematical Modeling of Gas Exchange in Modified Atmosphere Packaging. Transactions of the ASAE, 34(1), 239-45. Exama, A., Ami, J., Lencki, R.W., Lee, L.Z. and Toupin, C. (1993) Suitability of plastic films for modified atmosphere packaging of fruits and vegetables. J. Food ScL, 58(6), 1365-70. Haggar, P.E., Lee, D.S. and Yam, K.L. (1992) Application of an enzyme kinetics based respiration model to closed system experiments for fresh produce. J. Food Process Engineering, 15, 143-57. Hayakawa, K., Henig, Y.S. and Gilbert, S.G. (1975) Formulae for predicting gas exchange of fresh produce in polymeric film package. J. Food ScL, 40, 186-91. Isaka, T. (1988) Recent trends in use of far IR radiations: use on packaging films. Food lnd. (Shokuhin Kogyo, Jpn.), 31(24), 27. Jurin, V. and Karel, M. (1963) Studies on control of respiration of Mclntosh apples by packaging method. Food Technol. Xl, 104-8. Joyce, D.C. (1988) Evaluation of a ceramic-impregnated Plastic Film as a Postharvest Wrap. HortScience, 23, 1088. Kader, A. A. (1987) Respiration of gas exchange in vegetables. In: Post Harvest Physiology of Vegetables, J. Weichmann (ed.), Marcel Dekker, New York, Chapter 3. Kader, A.A., Zagory, D. and Kerbel, E.L. (1989) Modified atmosphere packaging of fruits and vegetables. CRC CHt. Rev. Food ScL Nut., 28(1), 1. Katzyoshi, T. (1992) Freshness keeping packaging. In: Handbook of Food Preservation. K. Umeda, K. Yasmoto, K. Utagawa, T. Yokoyama and T. Yamaguchi (eds), Creative, Tokyo, 365-74. Labuza, T.P. and Breene, W.M. (1989) Application of 'active packaging' for improvement of shelf-life and nutritional quality of fresh and extended shelf-life foods. / Food Proc. and Pres., 13, 1-69. Lebermann, K.W., Nelson, A.I. and Steinberg, M.P. (1968) Post-harvest changes of broccoli stored in modified atmosphere: I. Respiration of shoots and color of flower head. Food Technol., 22(4), 143-6. Lee, D.S., Haggar, P.E. and Yam, K.L. (1992) Application of ceramic-filled polymeric films for packaging fresh produce. Packaging Technology and Science, 5, 27-30. Lee, D.S., Haggar, P.E., Lee, J. and Yam, K.L. (1991) Model for fresh produce respiration in modified atmosphere based on principles of enzyme kinetics. J. Food ScL, 56(6), 1580. Lee, J. (1987) The design of controlled or modified packaging systems for fresh produce. In: Food Product-Package Compatibility, Proceedings, J.I. Gray, B.R. Harte and J. Miltz (eds), Technomic Publishing, Lancaster, PA, USA. Mannapperuma, J.D. and Singh, R.P. (1990) Micromodel optimization of modified atmosphere vegetable/fruit packaging. In: Proceedings of the Fifth International Conference on Controlled/Modified Atmosphere/Vacuum Packaging-CAP90, San Jose, Calif., January 17-19. Mannapperuma, J.D. and Singh, R.P. (1994) Design of Perforated Polymeric Packages for the Modified Atmosphere Storage of Fresh Fruits and Vegetables. 1991 IFT Annual Meeting, Paper 21-8. Mizutani, Y. et al. (1993) Microporous polypropylene sheets. Ind. Eng. Chem. Res., 32, 221-7. Meyers, R.A. (1985) Modified Atmosphere Package and Process. US Patent 4515266. Ohta, H., Nakatani, A., Saio, T., Nagota, Y., Yoza, K. and Ishitani, T. (1991) Gas Permeability of Commercial Plastic Films. Report of Ginki Chogoku National Agricultural Experimentation Station, 82, 43-6.
Powrie, W.D. and Skura, BJ. (1991) Modified atmosphere packaging of fruits and vegetables. In: Modified Atmosphere Packaging of Food, B. Ooraikul and M.E. Stiles (eds), Ellis Horwood, New York. Prince, T.A. (1989) Modified atmosphere packaging of horticultural commodities. In: Controlled/Modified Atmosphere/Vacuum Packaging of Foods, A.L. Brody (ed.), Food & Nutrition Press, Trumbull, Connecticut, 67-100. Robertson, G.L. (1992) Packaging of horticultural products. In: Food Packaging: Principles and Practice, Marcel Dekker, New York, 470-506. Shelekhin, A.B., Dixon, A.G. and Ma, Y.H. (1992) Adsorption, permeation, and diffusion of gases in microporous membranes. II. Permeation of gases in microporous glass membranes. / Membrane ScL, 75, 233-44. Singh, R.P. and Oliveira, F. (1994) Minimal Processing of Foods and Process Optimization. CRC Press, Boca Raton, Florida, 438-9. Solomos, T. (1994) Some biological and physical principles underlying modified atmosphere packaging. In: Minimally Processed Refrigerated Fruits and Vegetables, R.C. Wiley (ed.), Chapman & Hall, New York, 183-225. Song, Y.S., Kim, H.K. and Yam, K.L. (1992) Respiration of blueberry in modified atmosphere at various temperatures. J. Amer. Soc. Hort. Sci., 117(6), 925-9. Veeraju, M. and Karel, M. (1966) Controlling atmosphere in fresh-fruit package. Modern Packaging, 40, 168, 170, 172, 174, 254. Weichmann, J. (1986) The effect of controlled-atmosphere storage on the sensory and nutritional quality of fruits and vegetables. Hort. Rev., 8, 101-27. Yam, K.L., Haggar, P.E. and Lee, D.S. (1993) Modeling respiration of low CO2 tolerance produce using a closed system experiment. Foods Biotechnol., 2(1), 22-5. Yang, CC. and Chinnan, M.S. (1988) Modeling the effect of O2 and CO2 on respiration and quality of stored tomatoes. Trans. ASAE, 31, 920-5. Zagory, D. and Kader, A.A. (1988) Modified atmosphere packaging of fresh produce. Food Technology, 42(9), 70-7.
4
Active packaging in polymer films M.L. ROONEY
4.1
Introduction
Polymers constitute either all or part of most primary packages for foods and beverages and a great deal of research has been devoted to the introduction of active packaging processes into plastics. Plastics are thermoplastic polymers containing additional components such as antioxidants and processing aids. Most forms of active packaging involve an intimate interaction between the food and its package so it is the layer closest to the food that is often chosen to be active. Thus polymer films potentially constitute the position of choice for incorporation of ingredients that are active chemically or physically. These polymer films might be used as closure wads, lacquers or enamels in cans and as the waterproof layer in liquid cartonboard, or as packages in their own right. The commercial development of active packaging plastics has not occurred evenly across the range of possible applications. Physical processes such as microwave heating by use of susceptor films and the generation of an equilibrium modified atmosphere (EMA) by modification of plastics films have been available for several years. Research continues to be popular in both these areas. Chemical processes such as oxygen scavenging have been adopted more rapidly in sachet form rather than in plastics. Oxygen scavenging sachets were introduced to the Japanese market in 1978 (Abe and Kondoh, 1989) whereas the first oxygen-scavenging beer bottle closures were used in 1989 (see Chapter 8). The development of plastics active packaging systems has been more closely tied to the requirements of particular food types or food processes than has sachet development. This chapter surveys the range of polymer-based active packaging processes that have been reported, their chemical or physical basis and their application to foods and beverages. Attention is given to opportunities for, and obstacles to, either commercialisation or extension of the current range of application. Some processes which have become economically important are treated individually in other chapters. 4.2 Oxygen scavenging The removal of oxygen from package headspaces and from solution in foods and beverages has long been a target of the food technologist. Introduction of vacuum packaging and inert-gas flushing has provided solutions for some
Table 4.1 Food characteristics influenced by oxygen scavengers Characteristic
Targets
Microbiological status Infestation Chemical degradation Physiological changes
Moulds, aerobic bacteria Insects, larvae, eggs Rancidity, pigment/nutrient loss, browning Respiration
of the problems of distribution of oxygen-sensitive foods as described by Brody (1989). However, the opportunity to improve on the benefits gained by application of those technologies, as well as the chance to treat the problems of distribution of foods individually, has led to the current interest in oxygen-scavenging plastics. The properties of foods that can be influenced by the presence of oxygen scavengers are shown in Table 4.1. The growth of moulds is particularly important in dairy products such as cheese and in bakery products. Oxygen levels of 0.1% or lower are required to prevent the growth of many moulds. Bacterial growth and the growth of yeasts can be a problem in high wateractivity foods including meats and prepared dishes, as well as in juices. Oxygen scavengers can prevent oxidative damage to flavour and colour in a wide range of foods. Likewise, they can maintain atmospheres with oxygen concentrations too low for insect survival in agricultural and horticultural products. The list in Table 4.2 is indicative of the range of foods which could benefit from oxygen scavenging, their type of packaging and the forms in which the scavenger might be applied. The package converter can decide the nature and quantity of active components used in plastics packaging. This allows the opportunity for tailoring the packaging to the requirements of the product. Table 4.2 Potential applications of oxygen scavenging plastics Product
Packaging
Aseptic liquids Bakery products Beverages Beer Cheese Coffee, tea Cereals Dried Fruit Dried Foods/Nuts Fried snacks Fruit/Vegetables Milk powder Meat - fresh - processed
Cartonboard, bag-in-box film, coating, adhesive, ink Flexibles film, etc. Flexibles, bag-in-box film, etc. Crown seal liners Resin, organosol Flexibles film, etc. Closures, flexibles film, etc., resin Flexibles film, etc. Flexibles film, etc. Flexibles, closures film, etc., resin Flexibles film, etc. Flexibles film, etc. Flexibles film, etc. Flexibles film, etc. Flexibles film, etc.
Component
Pasteurised liquids
Closures, bottles
resin
Retorted foods
Can lacquers, trays, lidding
resin, film, etc.
Wagner (1990) lists a wide range of oxygen-sensitive prepared foods which are of increasing importance in consumer societies. Some are suitably packaged using existing processes. However, quality can often be retained longer if residual oxygen is removed. This would allow use of different packaging materials and distribution systems. Some foods, and particularly beverages, cannot be stabilised adequately with existing packaging technologies in order to allow use of the full range of desired distribution systems. This is particularly important when reduced levels of additives have been chosen for regulatory or marketing reasons. Koros (1990) has set out the maximum quantity of oxygen which a generalised range of foods can take up and still have a shelf-life of one year. These quantities are generally in the 1-200 mg/kg range. Abe and Kondoh (1989) have shown the need for oxygen removal by in-pack systems when the economic limit of around 0.5% is reached in the general case. This figure can vary in practice as residue levels of around 2% are often encountered when form-fill-seal (ffs) gas flushing is used commercially. Alternatively, less than 0.1% oxygen can be found in vacuum packs of beef primals where muscule respiration and bacterial action scavenge oxygen. The most appropriate method of removal of oxygen from a food package depends on the nature of the food, its prior processing and the packaging machinery and the way it is distributed. The factors which may need to be considered, and estimates of efficiency when sachets are used, are summarised in Table 4.3 which is based on a similar table devised by Hirata (1992), who compared sachet technologies with vacuum and gas-flush packaging systems. The expected benefits of use of oxygen-scavenging plastics are to minimise the materials cost by matching the quantity of scavenger to the need, and to keep the filling speed high. 4.2,1 Forms of oxygen-scavenging packaging In-pack oxygen scavenging involves use of a variety of forms of scavenger. Sachets merely inserted into the food package constitute most of the present systems in commerce. Alternatively, the scavenger can be hot-melt bonded to the inner wall of the package. This is done using the Mitsubishi Ageless scavenger sachet attached to the lid of the steamed rice packages Table 4.3 Comparison of headspace oxygen removal systems
a b c d e
System
Residual O2 kPa
Capital investment
Film cost
Filling speed
Vacuum N2 Flush a+b Scavenger b+d
< 1.5 1-2 < 1.0 <0.1 <0.1
High Medium High Low Medium
High High High Medium Medium
Low Medium Low High Medium
manufactured in Japan by Ajinimoto under the Take Out brand name. The self-adhesive label concept of US company Multiform Desiccants is used in the Marks & Spencer retail chain's preserved meat packs in the UK. Beyond the concept of adhesion of sachets to the package lies a wide range of possibilities and it is to this area that much of the recent research and development has been directed. Alternatives include package inserts in the form of cards or sheets, or layers coated onto the inner wall of the package. An example is the beverage crown seal liner currently used in the beer industry. This liner was developed by Advanced Oxygen Technologies Inc. and Zapata Industries Inc. under the name SmartCap and independently by W.R. Grace and Company and under the name Daraform. Both companies manufacture in the USA. Alternatively an iron-based system has been developed under the name Oxyguard by Toyo Seikan Kaisha Ltd in Japan. The industrial development and history of oxygen-scavenging closures is discussed in detail in Chapter 8. Oxygen-scavenging films or other plastics packaging materials are alternatives to sachets. Low molecular weight ingredients may be dissolved or dispersed in a packaging plastic or the plastic may be made from a polymeric scavenger. The scavenger may be incorporated into a solid which is dispersed in the plastic or may be introduced into various layers of the package such as in the form of adhesive, printing ink, lacquer or enamel, such as found in cans. Sachets can be a highly efficient form of oxygen scavenger but their nature does not favour contact with liquid foods or where cling of the package to the film may isolate the sachet from areas of oxygen entrapment or ingress. In such situations it would be preferable to have the scavenger in the packaging material. This allows all exposed surfaces of the food to be deoxygenated or protected from oxygen ingress by permeation. 4.2.2 Plastics packaging as media for oxygen scavenging Oxygen-scavenging packaging has to date been applied only in packs which have inherently a substantial barrier to oxygen ingress by permeation or leakage. Abe and Kondoh (1989) recommend that Ageless sachets should be used in packages with an oxygen transmission rate less than 20 ml/m2/atm/ day. Such requirements therefore rule out the common heat-seal polymers and thin layers of the more mediocre barriers like polyester (PET) and nylon 6. However, there can be situations such as in transportation of beef primal cuts in which a shorter period of absolute barrier to oxygen is desirable and these plastics might be used as the barrier. The patent of Speer and Roberts (1993) describes a system involving oxidation of poly(l,2-butadiene) and appears to be directed at that market. The commonly found oxygen barriers may be in the form of a single-layer package, as in the case of the PET or PVC bottle, or jar or cup, but are more
Table 4.4 Permeability coefficients P X 1010 (cm3 mm cm"2 sec"1 c m 1 Hg) @ 30 0 C Film material Polyvinylidene chloride Polyester (Mylar A) Polyamide (Nylon 6) Polyethylene (d = 0.960) Polyethylene (d = 0.922) Polystyrene Polyvinylchloride
N2
O2
CO 2
0.0094 0.05 0.10 2.7 19 2.9 0.40
O053 0.22 0.38 10.6 55 11.0 1.2
029 1.53 1.6 35 352 88 10
(Reproduced from Paine, 1962, with permission.)
likely be part of a multilayer package. Table 4.4 shows the oxygen permeability of a range of polymers. A more extensive tabulation of film permeability values can be found in the review by Bixler (1971). Oxygen-scavenging compounds can be dispersed or blended in highpermeability films such as plasticised PVC and polyethylene. Where the oxygen scavenger is molecularly dispersed in the polymer it is available to oxygen in its entirety, unlike there case where solid particles are used. The particular advantage of such polymers is that they allow rapid diffusion of oxygen and water (at elevated temperatures) from the headspace or food in to the reactive ingredients. Such a characteristic of the polymer helps offset the disadvantage of plastics-film-based compositions in comparison with sachets where the scavenging powder has a large reactive surface area exposed. Polyethylene and polyvinyl chloride (plasticised) are nominated as the reaction medium in crown closure liners for beverage bottles such as for beer, as described in detail in Chapter 8. The use of plastics as media in which to disperse or dissolve oxygen scavengers places a severe limitation on the number of reactions which can be involved in the scavenging process. Whereas molecules of the size of oxygen or water can diffuse at an adequate rate, larger molecules behave as if they were immobilised. The fact that a molecule can migrate fast enough to fail food-contact regulatory tests does not indicate that it can be used in an oxygen-scavenging system. Thus the use of iron oxidation in polymers has been a challenge for some years requiring all reagents to be intimately mixed. A further challenge is to establish whether the breakdown of iron particles on oxidation occurs as freely in a polymer matrix as it does in powder form in sachets. Labuza and Breene (1989) note that virtually all of the iron in commercial sachets is available for oxygen removal. The high gas and vapour permeability of the common heat-seal polymers allows them to be used as reaction media for oxygen scavenging in laminates. Indeed this has been proposed in a large number of patents such as those of Hofeldt and White (1989) and Farrell and Tsai (1985). The presence of the oxygen scavenger in the heat-seal layer allows maximum advantage to be taken of the full thickness of the physical barrier to oxygen permeation of the barrier layer. However, if the scavenger is incorporated
into the barrier layer as in the Ox-bar bottle of CMB Technologies pic (Folland, 1990) that portion closer to the outside becomes exposed to relatively large amounts of oxygen. The permeability of the polymer medium in which the scavenging reaction occurs need not be the limiting variable determining scavenging rate. Depending upon the chemistry involved, the oxidation reaction can be inherently slow, as with crystalline sulfites at room temperature or the reaction can require the presence of an additional species as with the rusting of iron. The permeability of the heat-seal layer to both oxygen and water vapour can be limiting as in the mixed sulfite/acetate layer in retort pouches patented by Farrell and Tsai (1985). In this case the water is needed in substantially larger quantity than the oxygen in order to dissolve the deliquescent potassium acetate in which solution the potassium sulfite dissolves and reacts with oxygen. An approach to overcoming any limitation on scavenging rate by the heatseal layer's permeability has been to use microporous polymers such as nonwoven polyolefins. Several recent patent applications describe claims of enhanced availability to the package atmosphere of granular reagents, as in that of Mitsubishi Gas Chemical Industry Co. Various sulfites can be held in a fibrous layer sandwiched between, for example, one layer of foil, and a second of a plastic or paper with an oxygen permeability greater than 7000 ml/m2/day atm. A second patent describes a package consisting of three layers. The outer layer is a plastics film, the middle is a perforated or nonwoven layer containing an oxygen scavenger and the inner layer of the package is a microporous film which resists the flow of liquids through its pores. This is claimed to be useful for packaging liquid foods (Ohtsuka et al, 1984). Such films cannot be used where transparency is required but may have application in many forms of packaging. Traditionally, poly(vinylidene chloride) copolymers have provided a water-insensitive oxygen barrier when used as a layer in laminates, coated films or coextrusions. The introduction of ethylene-vinyl alcohol copolymers (EVOH) and poly(vinyl alcohol) means that, together with traditional nylons, most oxygen barriers are now water sensitive. The permeability to oxygen of EVOH copolymers increases approximately 10-fold when exposed to a relative humidity change from about 40-100%. Such a change may render a material which is suitable for packaging an oxygen-sensitive food at low relative humidity into one which is most unsuitable at high relative humidity. The plasticising effect of water on the barrier properties of EVOH (or other hydrophilic barriers) is time dependent, especially if the hydrophilic layer is protected by a water-barrier layer such as polypropylene as in the case of retortable lunch-cups. When such retortable packs containing a wet food such as an entree are subjected to steam retorting, water is absorbed by the EVOH in such large quantities that the barrier layer becomes quite
permeable to oxygen. The rate of water release through the outer polypropylene layer becomes very slow on cooling, so the oxygen permeability can remain elevated for many weeks (Tsai and Wachtel, 1990). Although addition of desiccants to the polypropylene (Tsai and Wachtel, 1990) and mica platelets to the EVOH (Bissot, 1990) is used to reduce this impact there is an opportunity here to include an oxygen-scavenger layer in the coextrusion to absorb the oxygen, particularly during the period of enhanced permeability. In fact there has been a recent news report of the introduction of an oxygen scavenger into such packages by Toyo Seikan Kaisha Ltd. Thus active packaging has the potential to contribute to solving the permeability problem in two ways, providing: desiccants which absorb water in polypropylene; and oxygen scavengers which remove oxygen when it does pass through the hydrated barrier layer. Whereas elevated temperatures and high humidity have been used to advantage in the research of Farrell and Tsai (1985), the effect of temperature on the performance of oxygen scavengers in polymer-based films has been reported in only rare cases. W.R. Grace (1994) have investigated the effect of low temperatures on their (optionally photosensitised) metal-catalysed oxidation of syndiotactic poly(l, 2-butadiene). They have foumd that this polymer, and certain other low crystallinity polymers with glass transition temperatures below -15°C, scavenge at least 10 ml/m2/day O2 at 100C or lower. The photoinitiated system described by Rooney (1994) has been shown to function at 00C. 4.2.3 Brief history of oxygen-scavenging films The initial oxygen-scavenging packaging film was a multilayer described by Kuhn et al (1970) and by Warmbier and Wolf (1976). These systems were based on the earlier work of King (1955) and Abbott et al (1961) who applied palladium metal to the inside surface of can lids. The cans were flushed with mixtures of hydrogen (8%) in nitrogen to give a mixture in which the residual oxygen could react with hydrogen to form water on the palladium surface. The reaction of hydrogen with residual oxygen on palladium has been taken further by Hayashi et al (1986) who vacuum-metallised polyester film at 2.5 x 10"^ moles of Pd/m2 and laminated this to high density polyethylene. When a bag made from this laminate was filled with 500 ml of a mixture of hydrogen-nitrogen 8:92% by volume, this layer was found to be effective in catalysing the conversion of oxygen to water, reducing the oxygen content from 3-4 to 0.4% in 1 day. Due to the expense of this process it would be suitable for packaging high-valued items such as probes for an oxygen analyser, although production of a thinner metal layer may change future economics. The earliest investigation involving reaction of oxygen in homogeneous
polymer films was described by Rooney and Holland (1979), Rooney, Holland and Shorter (1981) and by Rooney (1981, 1982). This technique takes advantage of the ease of excitation of oxygen from its ground state to its first singlet excited state and has shown that use of polymers as scavengers or reaction media need not inhibit rapid oxygen removal. Very little investigation of the chemistry of oxygen-scavenging films has been published in peer-reviewed journals. However, numerous patents and some conference proceedings give sufficient detail to allow a comparison of the systems reported. The most evident trend in oxygen-scavenging system development during the past 20 years has been the increasing importance of patent applications for compositions and designs based on plastics. Very few of these have involved actual polymer oxidation but rather have required the reactive ingredients to be dispersed within the polymer matrix or to be sandwiched between film layers. An examination of patent applications worldwide gives the results shown in Figure 4.1 which shows the numbers of initial applications for new compositions or designs without consideration of whether additional applications for the same idea have been lodged in other countries. The histogram shows that whereas initially only sachet technologies were considered, there was a slow growth in the number of plastics-based systems devised until the numbers were equal for both types of system in 1993-94. The increasing number of plastics-based systems results from a substantial overall increase in the interest in oxygen-scavenging systems, not from a decrease in the numbers of sachet technology applications. This has resulted from a more lateral approach to potential reactions coupled with approaches
Plastics
Applications
Sachet
Years Figure 4.1 Patent applications for oxygen scavengers involving sachets and plastics.
to overcoming previous deficiencies. This is particularly evident in patent applications for systems involving oxidation of carbon-carbon double bonds in small molecules like squalene and fatty acids or in polymers like rubbers. In each case the transition-metal catalysed oxidation results in development of odorous compounds such as low molecular weight aldehydes which are adsorbed by zeolites, carbon or other adsorbents. In one example linseed oil, iron oleate, calcium carbonate and active clay are mixed with activated carbon to give a solid oxygen scavenger (Toppan, 1992). The advantage of this system is that water is not needed as a reagent. Some compositions, such as that of W.R. Grace and Co. for oxidation of squalene with a transition metal catalyst (Ebner et al9 1992), appear not to include an adsorbent for odorous products. Some patents involve claims of conventional antioxidants as oxygen scavengers. The use of bifunctional antioxidants at up to 2% in rigid poly(vinyl chloride) was claimed to reduce the permeability of that polymer 20-fold although the period of effectiveness was not reported (Wijbrief, 1971). These antioxidants are normally associated with reactions of primary products of reaction of molecular oxygen with polymers, so the mode of action of this process is still uncertain. W. R. Grace and Co. has applied for patents for the use of ascorbic acid dispersed into plastics such as the common heat-seal plastics or closure liners (Hofeldt and White, 1989). This process relies on the presence of water from the food or beverage as well as the presence of isoascorbic acid or a metallic sulfite such as sodium sulfite. The process is now commercial and follows the patent of Farrell and Tsai (1985) who patented the sandwiching of either a sulfite alone, or one mixed with potassium acetate, between the layers of a retortable pouch structure. Several other applications for oxygen-scavenging plastics containing ascorbic acid have subsequently been lodged. One combines oxygen scavenging with the antimicrobial action of a silver zeolite (Shimagawa Nenryo KK, 1992). CMB Foodcan pic has developed a novel system for use in blow-moulded bottles made from PET into which up to 7% MXD-6 nylon had been blended (Cochran et ai, 1991). An additional catalyst in the form of a polymer-soluble cobalt salt such as cobalt stearate was necessary to cause the nylon to react with oxygen. An example of the performance of a bottle made from the Ox-Bar (Trademark of CMB Technologies pic) has been given by Folland (1990). The results suggest that beer held in such a bottle would be saved from oxygen ingress via the bottle wall for at least 12 months thus providing oxygen-barrier performance functionally equivalent to that of glass. As with any form of active packaging, oxygen-scavenging plastics are designed to achieve a specific effect - in this case protection of a packaged food from oxygen. As it turns out, the shelf-life of carbonated beverages like beer is limited by the loss of carbon dioxide by permeation. Thus the effect of the oxygen-scavenging bottle would be to change the nature of the limiting
variable if used in those countries where beer has a high level of carbonation. This process has not reached the market place since the reaction products need to be defined in greater detail. What is particularly interesting in this case is that the use of a cobalt salt catalyst in PET appears to have satisfied some national regulatory authorities. 42.4
Chemistry of oxygen scavenging
Unlike most other forms of active packaging, oxygen-scavenging films must be stable in the oxygen-rich environment of air prior to use. This has presented a problem to chemists formulating such systems and surprisingly few methods of activation appear either in the patent literature or in commercial practice. In some instances activation might not be a necessary consideration if the package is prepared from all constituents immediately before filling. Such packaging systems would include blow-moulding of beverage bottles which often occurs on the premises of the beverage filler. Thus the catalyst and the oxidisable substrate can be kept apart until the bottle is blown, as in the OxBar process developed by CMB Technologies pic. A further factor is the nature of the packaging material carrying the oxygen scavenger. In the case of the PET bottle, containing MXD-6 nylon blended into the PET for instance, the oxygen permeability of the bottle is so low that a delay between blowing and closure is quite reasonable. A general guide to such circumstances is that if the plastic material actually carrying the oxygen scavenger is exposed to the outside air during its life as a package the system can probably be chosen not to have an additional activation step. The systems which have been developed for use (or prospective use) in coatings, laminations or other plastics layers with high oxygen permeability are normally activated by one of the following mechanisms. • • • •
Supplying a reagent on package filling, viz. hydrogen or water Supplying water as a solvent or swelling agent on filling. Continuous exposure to light as energy source. Brief exposure to light for activation of o chain reactions, e.g. autoxidation o rapid photoreduction of scavenger precursor
In each of the above processes several variants exist. For instance, the water can come from the food itself as described in the Oxyguard process of Toyo Seikan Kaisha Ltd, or in the variety of patents referred to in Chapter 1. This requirement is elaborated in the discussion on sachet technologies in Chapter 6.
Water needed as a reaction solvent or to burst micro-capsules can be supplied from the retorting steam in the case of retortable plastics packages which are processed at around 1200C. In this case water vapour permeation can be sufficient from the outside of the pack as well as from the food itself and here heat is also a trigger for commencement of the process. Tsai and Wachtel (1990) have shown that EVOH trays can take up as much as 2.8% extra water on retorting, and this diffuses out slowly over a period of some weeks. 4.2.4.1 Supplying a reagent. The oxidation of hydrogen by molecular oxygen requires metal catalysis at room temperature. Indeed such a process has long been used by microbiologists to generate low-oxygen-content environments for cultivation of anaerobic microorganisms. Conventionally a mixture of hydrogen (8%) in nitrogen is either flushed into the cultivation chamber or hydrogen is generated in a sealed anaerobic jar in the presence of an air headspace. The surface of palladium metal either as mesh or deposited on porous alumina catalyses the reaction to form water. The application of this process to packaging by King (1955) and Abbott et al. (1961) was particularly successful for removing oxygen which desorbs from canned spray-dried milk powder. The hydrogen content of the flushing gas is limited to 7% or less to avoid the risk of explosion. It should be noted that an explosion limit of 6.5% hydrogen is specified in plants for production of chlorine by electrolysis. The laminate bag described by Warmbier and Wolf (1976) consists of a polyester outer layer bonded to aluminium foil then Surlyn. Between this Surlyn and the heat-seal layer, also of Surlyn, is sandwiched a layer of powdered alumina upon which palladium metal has been deposited. The bag was flushed with the above-mentioned H2ZN2 mixture before heat sealing. On storage the milk desorbed oxygen which diffused, together with the hydrogen, through the Surlyn to react on the catalyst surface. The quantity of water which was formed was calculated by the authors to be insufficient to affect the food. There appears to have been very little use of this system, marketed as Maraflex 7F by American Can Company, with foods although it was used in the US space programme. The oxidation of iron in the presence of electrolytes has been well established in the sachet technologies reviewed in Chapter 6. Where patents describe incorporation of treated iron powder into plastics, water ingress is generally needed. Some specify the need for such materials to be used under retorting conditions. This is probably for the purpose of introducing sufficient water on to the iron surface to carry out the reactions described in Chapter 6. It is doubtful whether an adequate rate of oxygen scavenging can be achieved when the polyolefin heat-seal plastics are used as the reaction medium at room temperature.
4.2.4.2 Supplying water as a solvent or swelling agent. Some packaging applications potentially permit the use of scavenging components as heterogeneous additives to the packaging material. Particularly where reagents are in the crystalline form or coated with a low-permeability substance, the rate of reaction with atmospheric oxygen can be acceptably low for incorporation into packaging. When the package is exposed to high humidity and elevated temperatures the rate of transmission of water from either the food or the outside (e.g. as steam) can be sufficient to dissolve the reagent thus enabling oxidation to proceed rapidly. An early example of the dissolution of a salt was patented by Farrell and Tsai (1985). They incorporated a mixture of potassium acetate and sodium sulfite crystals (or potassium sulfite alone) between the barrier and heat-seal layers of a retort pouch laminate which had five layers overall. The potassium acetate is deliquescent and absorbs sufficient water to dissolve when the food-filled pouch is retorted. The sodium sulfite dissolves in the potassium acetate solution and reacts with oxygen diffusing into this solution from the food and especially from the retort atmosphere. The water vapour permeability of the laminate can increase 1000-fold from 210C to 121°C. The sulphite reacts with oxygen as in Equation 4.1. 2Na2SO3 + O2 -> 2Na2SO4
(4.1)
Although this process can result in the presence of an aqueous solution in the laminate, it should be possible to consider use of some form of binder for the solution after retorting. A process such as this which requires both heat and high humidity is really limited to applications which involve retorting or substantial heat-treatment of the packaged food. A recent patent describes the incorporation of a metallic salt and a complexation agent in separate microcapsules within a polymer film (Zenner et al.9 1992). On exposure of the film to an atmosphere of high relative humidity, as in the headspace of some food packs, the microcapsules absorb water and swell. This results in bursting of the microcapsules and mixing of the complexing agent and the salt under moist conditions. The resultant metal chelate forms an oxygen adduct with a binding constant greater than 105M'1. The presence of such a strong adduct is effectively equivalent to oxygen removal by means of an irreversible reaction. The reaction of oxygen with a metal complex can also involve a weak coordinate bond without converting the metal ion to a higher oxidation state. This type of reaction is the basis of blood oxygenation and can frequently be reversed, at least to some extent (see Chapter 1). This type of oxygen absorber was the subject of extensive research by Aquanautics Inc in the USA, the predecessor of Advanced Oxygen Technologies Inc. Ascorbic acid and isoascorbic acid react readily with oxygen, more rapidly at high pH. The use of metallic salts of these acids is described in
patents from many sources. The chemistry of oxidation of ascorbic acid is complex, consisting of a series of consecutive and concurrent reactions, depending upon the conditions. The steps involved in the commercially accepted scavenging compositions have not been described in the scientific literature. It has been observed in the author's laboratory that cellulose acetate films containing dissolved ascorbic acid turn light-brown in colour after standing in darkness under ambient temperature and humidity conditions for one to two years. The steps involved in ascorbic acid oxidation in solution have been reviewed by Tannenbaum (1976) who summarised the reactions involved in the oxidation of ascorbic acid. A simpler summary is shown in Figure 4.2. The initial step is the formation of dehydroascorbic acid and this step is strongly dependent upon pH. Ascorbic acid can be regenerated by reaction with mild reducing agents such as metallic sulfites. It is particularly interesting that the patent of Hofeldt and White (1989) describes the optional use of sodium sulfite in combination with ascorbic acid (and its isomers) in their oxygen-scavenging closure liner for use with beverages such as beer. There is thus the potential for use of ascorbic acid as an intermediate in the
2NaOH HOHC
HOHC
CH2OH
BROWNPRODUCTS
Figure 4.2 Oxidation of ascorbic acid.
oxidation of sodium sulfite to the sulfate. Whether this occurs in the ambient or low-temperature storage of beer has not been described in the literature. Given the heterogeneous nature of the reaction mixture it is doubtful whether this desirable regeneration occurs to a very great extent in commercial practice without the formation of an aqueous solution as described by Farrell and Tsai (1985). As can be seen from the summary in Figure 4.2 one of the reaction products of oxidation of ascorbic acid to dehydroascorbic acid is hydrogen peroxide or initially the hydroperoxy free radical. Further hydrogen abstraction by this radical forms hydrogen peroxide. The use of sodium sulfite, or another reducing agent, would therefore seem to be desirable if the ultimate re-introduction of an oxidising agent is to be avoided. The role of ascorbic acid as a promoter of browning when oxidised in foods is already known (Tannenbaum, 1976). The use of ascorbic acid or its isomers or salts is the basis of the W.R. Grace and Co. and Zapata Inc oxygen-scavenging closures in common use in some beer bottles. 4.2.4.3 Continuous exposure to light as an energy source. Some oxidation reactions which do not occur when oxygen is in its unexcited (ground) state can be brought about by the process of photosensitisation, which involves transfer of visible light energy to oxygen via the intermediacy of a dye. A polymer film can be the medium for these photosensitised oxygenscavenging reactions. Such a film must contain a photosensitising dye and an electron-rich oxidisable compound termed a singlet oxygen acceptor. The oxygen-scavenging process occurs when the film is illuminated with UV, visible or near infra-red irradiation of appropriate wavelengths. The steps in the process are shown below, and these occur within the matrix of the polymer film. (4.2) (4.3) (4.4) (4.5) (4.6) (4.7) When a suitable photosensitiser, D, absorbs light (hx>) it is excited to a short-lived higher energy singlet state, *D, (Equation 4.2) which largely converts to the longer-lived triplet excited state, 3D, (Equation 4.3). In this form the dye can pass the excitation to oxygen by the process of triplet-
triplet energy transfer (Equation 4.4). The oxygen diffusing into the polymer from the package headspace (for instance) needs to come very close to the excited, immobile dye molecules during the triplet lifetime of the latter. This is of the order of 10-1000 microseconds. Thus, for such a process to be useful the polymer matrix (such as an inner layer of a laminate) would need to be very permeable to oxygen. Once excited to its singlet state the oxygen can react with any electronrich acceptors, A, present in the polymer matrix (Equation 4.5) provided they are within the distance the singlet oxygen can diffuse before it is quenched back to the ground state (Equation 4.6). The process occurs only within the lifetimes of the excited sensitiser (Equation 4.7) and singlet oxygen and so requires continuous illumination. The distance singlet oxygen can diffuse before quenching in a polymer matrix is of the order of IOOA depending on the polymer permeability and other factors (Turro et a/., 1981). This chemistry has been used in the laboratory as an oxygen-scavenging process where the polymer matrix is, for instance, cellulose acetate or ethyl cellulose (Rooney et al., 1981; Rooney, 1982). The sensitisers include erythrosine or me^o-tetraphenylporphine and the acceptors are bis(furfurylidene)pentaerythritol and ascorbic acid. It was found that the permeability of the polymer film is an important determinant of scavenging rate, with the ethyl cellulose being a better matrix for rapid scavenging than cellulose acetate. Ethyl cellulose has an oxygen permeability coefficient at 25°C of 7 Barrers (cm3 (STP) cm/cm-2 s"1 cm"1 Hg x 10~10) compared with that of 0.7 Barters for cellulose acetate (Bixler, 1971). It was also found that the rate of scavenging of oxygen from a pouch by the film is limited by light intensity initially but becomes diffusion limited when the oxygen partial pressure reaches low values (Rooney et al.9 1981). The light intensity used was 2 x 10 5 -7 x 105 Lux. It was subsequently shown that the rate of oxygen scavenging reaches a maximum at a concentration of tetraphenylporphine above 10~3 M but less than 10~2 M. When the scavenger film was used as a roll it was found that the rate was dependent upon the length of film in the roll consistent with increased access of oxygen to the film. The singlet oxygen acceptor does not need to be a small molecule dissolved in the polymer. It has been shown the double bond of natural rubber can be photo-oxidised at a rate sufficient to bring about rapid oxygen scavenging from the headspace of a package. Figure 4.3 shows the rate of scavenging of oxygen from air, 20 ml, in a 10 cm x 10 cm pouch coated on the inside with either natural rubber dyed with tetraphenylporphine, 10~3 M, or ethyl cellulose 0.6 M with respect to PEF and 10~3 M with respect to tetraphenylporphine. The rubber scavenged oxygen substantially faster than the PEF in the ethyl cellulose. This was interpreted as being due to the higher concentration of double bond (13.5 equivalentsflitre) in the rubber than the concentration of furan rings (1.2 equivalents/litre of film) in the
% Oxygen
(bifunctional) PEF, although the higher permeability of rubber towards oxygen (24 Barrers) than that of ethyl cellulose (7 Barrers) is also likely to contribute. It is significant that the reaction occurring in natural rubber occurs on the polymer chain and therefore is not dependent on the free rotation of the acceptor. The photo-oxidation of the tacky natural rubber layer resulted in rapid crosslinking as indicated by the loss of tack within minutes. An unpleasant odour was also generated. The oxidation of the rubber continued on dark storage resulting in formation of a brittle powdery film. The potential for use of synthetic rubbers in place of natural rubber was investigated in order to determine whether the nature of the rubber monomers affects the rate of scavenging. The results in Figure 4.4 show the rate of oxygen scavenging by poly(dimethylbutadiene) (PDMB), cispolybutadiene (PB) and ra-polyisoprene (PI). Based on the reactivity of simple low-molecular weight analogues of these polymers, it would be expected that the more highly methylated rubber poly(dimethylbutadiene) would react more rapidly with singlet oxygen. However, the inverse relationship is observed, presumably due to the low oxygen permeability of PDMB of 2.1 Barrers compared with values of 20 Barrers and 24 Barrers for PB and PI. Thus permeability rather than reactivity can be the important
Time (min)
Figure 4.3 Oxygen scavenging by natural rubber •, and PEF 0.5M in ethyl cellulose a, both dyed with tetraphenylporphine, 103M. Volume of air, 20ml, pouch area 100cm2. (Reprinted from Rooney, 1982b, with permission.)
oxygen (%)
time (minutes) Figure 4.4 Oxygen scavenging by polydienes, poly(dimethylbutadiene) +, cis-polyisoprene O, cispolybutadiene A. Coatings on inner surface of pouches of area 100 cm2, volume of air 20 ml.
variable in an oxygen-scavenging polymer system as was found to be the case with PEF dissolved in cellulose acetate and ethyl cellulose. The use of multiple dyes in oxygen-scavenging films has been investigated, since illumination with white light of polymer films containing only one dye is wasteful of potentially usable energy (Rooney et ai, 1994). It has been found that even dyes which are not photosensitisers can harvest energy and transfer this energy to a photosensitiser also in the film. Thus curcumine, a natural food colour and a poor photosensitiser (Chignell et ai, 1994), was shown to enhance the rate of oxygen scavenging photosensitised by eosine in ethyl cellulose. The effectiveness of such a film in suppressing rancidity in sunflower oil has been investigated and found to be substantial at 230C and 37°C (Maloba et al., 1994).
4.2.4.4 Brief exposure to light for activation. 4.2.4.4.1 Chain reactions. Continuous exposure of a package to light places an intolerable restriction on the way in which most foods would need to be stored and distributed. It is not surprising therefore that the use of light to activate reactive polymer-based systems has been investigated. The problem, inherent in all oxygen-scavenging systems, of using compounds reactive towards oxygen but stable towards thermal processing of the packaging material needs to be addressed. The use of autoxidation of fatty acids for oxygen scavenging has already been claimed but this process seems likely to result in formation of the same volatile oxidation products which are undesirable in foods (Frankel, 1982). Some sachet patent applications describe the use of adsorbents in the sachets to bind volatile off-flavours generated in this way. Mitsubishi Gas and Chemical Company has claimed that activated alumina, silica gel or charcoal absorbs any odour formed on the oxidation of fatty acids or oils catalysed by transition metal compounds in the presence of alkaline earth bases (Inoue and Komatsu, 1988). Toppan Printing Company (1992) claimed a similar composition involving addition of activated clay. The benefit of such systems is independence from water for the reaction. The research effort put into development of photodegradable plastics packaging over the past two decades has provided a background to one approach to this problem Rabek and Ranby (1975) show how a transition-metal metal salt and a photosensitiser dispersed in a plastic can cause it to degrade in darkness once exposed to sunlight for some days. This form of photodegradation involves a substantial extent of hydrogen abstraction from the polymer backbone coupled with hydroperoxide formation, particularly on tertiary carbon atoms. Subsequent breakage of the hydroperoxy bond leads to formation of either keto or aldehyde groups which become additional photosensitisers or result in polymer chain scission. It is in the prevention of the latter reaction that the opportunity for development of oxygenscavenging polymers exists. The scavenging of oxygen including oxidation of hydrocarbon polymers impregnated with salts of transition metals has been described by Speer et al (1993). The novelty of their method is that the oxidation process is activated by brief exposure to light of wavelengths less than 750 nm. The optional use of a photosensitiser is reported to increase the rate of activation, particularly in the presence of an antioxidant. As with the photodegradable plastics processes, the chemistry involves initiation of the free radical process on the polymer chains by changes in the oxidation state of the transition metal ion, preferably cobalt. The redox reaction on the polymer chain involves either hydrogen abstraction in polypropylene or polyethylene or oxygen attack on the double bond in the case of poly(l,2-butadiene).
The hydroperoxide formed by reaction of either the polymer free radical or by direct attack on the polymer decomposes to form ketones, aldehydes, peroxides or carboxylic acids, in some cases with chain scission. In the case of poly(l,2-butadiene) it is the aim to localise the oxidation to the side chain containing the double bond. Speer et al. (1993) report that this polymer retains its physical properties even when most or all of its oxidisable groups have been reacted. Since this is a typical autoxidation that is also thermally activated, the polymer needs to be stabilised against premature initiation in the extruder or moulding machine in package fabrication. Antioxidants provide this protection. The quantity of antioxidant is chosen both to provide this protection and to retard the autoxidation chain reaction for a selected period of time between activation and package filling. Removal of antioxidant from the polymer appears likely to increase the likelihood of taint formation due to unrestricted polymer oxidation. Attention should probably be given to inclusion of odour absorbents in the composition as proposed by Toppan (1992). The manner in which such a system could be used is particularly broad but one suggested is to laminate or coextrude the reactive layer with an oxygen-barrier layer. Such an application would be appropriate for many packaging processes such as in the storage of cheese, nuts and meats such as beef primal cuts. The use of such compositions at the low temperatures required for storage of meats and cheese requires that the reactive layer be readily permeable to oxygen. Speer and Roberts (1993) have specified that the compositions containing their oxidisable components should be largely amorphous and have a glass transition temperature below -15°C. 4.2.4.4.2 Rapid photoreduction of a scavenger precursor. Research into systems involving this form of triggering indicate that this approach overcomes some of the limitations of other methods already proposed (Rooney, unpublished results). 4.2.5 Chemical barrier to oxygen permeation Implicit in the use of oxygen scavenging inserts (sachets) or closures is the effect these have on the consumption of oxygen as it enters the product during its storage life. The results in Table 4.5 show the calculated impact of Table 4.5 Calculated life of chemical oxygen barriers Film _ _ _ _ PET 25 nm/LDPE 25 \x.m PVDC-coated OPP 25 jjum Metallised PET 12 ^m/LDPE 25 |xm
OTR cm3 m~2 day"1 atm~l _ _
Barrier life (days) _
63 9 0.5
56 362 7250
the oxygen transmission rate of packaging materials on the period of effective oxygen scavenging by a commercial oxygen-scavenging sachet with an absorption capacity of 50 ml of oxygen. The packages are 10 cm x 20 cm and initially contain 100 ml air. LDPE is low-density polyethylene, OPP is oriented polypropylene, PET is polyester and PVDC is poly(vinylidene chloride). However, in a food-packaging situation where the food is tightly packed, the sachet can be expected to deoxygenate only a small portion of the headspace and combat permeation of oxygen that is accessible to it. Close-fitting packages such as vacuum packs for block cheese and meats or aseptic cartons of beverages are examples where the headspace is very small and oxygen permeation is the prime cause of quality loss. It is in such circumstances that oxygen-scavenging plastics films are particularly needed. The use of the oxygen-scavenging reaction to intercept oxygen diffusing through the package wall is an example of a chemical barrier as distinct from the physical barrier normally provided by aluminium foil and vacuum metallising or crystalline polymers such as poly(vinylidene chloride). The use of a chemical oxygen barrier offers the opportunity to cheapen barrier packaging for relatively short shelf-life products such as wholesale or export units or for some fresh foods. This can be achieved by using a barrier film of intermediate performance coupled with a chemical barrier. This combination was investigated by Rooney and Holland (1979) and the results in Figure 4.5 demonstrate a period of total barrier to oxygen permeation from air at 25°C. A laminate of polyethylene/nylon 6/cellulose acetate containing oxygen-scavenging reagents was used to separate the two compartments of a glass permeability cell. When the ethyl cellulose layer contained 0.5 M fe-furfurylidenepentaerythritol, illumination with fluorescent light resulting in a barrier life of around 30 days was followed by a period of oxygen transmission at a rate less than that found in darkness. In this case the chemistry used was a singlet oxygen reaction which requires constant illumination to photo-excite the oxygen. Other chemistries can be used to achieve similar reactions in darkness and Speer et al (1993) identified this application for their photoinitiated chain reactions in rubbers, and Cochran et al (1990) devised the Ox-Bar process largely for this purpose. The use of oxygen-scavenging sachets to allow the use of a cheaper packaging material has been described by Alarcon and Hotchkiss (1993) and their work is reviewed in Chapter 6. Although use of multilayers of plastics films appears the most appropriate approach to solving permeability problems, there have been early attempts to carry out scavenging reactions in liquid layers. Oxygen scavenging in liquid layers was one of the earliest processes described in the patent literature. Cook (1969) claimed that bilayers of plastics films separated by a layer of a solution of one of several antioxidants in high boiling point solvents
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in darkness
% Oxygen
illuminated
Time (days)
Figure 4.5 Barrier to oxygen permeation of polyethylene/Nylon 6/cellulose acetate laminate. Cellulose acetate 26 |xm thick containing PEF (0.5 M) and erythrosine, 5 X 10~3 M. Oxygen concentration measured in a cell on the cellulose acetate side of the laminate, air on the other side.
demonstrated a reduced oxygen permeability. Whereas the mechanism was unknown, all of the common antioxidant classes were claimed to be effective. Use of an aqueous solution of an oxygen-scavenging reducing sulfite as a layer between two polymeric layers has been claimed by Scholle (1976). As with the organic system of Cook (1969), the purpose was to reduce the oxygen permeability of the laminate. In the case of Cook's system the role appears to have been to improve shrinkable packaging, as for meat, whereas the Scholle system was particularly suitable for bag-in-box liners since the packages consist of, at least, a collapsible duplex which is sealed together only at the edges where the liner forms a bag (and around the tap fitting). Such a system requires an oxygen scavenger for oxygen-sensitive products due to the gas space between the two film layers. 4.3 Moisture control films Moisture affects the gas and vapour permeability of hydrophilic plastics packaging films (Davis, 1966). Fruit cakes packed in films with an appropriate water vapour permeability can have long shelf-lives because the surface dries somewhat. This creates conditions unfavourable to mould growth. Nitrocellulose-coated regenerated cellulose has been used because it
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in darkness
% Oxygen
illuminated
Time (days)
Figure 4.5 Barrier to oxygen permeation of polyethylene/Nylon 6/cellulose acetate laminate. Cellulose acetate 26 |xm thick containing PEF (0.5 M) and erythrosine, 5 X 10~3 M. Oxygen concentration measured in a cell on the cellulose acetate side of the laminate, air on the other side.
demonstrated a reduced oxygen permeability. Whereas the mechanism was unknown, all of the common antioxidant classes were claimed to be effective. Use of an aqueous solution of an oxygen-scavenging reducing sulfite as a layer between two polymeric layers has been claimed by Scholle (1976). As with the organic system of Cook (1969), the purpose was to reduce the oxygen permeability of the laminate. In the case of Cook's system the role appears to have been to improve shrinkable packaging, as for meat, whereas the Scholle system was particularly suitable for bag-in-box liners since the packages consist of, at least, a collapsible duplex which is sealed together only at the edges where the liner forms a bag (and around the tap fitting). Such a system requires an oxygen scavenger for oxygen-sensitive products due to the gas space between the two film layers. 4.3 Moisture control films Moisture affects the gas and vapour permeability of hydrophilic plastics packaging films (Davis, 1966). Fruit cakes packed in films with an appropriate water vapour permeability can have long shelf-lives because the surface dries somewhat. This creates conditions unfavourable to mould growth. Nitrocellulose-coated regenerated cellulose has been used because it
prevents contamination with moulds and allows substantial water vapour permeation. There are, however, several sets of circumstances in food packaging where greater selectivity in control of liquid and gaseous water is required. These include: • • • •
Transpiration of horticultural produce. Melting of ice, e.g. in fish transportation. Temperature fluctuation in high erh food packs. Drip of tissue fluid from cut meats and produce.
The problems which arise from these circumstances are the result of build-up of liquid water. Alternatively, the natural water level in the tissue can be undesirably high near the surface for microbiological stability under the chosen storage conditions. The results of the presence of water in unwanted quantities include bacterial (yeast) and mould growth as well as fogging of films and mobile blood and tissue fluid. Moisture migration via the vapour phase can result in transient formation of regions without adequate preservative or where the aw is high even though the food is packed at lower aw. Some of the ways of addressing these problems via packaging are considered in detail in terms of either liquid water control or humidity buffering. 4.3.1 Liquid water control Temperature cycling of high erh foods has led most film manufacturers to use heat-seal plastics with anti-fog additive. These additives are blended with the resin before extrusion and migrate to the surface after film formation. The additives are chosen for their amphiphilic nature with the non-polar chain in the plastic and the polar end group at the interface. The result is a lowering of the interfacial tension between water condensate and the plastic film. The fog droplets therefore coalesce and form a transparent film on the plastic. This film may even flow on sloped surfaces and gather at the bottom of the pack in extreme circumstances. Anti-fog treatments are a cosmetic form of active packaging, assisting the customer to see the packaged food clearly. There is no change in the availability of liquid water in the package and this has the potential to cause produce spoilage unless managed by one of the processes described below. Other active packaging systems go further in removing liquid water from food contact. Several companies manufacture drip-absorbent sheets, which may have a variety of other names. Basically they consist of two layers of a microporous or non-woven polymer, such as polyethylene or polypropylene, between which is placed a superabsorbent polymer in the form of free-flowing granules. The duplex sheet is sealed at the edges and is normally quilted to
allow the water absorbent to be held in place rather than aggregating towards one edge of the sheet when tilted towards one edge. Such sheets are then used either as pads under whole chickens or chicken pieces to absorb drip, thereby preventing discoloration of either the meat or the white foam tray. Given the interest in reducing the volume of solid waste by decreasing foam polystyrene usage, the use of drip-absorbers may become even more necessary to prevent water damage to the more hydrophilic alternatives. Large sheets are used for absorption of melted ice in the packaging of seafood for air transportation. The superabsorbent polymer used in the Thermarite® Pty Ltd sheet manufactured in Australia is capable of absorbing at least 100 times and possibly as much as 500 times its own weight of liquid water depending dramatically upon salinity (Malouthi et al., 1994). Another product of this type is Toppan Sheet manufactured in Japan. A recent patent application in Japan describes a water-absorbent sheet which has a high barrier to oxygen for use with fresh meat (Showa Denko KK, 1990), This would necessitate use of a sealed outer pack. The preferred polymers used to absorb the water are polyacrylate salts, although graft copolymers of starch can also be used. Such copolymers consist of a polysaccharide backbone with synthetic polymer chains radiating from the starch backbone. The strong association of the polysaccharide chains is disrupted allowing the starch to exercise its affinity for water by hydrogen bonding. Such polymers tend to become slimy when swollen with large amounts of water. The swelling of the polymer on hydration results in substantial distortion of the duplex sheet, an effect that is controlled somewhat by the quilted seal pattern. This effect can be controlled by choice of the amount and the capacity of the superabsorbent polymer. The significance of such active packaging in seafood shipment by air lies in the removal of the potential for spillage of salt water from cartons in aircraft holds. The aluminium construction of aircraft makes them easily susceptible to corrosion damage, at great cost. 4.3.2 Humidity buffering An entirely different approach to the control of excess moisture in food packages is to intercept the moisture in the vapour phase. This approach allows the food packer or even the householder to reduce the surface concentration of water in a food by reducing the in-pack relative humidity. This can be done by placing one or more humectants between two layers of a plastic film which is highly permeable to water vapour. An example of this type of product is Pichit manufactured by Showa Denko in Japan. The film duplex is described as containing an alcohol, described by Labuza (1989) as propylene glycol and a carbohydrate, both of which are humectants.
Table 4.6 Water vapour uptake by Pichit and Toppan sheet YimQ (h) __ 3 4 24 26
Weight loss from petri dish (%) Pichit -_ 2.4 3.6 15.2 22.0
Toppan sheet _ 0.7 1.2 8.4 8.8
The effectiveness of such a combination material in comparison with a superabsorbent polymer absorbent for liquid water is shown in Table 4.6. Distilled water 5 g was placed in each of two petri dishes (with lids) and these were enclosed with either Pichit or Toppan Sheet (450 cm2) in a highbarrier package. The packages were opened at intervals and the petri dishes were weighed to give the per cent loss of weight of water, shown in Table 4.6. Thus, whereas the superabsorbent polymer has a far greater capacity for liquid water uptake, the humectant-based film more rapidly absorbs water vapour. Pichit is marketed for home use in roll or single sheet form for wrapping pieces of flesh food such as fish or chicken to reduce the aw proximate to the food. At present there is a lack of experimental verification of the significance of this effect. Louis and de Leiris (1991) suggest two to three days of fresh storage is possible. It has been suggested by Labuza (1989) that the permeability of poly(vinyl alcohol) to propylene glycol may be sufficient to allow some of the latter to diffuse to the food surface. If this is the case, some antimicrobial action might be expected. Propylene glycol is a GRAS substance in the USA. Louis (1991) reports the availability of an additional moisture control film which plays the same role as Pichit in the Japanese market. The potential for use of such materials for quality retention in the home appears to be considerable and this type of film should be subjected to detailed objective evaluation for use in the domestic situation where excess portions of flesh foods need refrigerated storage for some days. The potential for washing the surfaces of sheets like Pichit, followed by re-use, is an attractive incentive for active packaging use in the home provided absence of microbial contamination can be demonstrated. A different approach to humidity buffering has been under investigation for use in the distribution of horticultural produce. Currently the packages are wholesale fibreboard cartons of produce, usually with a polyethylene liner or consisting of the very expensive waxed fibreboard without a liner. A recent development has been the water-barrier coating of the inside of fibreboard cartons to allow moist produce to be placed directly into the carton. Besides the introduction of liquid water with the produce, packing into closed spaces allows the build-up of water vapour. The produce
continues to lose water by evaporation during distribution and the relative humidity in the lined carton can reach close to 100%. Since temperature cycling is very difficult to avoid during handling, there is every likelihood of condensation, and, with this, the growth of microorganisms on fruits and vegetables. Two widely different approaches have been taken to buffering the humidity in the cartons in order to prevent condensation while not concurrently causing desiccation of the produce. One is to include microporous bags or pads of inorganic salts and the other is to line the carton with a protected layer of a solid polymeric humectant. The approach of using microporous sachets of inorganic salts has been used in the US tomato market in recent years. This was proposed by Shirazi and Cameron (1992) who showed that an equilibrium relative humidity of around 95% above produce can be reduced to around 80% by use of sachets of desiccant salt such as sodium chloride. The application was extended by Hudson (1991). Both these studies involved use of heat-sealed liners which were concurrently being evaluated for their performance to maintain equilibrium modified atmospheres (EMA) generated by the fruit (see Chapter 3). The control of moisture is one of the main impediments to the introduction of EMA packaging. Indeed it is common to package potatoes in perforated sacks to prevent unwanted build-up of condensation. The most recent alternative involves the use of the carton as the active package rather than an insert. This approach lends itself to combination with EMA generation less readily as the humidity is buffered at the interface with the fibreboard. The designs of Patterson and Joyce (1993) involve: an integral water vapour barrier layer on the inner surface of the fibreboard; a paper-like material bonded to the barrier and which acts as a wick; and a layer highly permeable to water vapour but unwettable next to the fruit or vegetable. The latter layer is spot welded to the layer underneath. Accordingly, the multilayer of material on the inside of the carton is able to take up water in the vapour state when the temperature drops and the RH rises. When the temperature rises the multilayer releases water vapour back into the carton in response to a lowering of the RH. The condensation control system therefore acts as an internal water buffer. The critical characteristic of the system is the capacity of the wick layer for water. The performance of this system is demonstrated in Table 4.7 taken from Patterson et al. (1993) which shows the results of a comparison of the free Table 4.7 Free water in carrot packs on cooling Packaging Polyethylene liner Condensation control carton •Standard error of the mean (n = 5)
Cooled from 10 to 3°C
Cooled from 22 to 3°C
10.1 (0.6)* 0.0 (0.0)*
22.9 (0.7)* 0.2 (0.2)*
water found in cartons of carrots with a conventional liner with the condensation control multilayer. The cartons containing carrots, 6 kg, were cooled to 3°C from either 100C or 22°C and after 3 days the amount of free water in the boxes and on the carrots was measured. The results demonstrate the capacity of such a system for water uptake under circumstances likely to be found in commerce. Shrink wrapping is an alternative approach to the use of active packaging systems for control of condensation on spherical fruits such as citrus. BenYehoshua (1989) has reviewed this field, to which he has made such a large contribution. 4.4 Removal of taints and food constituents The interaction of packaging plastics with food aroma has long been recognised, especially through the flavour 'scalping' which is of considerable commercial interest. Hirose et al. (1989) demonstrated the impact of the nature of the metal ion in Surlyn film layers in aseptic brick-packs on the scalping of limonene from orange juice. In periods as short as 2 weeks at 24°C, almost 30% of the limonene was found in the Surlyn-1601 and 20% in the polyethylene layer in brick packs. Commercially, plastics packaging has not been used to remove selectively components of the flavour or aroma of foods which are considered undesirable, but a potential opportunity has been available for over a decade. Some varieties of orange, such as the Navel, contain a tetraterpenoid of the formula C26H30O8 which is initially present in the albedo but which is extracted into the juice on standing or heating. Thus the juice of such oranges becomes bitter on pasteurisation when this compound, limonin, reaches concentrations exceeding 8-12 mg/kg (Chandler et al, 1968). Processes have been developed for debittering such juices by passing them through columns packed with cellulose triacetate or nylon beads (Chandler and Johnson, 1979). A simple active packaging process was suggested by the same authors, who proposed that since the juice extracts the bitter principle on standing for 24 h, inclusion of the absorbent in the packaging might remove it as it is extracted. To this end they proposed using their absorbents in film form such as cellulose triacetate or as acetylated paper. They showed that a 1-litre plastic bottle coated internally with cellulose acetate-butyrate reduced the limonin content of 500 ml of juice from 42 to 11 mg/kg after 3 days' refrigeration. Similarly, when cans lined with acetylated filter paper containing juice with 14.9 mg/kg limonin were spin cooked and allowed to stand, the juice was only slightly bitter after 4-13 days. It appears that this process has not been taken up commercially although it offers considerable potential for freshly squeezed Navel juice marketing
(Johnson, R.L., private communication). On the other hand, the sorption of limonene oil from packaged juices by heat-seal layers has been the subject of several studies (Mannheim et al, 1989; Hirose et al, 1989). Thus opportunities for active packaging can be closely related to problems of food-package interactions. A closely related goal had long been achievable commercially in the tinplate canning of foods in which protein degradation resulted in the release of sulfur compounds from the food. These sulfur compounds cause the phenomenon of 'sulfur staining' on the tinplate and so it has been the practice to disperse zinc oxide in tinplate lacquers to intercept such compounds reacting with them before they can diffuse to the tinplate surface. The remaining methods described in the literature to date for removal of taints or off-flavours have largely involved incorporation of ingredients with a specific interaction or reaction with a functional group known to be present in the taint or undesirable food component. Two types of taints amenable to removal by active packaging have been identified by researchers responsible for current commercial products. These are amines resulting from protein breakdown in fish muscle and aldehydes formed from the breakdown of peroxides which result from the initial stages of autoxidation of fats and oils. The formation of aldehydes can make a wide variety of oil-containing foods organoleptically unacceptable well before there is significant damage to the nutritional or functional properties of the food. Examples of such products would be fried snackfoods such as potato crisps, biscuits and cereal products. Early developments occurred in Japan where there was seen to be a need to remove amine smells from fish which was stored in domestic refrigerators. The amines formed in fish muscle degradation include strongly basic compounds and thus are potentially strong in their interaction with acidic compounds such as citric or other food acids. Hence the earliest work involved incorporation of such acids in heat-seal polymers such as polyethylene and extruding them as layers in packaging (Hoshino and Osanai, 1986). A later approach to removal of amines odours has been provided by the ANICO Company Ltd in Japan under the trade name ANICO BAG. Bags made from film containing ferrous salt and an organic acid such as citric or ascorbic acid are claimed to oxidise the amine or other oxidisable compound as it is absorbed by the polymer. If these materials can be shown to be effective there is an opportunity to determine which variables optimise their rate and extent of reaction. Questions which would need to be answered are What is the nature of the products of such reaction and What is their fate. Questions such as these are of particular interest to regulatory authorities, and the potential for several active packaging systems to generate mobile reaction products is considered in Chapter 11.
Removal of aldehydes such as hexanal and heptanal from package headspaces is claimed by Dupont Polymers, Packaging Division, for their recently introduced tie layer Bynel IXPlOl which is a high-density polyethylene (HDPE) resin masterbatch. This masterbatch is blended (2.5-12%) with unmodified HDPE or other linear polyethylenes to form an intermediate layer in coextrusions. It is specified that the heat-seal layer should not be a 'good to excellent gas barrier' (Dupont, 1993). It is interesting to note that the use of a form of active packaging can place restrictions on other components of the packaging. The restriction in this case, and many others, is that the extrusion temperature should not exceed 2200C to avoid fuming. The chemistry of the process is not described but such a process would require the reaction with the aldehyde to be effectively irreversible at least over the temperature range the package is likely to encounter. One such reaction would be the formation of a Schiff base by reaction of the aldehyde with an amino group. The amino group would need to be rather stable to heat and oxygen in order to remain unaffected after the extrusion at temperatures up to 2200C in an air atmosphere. There may well be a wide range of food constituents which can be removed by making use of specific interactions with selected packaging components or by chemical reaction with them. A fertile research field would seem to be open especially with liquid foods since solubility and diffusion of food constituents in the packaging can be utilised so that the removal process is not limited to compounds with a significant vapour pressure at distribution temperatures. It will be necessary for industry and regulators to ensure that such processes are not used to conceal the marketing of sub-standard or even dangerous products if for instance microbial odours were to be scavenged. The tainting of foods by compounds originating either in the packaging material itself (e.g. monomers) or outside but permeating the packaging material continues to be a source of problems for the food industry. The approaches described above may contribute to their solution but additional approaches have been described in the patent literature. These are formation of chemical barriers as distinct from physical barriers like aluminium foil or crystalline polymers. Myrcene (7-methyl-3-methylene-l,6-octadiene) has been found to react with traces of styrene or acrylonitrile when the latter are present in acrylonitrile-butadiene-styrene copolymers or blends being extruded (Tokas, 1979). Addition of myrcene is claimed to introduce no new taint while reducing residual levels of the monomers. In another patent, foods packaged in plastics consisting of more than one layer are claimed to be protected against taints from the outside by inclusion of the appropriate adsorbent in the outer layer (Kiru Kogyo KK, 1994). The adsorbent is chosen for expected taints and is kneaded onto the outer layer, apparently retaining the taints there. The period of this type of equilibrium adsorption
needs to be established, especially when a package is subjected to temperature changes. This may have application in areas where products are to be stored or shipped together with odorous products which are inadequately packaged. A compound which while not a taint is often found undesirable in packages of respiring horticultural produce is the gaseous auxin ethylene. The potential benefits resulting from such removal are particularly great and are discussed separately in Chapter 2. 4*5 Ingredient release Active packaging materials considered so far have exerted their action on the packaged food by removing unwanted components of either the food or of the headspace enclosed with the food. Another form of interaction is by release of desirable ingredients into the food from the packaging materials or from inserts packaged with the food. Some substances released commercially or which have been the subject of investigation are listed in Table 4.8. Most are used for their antimicrobial activity, although sulfur dioxide also serves as a chemical stabiliser of colour and flavour by preventing progress of the Maillard reaction which causes non-enzymic browning of products such as dried fruit and wines (Davis, 1975; Davis et aL9 1978). Hinokitiol, also known as p-thujapricin (Hirata, 1992), derived from cypress bark, is an additional antimicrobial compound specific to the Japan market. Processing of foods often results in loss of flavour by degradation or evaporation. Another mechanism of flavour loss is the scalping of some flavour components by plastics used in packaging (Mannheim et aL, 1989). There is therefore the opportunity to replace these lost food constituents by diffusion from the packaging, especially where scalping or flavour degradation occurs after packaging. There is, however, the question of whether a food is being sold as fresh when this is not so. This is more a legal matter of consumer protection than a technical one as the question arises of when Table 4.8 Substances emitted by Active Packaging Substance
Purpose
Carbon dioxide
antimicrobial
Ethanol Silver ion Organic acids Sulfur dioxide Benomyl Flavours Hinokitiol BHT Enzymes
antimicrobial antimicrobial antimicrobial antimicrobial antimicrobial fortification antimicrobial antioxidant various
Source film sachet sachet film film sachet film film film film film
Reference Rooney, 1989 Abe, 1990 Abe, 1990 Hirata, 1992 Hotchkiss, 1993 ICI Australia Halek and Garg, 1988 Venator, 1986 Abe, 1990 Han et a/., 1987 Budny, 1990
flavour addition to a food fabricated from many ingredients becomes contrary to consumer interests. Release of very few flavours has been investigated to date, although the ease of oxidation of many flavours suggests an opportunity to provide slowrelease flavour precursors in the packaging material. The manufacture of flavour concentrates in the common commodity plastics has been described by Venator (1986). Master batches of plastics with concentrations of up to 40% of the flavour have been marketed with a view to obviate the effects of flavour scalping. 4.5.1 Antioxidant release from plastics Two factors acting concurrently are likely to influence the use of packaging materials as sources of antioxidants in some foods. The first of these is the need of the industry to respond to pressure by some consumer advocates for reduced use of food additives (Smith, 1993). The second is the renewed interest of plastics resin manufacturers in using natural, or other approved food antioxidants in polymer stabilisation replacing some of those developed specifically for plastics. Dilaurylthiodipropionate and its base acid, thiodipropionic acid, are approved food additives in some countries and are used as stabilisers in food-grade polyethylene (Anon., 1992). The potential for evaporative migration of antioxidants into foods from their packaging plastics has been studied by CaIvert and Billingham (1979) who developed a theoretical model. This work has been taken further by Han et al. (1987) who determined the effect of temperature on both the diffusion coefficient of butylated hydroxytoluene (BHT) in HDPE and the rate of its evaporation into the package of oat flakes. It was found that at 39°C only 55% of the original BHT remained in the film after 1 week. The loss by outward migration was 70%, and 25% of the BHT was found in the cereal. After 6 weeks the HDPE film was free of antioxidant and 19% of that originally in the film remained in the cereal. The workers compared the impact of two starting levels in the film on oxidation of the cereal oil and found that with an initial 0.32% BHT there was less oxidation than with 0.02%. These results of Han et al (1987) demonstrate the potential for release of antioxidant into foods provided the rate of diffusion can be matched to the food's needs. The outward loss can be controlled by use of a layer of film with low permeability to the antioxidant or by use in a closure. In the case of liquid foods or solids with close-fitting packaging the process could be based on diffusion alone and not require the antioxidant to be able to evaporate. Commercial use of this approach to antioxidant release has been reported by Labuza and Breene (1989) who noted that waxed paper has sometimes
been used as a reservoir for antioxidant release by the US cereal industry. Other antioxidants might also be used in this way. Lignert and Eriksson (1980) found that Maillard reaction products have a strong antioxidant function. This work has been extended by Anese et al (1993). The potential for applying such antioxidants to foods via packaging materials has been suggested (Eriksson, private communication). The patent of Goyo Shiko KK (1993) describes application of amino acids and saccharides which produce reducing sugars on decomposition and perform as oxygen scavengers when used in coating or lamination of packaging films. These compositions would be expected to undergo the Maillard reactions but their rate would depend upon the thermal conditions used in preparing the packaging and any thermal processing of the packaged product. The inventors nominate liquid foods in cans as a target product area and, given the hydrophilic nature of the polymers involved, it appears likely that Maillard reaction products could be extracted into the food to act as antioxidants as well as scavengers. It has recently been shown that polyethylene bottles stabilised with vitamin E cause less noticeable flavour in distilled water than bottles stabilised with either BHT or Irganox (a hindered amine antioxidant for polymers). There is an opportunity to investigate whether oils can be stabilised by diffusive addition of antioxidant at sustained low levels from the packaging. The observation of Han et al (1987) that BHT was lost outwards from HDPE film packs points to an otherwise unrelated opportunity for active packaging. There has been constant pressure from importers of grains and other agricultural products for reduced levels of pesticide residues in these products at the time of delivery. Packages such as sacks and lined cartons containing these products are often attacked by insects during warehousing and transport. The inclusion of low-toxicity fumigants such as pyrethrins in an outer layer of packaging material offers the opportunity to achieve sustained insecticidal activity without substantial addition of fumigant to the food. Highland and Cline (1986) found that polypropylene containing 203 mg m~2 of permethrin provided rather similar resistance to attack by burrowing insects to that provided by polyester which has a harder, slippery surface. Their work involved exposing: polyethylene; polypropylene/polypropylene/polyethylene laminate, the latter containing permethrin in the outer layer; and polyester/polyethylene pouches of many foods, to four insect populations. The permethrin-treated pouches were resistant to two insect species for 24 months. The effect was not entirely consistent since the treated pouches failed at 24 months with one insect species and were better than polyester with one of the remaining species and worse than polyester with the other. The authors concluded that the order of descending resistance of films to the insects was permethrin-treated film, polyester film, polyethyl-
ene film and worst, polypropylene film. This work might be developed to provide a low-cost answer to some of the major problems of fumigation, including the cost of repeated fumigations with methyl bromide approximately bi-monthly, as well as reducing the exposure of staff to fumigant application conditions. The permethrin or alternative treatments would need to be submitted for regulatory approval. It has been suggested recently that enzymes might be released into foods from packaging materials, probably to achieve effects such as antimicrobial action. Labuza and Breene (1989) have reviewed the potential for release of bound enzymes into foods. Enzyme inhibitors might also be bound to a film surface. An example would be the binding of the inhibitor of methyl esterase to the package surface to bind methyl esterase. The result might be prevention of cloud drop in fresh orange juice (R.L. Johnson, private communication). The subject of enzymes in active packaging is discussed in Chapter 7. The use of enzymes in edible coatings is discussed in Chapter 5. 4.6 Permeability modification There are a number of circumstances in which it is desirable for the packaging material to be more permeable to one substance than to another. The importance of predicting the requirements of plastic film's permeability to carbon dioxide and oxygen in packaging of fresh horticultural produce has been discussed in Chapter 3. It is possible to modify the permeability of a window in a package and to control gas exchange through this limited area. A variety of semipermeable patches were initially developed by the Hercules company in the USA. Subsequently the effect of sorbed water on cellulosic patches has been claimed to give selectivity in gas permeability matched to the respiration rates of some produce. There are other substances which need to be selected for entry into plastics packaging. Of particular interest is the ability to selectively transmit smoke flavours through films which are useful as skins for ham and other preserved meats. These are films of polyamide alloy which are highly permeable to water and oxygen under conditions of high temperature and high humidity and highly impermeable to oxygen at room temperature under dry conditions (Nishini and Yoshii, 1988). Hirata (1992) described one such type of film as having an oxygen transmission rate of 8 ml/m2/day at 200C, 60% RH and a water vapour transmission rate of (WVTR) 60 g/m2/day at 400C, 90% RH. The low oxygen permeability is necessary for colour retention of preserved meats. This high WVTR should favour rapid transmission of polar flavours. Kureha Chemical Co. (1986) described a polyamide film for this purpose with an OTR in the range 50-300 ml/m2/day for film thicknesses of 5-50 |xm.
4.7
Current use commercially
The commercial development of plastics-based active packaging has not occurred evenly either geographically or in terms of their field of application. The major field to date has been horticulture in which several forms of enhanced permeability films have been commercialised for both trade and home use. This success, in spite of a lack of soundly based evidence of effectiveness in many cases, owes much to substantial prior scientific research. This research into equilibrium modified atmosphere packaging has been based on the work of Kader (1980) and earlier workers including Jurin and Karel (1963). Since some of the active packaging technologies currently commercial are scarcely improvements on existing technologies there will probably be significant realignment in the marketplace. The need for active packaging solutions to problems in the storage and distribution of horticultural produce may well ensure that the development of the most soundly based technologies is commercialised. Ethylene scavenging, condensation control and equilibrium modified atmosphere packaging will continue to be emphasised. Active packaging for processed foods is still based largely on sachet technologies, with the exception of moisture control packaging. It is in the processed food field that commercial development of plastics-based systems can be expected to be substantial over the next few years. Perhaps Toyo Seikan Kaisha's planned manufacture of an oxygen-scavenging laminate for the semi-aseptically packaged boiled-rice market by Sato Food Industry Co. Ltd is the first example. This tray has been described as 'epoch making', causing, as is planned, a 100 000 meals/day operation to be more cost effective (Anon., 1994). The substantial impact on the marketplace of oxygen-scavenging crown seals for beverages, metallised polyester microwave susceptors and timetemperature indicator strips is discussed in other chapters. 4.8 Regulatory and environmental impacts Since plastics-based active packaging involves not only changing current materials somewhat but also inclusion of reactive components in some cases, regulatory authorities must become involved in many developments. Indeed regulatory considerations appear to have caused the delay in the introduction of a chemical barrier to oxygen into PET beverage bottles. The effect of environmental considerations on plastics-based active packaging will vary with the nature of the product/package combination. Commodity films used for produce packaging may become the object of recycling schemes, especially in the European Union, and so additional ingredients will need to be evaluated for their impact. Barrier packages used
for processed foods at present are generally not amenable to economic recycling. The benefits of active packaging in terms of food quality, safety and shelf-life extension will need to be considered in a holistic approach to environmental impact assessment. A study by Kooijman (1994) of food packaging in the Netherlands demonstrated that 'the packaging sub-system cannot be studied or optimised in isolation'. Accordingly, active packaging plastics should be judged on the basis of their contribution to the quality and safety of food. References
Abbott, J., Waite, R. and Hearne, J.F. (1961) Gas packing milk powder with a mixture of nitrogen and hydrogen in the presence of palladium catalyst. J. Dairy Res., 28, 285-92. Abe, Y. and Kondoh, Y. (1989) Oxygen absorbers. In: Controlled/Modified Atmosphere/ Vacuum Packaging of Foods, A.L. Brody (ed.), Food and Nutrition Press, Trumbull CT, p. 151. Alarcon, B. and Hotchkiss, J.H. (1993) The effect of Fresh-Pax oxygen-absorbing packets on the shelf-life of foods. Technical Report., Dept. Food Sci., Cornell Univ., NY, USA. pp. 1-7. Anese, M., Pittia, P. and Nicoli, M.C. (1993) Oxygen consuming properties of heated glucose/ glycine aqueous solutions. Ital. J. Food ScL, 1, 75-9. Anon. (1992) Plastics for food contact. Australian Standard 2070. Anon. (1994) Use of a New Container requiring no deoxidizer. Packaging Japan, July, 19. Ben-Yenhoshua, S. (1989) Individual seal-packaging of fruit and vegetables in plastic film. In: Controlled/Modified Atmosphere/Vacuum Packaging of Foods, A. Brody (ed.), Food and Nutrition Press, Inc., Trumbull, CT, pp. 101-18. Bissot, T.C. (1990) Performance of high-barrier resins with platelet-type fillers. In: Barrier Polymers and Structures, WJ. Koros (ed.), American Chemical Society, Washington DC, USA, pp. 225-38. Bixler, HJ. (1971) Barrier properties of polymer films. In: Science and Technology of Polymer Films, Vol. 2. HJ. Bixler and OJ. Sweeting (eds), Wiley-Interscience, New York, pp. 85-130. Brody, A.L. (1989) Modified atmosphere packaging of seafoods. In: Controlled/Modified Atmosphere/Vacuum Packaging of Foods A.L. Brody (ed.), Food and Nutrition Press, Trumbull, p. 59. Budny, J. (1990) The Use of Enzymes in Packaging for Selective Removal of Lactose, Cholesterol and Oxygen from Foods. Proceedings Pack Alimentaire '90. Innovative Expositions Inc., Princeton, NJ., Session B-2. Chandler, B.V., Kefford, J.F. and Ziemelis, G. (1968) Removal of limonin from bitter orange juice. /. ScL FoodAgric, 19, 83-6. Chandler, B.V. and Johnson, R.L. (1979) New Sorbent gel forms of cellulose esters for debittering citrus juices. /. Sci. Food Agric, 30, 825-32. Chignell, CF., Bilski, P., Reszka, KJ., Motten, A.G., Sik, R.H. and Dahl, T.A. (1994) Spectral and photochemical properties of curcumin. Photochem. Photobiol., 59, 295-302. Cook, J.M. (1969) Flexible film wrapper. US Patent 3429717. Davis, E.G. (1966) Texture changes in salted peanuts in flexible film and tinplate containers. Food TechnoL, 20, 92-4. Davis, E.G. (1978) The performance of liners for retail wine corks. J. Food TechnoL, 13, 235-41. Davis, E.G., McBean, D.McG. and Rooney, M.L. (1975) Packaging of foods that contain sulfur dioxide. CSIRO Food Res. Q., 35, 57-62. Dupont Polymers (1993) Bynel IPXlOl. Interactive packaging resin. Temporary Product Data Sheet, November. Ebner, CL., Morgan, CR., Roberts, W.P., Speer, D.V. and Speer, V.E. (1992) Composition for scavenging oxygen especially to protect oxygen-sensitive products, e.g. food - comprises
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155283. Isherwood, F. A. (1943) Removing oxygen from a container containing vacuum or gas packed food in which a metal (ex. Fe) absorbs oxygen to form an oxide. British Patent 553991. Jurin, V. and Karel, M. (1963) Studies on control of respiration of Mclntosh apples by packaging methods. Food Technol, 17(6), 104-8. Kader, A.A. (1980) Prevention of ripening in fruits by use of controlled atmospheres. Food Technol, 34(3), 51-4. King, J. (1955) Catalytic removal of oxygen from food containers. Food Manuf. 30, 441. Kiru Kogyo KK and Marutani Kakoki KK (1994) Functional container for medical supplies or foods - having a plastic functional layer containing adsorbent e.g. silica gel, zeolite on inner surface of outer layer of sealed container. Japanese Patent 6080163. Kooijman, J.M. (1994) Environmental assessment of food packaging: impact and improvement. Packag. Technol ScL9 7, 111-21. Kuhn, P.E., Weinke, K.F. and Zimmerman, P.L. (1970) Oxygen Scavenging System for Flexible Packaging of Whole Dry Milk. Proceedings of 9th Milk Concentrates Conference, Pennsylvania State Univ., September 15th. Kureha Chemical Co. (1986) Packaging film for smoking food has at least one layer and includes nylon layer of predetermined thickness, water absorption and oxygen permeability. Japanese Patent 60180832. Labuza, T.P. and Breene, W.M. (1989) Applications of 'Active Packaging' for improvement of shelf-life and nutritional quality of fresh and extended shelf-life foods. J. Food Proc.
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Applications. PhD Thesis, University of Western Sydney - Hawkesbury, NSW, Australia. Mannheim, C.H., Miltz, J. and Passy, N. (1989) Interaction between aseptically filled citrus products and laminated structures. In: Food and Packaging Interactions, J. Hotchkiss (ed.), ACS, pp. 68-82. Ohtsuka, S., Komatsu, T., Kondoh, Y. and Takahashi, H. (1984) Oxygen absorbent package has absorbent in three layer wall including microporous plastics film or nonwoven fabric. US Patent 4485133. Paine, F.A. (1962) Fundamentals of Packaging, Blackie and Son, Glasgow, 62. Patterson, B.D., Jobling, J. and Moradi, S. (1993) Water relations after harvest - new technology helps translate theory into practice. Proceedings of the Australasian Postharvest Conference, University of Queensland, Gatton, Queensland, Australia, pp. 99-102. Patterson, B.D. and Joyce, D.C. (1993) A package allowing cooling and preservation of horticultural produce without condensation or desiccants. International Patent Application PCT/AU93/00398. Rabek, J.F. and Ranby, B. (1975) Photodegradation, Photo-oxidation and Photostabilization of Polymers, John Wiley & Sons, London. Rooney, M.L. (1982) Oxygen scavenging from air headspaces by singlet oxygen reactions in polymer media. J.Food ScL, 47, 291-4, 298. Rooney, M.L. (1982b) Oxygen Scavenging: a novel use of rubber photooxidation. Chem. Ind., 197-8. Rooney, M.L. (1994) Oxygen-Scavenging Plastics Activated for Fresh and Processed Foods. Abstracts Annual Convention of the Institute of Food Scientists and Technologists, Atlanta. Rooney, M.L. and Holland, R.V. (1979) Singlet oxygen: an intermediate in the inhibition of oxygen permeation through polymer films. Chem. Ind., 900-1. Rooney, M.L. Holland, R.V. and Shorter, A J . (1981) Removal of headspace oxygen by a singlet oxygen reaction in a polymer film. J. ScL Food Agric, 32, 265-72. Rooney, M.L., Maloba Wakwabubi, F., Wormell, P. and Nguyen, M. (1994) Switched-on Active Packaging for Foods. Abstracts 27th Annual Convention, Australian Institute of Food Science & Technology, Canberra, Australia, 9-13 May. Scholle, W.R. (1976) Multiple wall packaging material containing sulfite compound. US Patent 4041209. Shimagawa Nenryo KK (1992) Freshness maintaining packaging body - has a free-oxygen absorber on outside and an oxygen penetrating resin material containing silver ion substituted alumino-silicate formed on inside. Japanese Patent 4,072,168. Shirazi, A. and Cameron, A.C. (1992) Controlling relative humidity in modified atmosphere packages of tomato fruit. HortScience, 13, 565-9. Showa Denko KK (1990) Heat shrinkage laminate wrapping material -for meat has water absorbing agent and water permeable film with oxygen barrier layer. Japanese Patent 2192941. Speer, D.V., Roberts, W.P. and Morgan, C R . (1993) Methods and compositions for oxygen scavenging. US Patent 5211875. Speer, D.V. and Roberts, W.P. (1993) Improved oxygen scavenging. PCT Application, US 93/09125. Tallgren, H. (1938) Keeping food in closed containers with water carrier and oxidizable agents such as Zn dust, Fe powder, Mn dust etc. British Patent 496935. Tannenbaum, S.R. (1976) Vitamins and minerals. In: Food Chemistry, VoI 1., O. Fennema, (ed.), Marcel Dekker, Inc., New York, 347-86. Tokas, E.F. (1979) Dry blend of styrene polymer and myrcene scavenger - with low residual monomer content, useful as packaging. US Patent 4180486. Toppan Printing Company (1992) Oxygen absorbent composition which functions in dry atmospheres - contains unsaturated fatty acid, oxidation catalyst, active clay and deodorant. Japanese Patent Application 4298231. Tsai, B.C. and Wachtel, J.A. (1990) Barrier properties of ethylene-vinyl alcohol copolymer in retorted plastic food containers. In: Barrier Polymers and Structures, W J . Koros (ed.), American Chemical Society, Washington D C , 192-202.
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5
Edible films and coatings as active layers B. CUQ, N. GONTARD and S. GUILBERT
5.1
Introduction
Edible films and coatings are traditionally used to improve food appearance and conservation. The most common examples are wax coatings for fruit (used in China since the 12th century), chocolate coatings for confectionery, lipid films to protect meat products, and soy milk-based lipoprotein films to improve the appearance and preservation of certain foods in Asia. Formulations for edible films or coatings must include at least one component able to form a suitably cohesive and continuous matrix. The basic materials can be classified in three categories: polysaccharides, proteins and lipidic compounds. Polysaccharides (vegetable and microbial gums, starches, celluloses and derivatives, etc.) have good film-forming properties. Films formed from these hydrophilic compounds provide efficient barriers against oils and lipids (Murray et al., 1972), but their moisture barrier properties are poor. Although not as extensively studied, protein-based films have highly interesting properties. Many protein materials have been tested: collagen, zein, wheat gluten, ovalbumin, soybean, casein, etc. (Guilbert and Biquet, 1989). The mechanical and barrier properties of these films are generally better than those of polysaccharidebased films; this is due to the fact that, contrary to polysaccharides which are monotonous polymers, proteins have a specific structure which confers larger potential functional properties (Guilbert and Graille, 1994). Many lipidic compounds, such as animal and vegetable fats (natural waxes and derivatives, acetoglycerides, surface-active agents, etc.), have been used to make edible films and coatings (Guilbert and Biquet, 1989; Kester and Fennema, 1986). They are generally used for their excellent moisture barrier properties, but there can be problems concerning stability (particularly oxidation), texture and organoleptic quality (opacity, waxy taste). Edible films and coatings formed with several compounds (composite films) have been developed to take advantage of the complementary functional properties of these different constitutive materials and to overcome their respective drawbacks. Most composite films studied to date combine a lipidic compound and a hydrocolloid-based structural matrix (Cole, 1969; Daniels, 1973; Gontard et al.9 1994a; Guilbert, 1986; Kamper and Fennema, 1984a, b). Coatings are formed directly on the food product using either liquid filmforming solutions (or dispersions) or molten compounds (e.g. lipids). They
can be applied by different methods: with a paint brush or by spraying, dipping-dripping, fluidizing, etc. Films are preformed separately from the food product. They can be produced, for instance, by drying a film-forming solution on a drum-drier, by cooling a molten compound, or through standard techniques used to form synthetic packagings, e.g. thermoforming or extrusion techniques for thermoplastic materials (Guilbert and Biquet, 1989; Kester and Fennema, 1986). The production process from a film-forming solution generally includes a first step with macromolecule solubilization in a solvent medium (often water-, ethanol- or acetic acid-based) which can contain several additives (plasticizers, crosslinking agents, solutes, etc.). The film-forming solution is spread in a thin layer, usually followed by a drying treatment. The functional properties of the film are dependent on a number of parameters (Gontard, 1991; Gontard et al.9 1992): formulation (characteristics and concentration of the basic and secondary components, pH, denaturing conditions, etc.), film-forming conditions (type of surface upon which the film-forming solution is spread, drying conditions) and conditions in which the film is used (temperature, relative humidity). The degree of cohesiveness of the matrix is a critical parameter affecting the functional properties of edible films (Banker, 1966). It is sometimes difficult to obtain adequate adhesion of the film to the food product, for instance when a hydrophobic film-forming material is used to protect a hydrophilic food product. In such cases, surface-active agents can be coated on the food or added to the film-forming solution, or a material capable of adhering to both components can be applied as an intermediate precoating (e.g. precoating with cocoa before sugar-coating peanuts). An edible film is an integral part of the food product it encloses and therefore must have neutral sensorial properties (or compatible with product nature) so as not to be detected during consumption. Application of an edible barrier layer is an easy means to structurally strengthen certain foods, to reduce particle clustering and to improve the visual and tactile features on the surface of the product. For example, Allen et al. (1963b) used alginate and cornstarch coatings to improve texture juiciness, general appearance, surface texture and color of beef steaks and pork chops. Edible films can also be used to package components or additives that are to be dissolved in hot water or food mixes (Daniels, 1973; Kroger and Igoe, 1971). Edible films or coatings act as an additional parameter for improving overall food quality and stability. They represent one way to apply hurdle technology to solid foods without affecting their structural integrity (Guilbert, 1994; Guilbert et aL, 1995). Many functions of edible films are the same as those of synthetic packaging (water, gas and solute barriers, mechanical properties, opacity, etc.). However, they must be chosen according to their specific application (i.e. type of food product and main deterioration mechanisms).
Films with substantial gas and moisture barrier properties are required for many applications: to control gas exchange for fresh foods and oxygen exchange for oxidizable foods and to reduce moisture exchange with the external atmosphere (Guilbert and Biquet, 1989). Retention of specific additives in edible films can lead to a functional response generally confined to the surface of the product (modification and control of surface conditions), (De Savoye et ai9 1994; Guilbert, 1988; Torres et aL, 1985a, b; Torres and Karel, 1985). Oil and solute penetration into foods during processing can also be limited by edible coatings (Daniels, 1973; Guilbert and Biquet, 1989). In this chapter, we will expand on the use of edible films and coatings as active layers, i.e when the edible film contributes by itself to the preservation. Figure 5.1 gives a schematic representation of food preservation with edible films and coatings as active layers when the first mode of deterioration results from respiration, from dehydration or moisture uptake, or from surface microbial development or oxidation. The protective features of edible films and coatings are dependent on gas and water vapor barrier
Food additives (e.g. antioxygen and antifungic agents)
Diffusion of food additives
Gas Transfers
Storage
Water Transfer
Food additives in
AOffiWOLM
SURFACE RETENTION of food additives
CONTROL of Gas Transfers
Storage
CONTROL of Water Transfer Figure 5.1 Schematic representation of food preservation with (top) or without (bottom) edible films and coatings as active layers, when the first mode of deterioration results from respiration (a), from dehydration or moisture uptake (b), or from microbial development or oxidation (c).
properties, on modification of surface conditions and on their own antimicrobial properties. 5.2 Use of edible active layers to control water vapor transfer Moisture transfers due to water vapor pressure or concentration gradients have major effects on the organoleptic and microbiological qualities of food products. For example, products such as raisins (Bolin, 1976; Kochnar and Rossell, 1982; Lowe et a/., 1963; Marston, 1983; Watters and Brekke, 1961), candies and chocolate-coated products (Andres, 1984; Barron, 1977; Cosier, 1957; Feuge, 1970; Jokay et al., 1967), dry crackers, biscuits, pizza crust, and filled bakery crust (Dhale, 1983; Heiss, 1968; Katz and Labuza, 1981; Labuza, 1985) may become unsatisfactory as a result of moisture gain or loss. Moisture exchanges are difficult to control in multicomponent foods such as mixtures of dehydrated foods, jelly-filled cookies, pies, and pizzas. Moisture transfer can be limited by reducing the vapor pressure gradient between components, e.g. using aw lowering agents (salts, sugars, polyols, etc.). However, this is not always possible and may result in drastic modifications of the sensory and physiochemical characteristics of the product (Guilbert, 1984, 1985; Karel, 1976). Another solution consists in using edible films or coatings with good moisture barrier properties to separate compartments and thus control further moisture transfer (Guilbert, 1986; Kamper and Fennema, 1985). Surface drying on some fresh and frozen foods or, inversely, moisture uptake in dry or semi-moist foods, can also be hindered by using films with a low water permeability (as schematized in Figure 5.1). Permeability is defined as a state which permits the transmission of permeants through materials (Mannheim and Passy, 1985). When there are no pores, faults or membrane punctures, permeability P, is equal to the product of the diffusion coefficient D, representing the mobility of permeant molecules in the polymer, and the solubility coefficient S, representing the permeant concentration in the film in equilibrium with the external pressure: P = DxS In practice, water, gas or solute permeability, P, through a membrane is determined by steady-state measurements: P =
AWx
AtAAp
where AW is the permeant weight that passes through a film of thickness x and area A; where At is the time and Ap is the differential partial pressure across the film. The diffusion coefficient can be obtained by taking
measurements before the steady state is reached. The solubility coefficient can either be calculated from P and D, or measured in a separate experiment (sorption isotherms). The diffusion and solubility of permeants are affected by temperature and by the size, shape and polarity of the diffused molecule. Moreover, these two parameters depend on film characteristics, including the type of forces influencing molecules of the film matrix, the degree of cross-linking between molecules, the crystallinity, the presence of plasticizers or additives, etc. (De Leiris, 1985; Gontard et ah, 1993; Kumins, 1965; Pascat, 1985; Schwartzberg, 1985). Permeability is only a general feature of films or coatings when the diffusion and solubility coefficients are not influenced by permeant content, i.e. when Fick's and Henry's laws apply. In practice, for most edible films, the permeant interacts with the film and the D and S coefficients are dependent on the difference in partial pressure. For instance, in relation to the water vapor permeability of hydrophilic polymer films, the water solubility and diffusion coefficients increase when the water vapor differential partial pressure increases because of the moisture affinity of the film (nonlinear sorption isotherm) and because of increased plasticization of the film due to water absorption (Gontard et ai, 1993; Schwartzberg, 1985). The film thickness can also influence permeability when using film-forming materials that do not behave ideally. The permeability of an edible film is thus defined as a property of the film-permeant complex, under specified temperature and water activity conditions. Water vapor permeabilities of some edible and synthetic films are given in Table 5.1. Permeability is clearly high in edible films formed from hydrophilic materials. These films can only be used as protective barrier layers to limit moisture exchange for short-term applications or in lowmoisture foods such as dried fruits (Forkner, 1958; Swenson et aLy 1953). Lipidic compounds are often used to make moisture barrier films and coatings (Table 5.1). For instance, coating fresh fruit and vegetables with wax reduces desiccation-induced weight loss during storage by 40-75% (Kaplan, 1986). Water is not very soluble or mobile in lipid-based films because of the low polarity and dense, well-structured molecular matrixes that can be formed by these compounds. Moisture resistance of lipid films is inversely related to polarity of the lipids. Hydrophobic alkanes and waxes, such as paraffin wax and beeswax, are the most effective barriers. More hydrophilic lipids, such as fatty acids, are less resistant to water vapor transmission since their polar groups attract migrating water molecules and thereby facilitate water transport. The moisture barrier capacities of different films can be classified in increasing order of efficiency, as follows: liquid oils < solid fats < waxes (Gontard et ah, 1994a; Kamper and Fennema, 1984b). This efficiency order has been
Table 5.1 Water vapor permeability of various films
Film Starch, cellulose acetate(1) Wheat gluten(3) Casein-gelatin(20) Wheat gluten(l5) Sodium caseinate(2) Cornzein(33> HPC and PEG(I8) MC and PEG(I8) MC and PEG(5) Corn zein(18) HPMC(13) Glycerol monostearate(16) MC (I6) Wheat gluten and glycerol(9) Wheat gluten and oleic acid(10) Wheat gluten and carnauba wax(l0) Wheat gluten(8) HPC (I9) Wheat gluten and soy protein(7) Wheat gluten and mineral oil(8) Corn zein and oleic acid(12) HPMC and palmitic acid(l4) Dark chocolate(4) MC and beeswax bilayer
Water vapor permeability (X 10'2 mol m m~2 s"1 Pa"1)
T (0C)
Thickness (X 103m)
RH % conditions
142 69.7 34.3 34.2 24.7 22.8 13.7 13.6 7.78 6.45 5.96 5.85 5.23 5.08 4.15 3.91 3.11 2.89 2.84 2.28 1.48 1.22 0.707 0.199 0.0482 0.0360 0.0320 0.0230 0.0185 0.0122 0.0122 0.000289
38 26 30 21 25 26 21 21 25 21 27 21 30 30 30 30 23 30 23 23 38 25 20 25 38 25 25 30 25 38 25 38
1.190 0.250 0.400 0.130 0.100 0.025 0.200 0.019 1.750 0.075 0.050 0.050 0.050 0.127 0.075 0.075 0.125 0.040 0.040 0.610 0.025 0.051 0.100 0.090 0.100 0.025 0.120 0.025
100-30 100-50 60-22 85-00 100-00 100-50 85-00 85-00 52-00 85-00 85-00 100-75 11-00 100-00 100-00 100-00 11-00 11-00 11-00 11-00 95-00 97-67 81-00 100-00 95-00 97-00 100-00 100-00 100-00 97-00 97-00 95-00
(According to Allen et al, 1963a(1); Avena-Bustillos and Krochta, 1993(2); Aydt et al, 1991(3); Biquet and Labuza, 1988(4); Donhowe and Fennema, 1993a(5); Donhowe and Fennema, 1993b(6); Gennadios et al, 199(F; Gennadios et al, 1993a(8); Gontard, 1991(9); Gontard et al, 1994a(10); Greener and Fennema, 1989a(ll); Guilbert and Biquet, 1989(l2); Hagenmaier and Shaw, 1990(13); Kamper and Fennema, 1984a(l4); Kester and Fennema, 1989a(15); Landman et al, 1960(16); Myers et al, 1961(17); Park and Chinnan, 1990(l8); Park et al, 1991(19); Schultz et al, 1949(20).) (HPC = hydroxypropylcellulose; HDPE = high-density polyethylene; HPMC = hydroxypropyl methylcellulose; LDPE = low-density polyethylene; MC = methylcellulose; PEG = polyethylene glycol; RH = relative humidity; T = temperature).
confirmed by Kester and Fennema (1989a) in a study on the resistance of various lipids, heated and adsorbed into filter papers, to water vapor transmission. Composition, fusion and solidification ranges, lipid crystalline structure, in addition to the interaction with water, oxygen and other components of the food product influence the physico-chemical, functional and organoleptic properties of lipid-based films (Fennema et al, 1994; Kamper and Fennema, 1984a, b; Kester and Fennema, 1989a).
In general, the rate of transmission of water through a lipid film increases as the length of the lipid hydrocarbon chain is decreased and the degree of unsaturation or branching of acyl chains is increased (Archer and Lamer, 1955; Fettiplace, 1978; Fettiplace and Haydon, 1980; Kamper and Fennema, 1984b and 1985). This is a consequence of enhanced mobility of hydrocarbon chains and less efficient lateral packing of acyl chains caused by a reduction of interchain van der Walls' interaction (Jain, 1972; Taylor et al., 1975). However, specific information on the water vapor barrier properties of films of more hydrophobic lipids is lacking, and almost all the information available has been gathered using a 100-0% RH gradient which is not commonly encountered during storage of foods under commercial conditions. The water vapor barrier properties of a lipid-hydrocolloid composite film is generally determined by the potentials of its component parts (Table 5.1). The coating operation used, i.e. emulsion (suspension or dispersion of nonmiscible compounds), successive layers (multilayered films) or solutions with a common solvent, affects the water vapor barrier properties of these films (Table 5.2). According to Schultz et al (1949), Martin-Polo et al (1992), Debeaufort et al (1993) and Gontard et al (1994a), who investigated the moisture permeability of multicomponent films composed of methylcellulose, pectinate or gluten and various lipids (waxes, fatty acids, etc.), it is better to form two successive layers than to apply a dispersion in solvent. Kamper and Fennema (1984a, b) carried out detailed studies of films, composed of soluble cellulose esters and a mixture of palmitic and stearic acids, and Table 5.2 Effect of coating operation on water vapor permeance of edible multicomponent films composed of MC and paraffin wax (weight ratio 1:1); gluten and beeswax (at 2.4 mg/cm2); and HPMC and stearic-palmitic acids (at 9 mg/cm2) Water vapor permeance (X 108 mol m~2 s'1 Pa'1) _
T (0C) _
MC and paraffin wax (emulsion)(1) MC and paraffin wax (bilayer)(1)
7.39 0.251
25 25
0.090 0.130
84-22 84-22
Glutten(2) Gluten and beeswax (emulsion)(2) Gluten and beeswax (bilayer)(2)
10.1 0.727 0.0257
30 30 30
0.050 0.120 0.090
100-00 100-00 100-00
25 25 25
0.040 0.040 0.125
85-00 85-00 85-00
Film _ _
HPMC(3) HPMC and fatty acids (emulsion)(3) HPMC and fatty acids (bilayer)(3)
8.02 0.00643 1£7
Thickness RH % (X 103 m) conditions _ _ 84_22
(According to Debeaufort et al., 1993(l); Gontard et aLy 1994a(2); Kamper and Fennema, 1984b(3).) (HPMC = hydroxypropylmethylcellulose; MC = methylcellulose; RH = relative humidity; T = temperature.)
demonstrated that application of emulsions resulted in reducing moisture permeability by 10-fold relative to bilayer systems. Variations in homogeneity and/or structure (size, form and orientation of the crystals) of the lipid layer related to the film-forming process and to coating operation, could explain this discrepancy. Permeability of composite films decreases substantially when the proportion of lipids increases. For instance, an increase of glyceride sucro ester (beeswax, stearic acid or monoglycerides) contents in gluten- (caseinate- or hydroxypropyl methylcellulose-) based composite films, causes a substantial reduction in water vapor permeability (Figure 5.2), (Avena-Bustillos and Krochta, 1993; Gontard et al, 1994a; Hagenmaier and Shaw, 1990). The number of hydrophobic residues (from lipid derivatives) in the matrix affects the water interaction potential, and consequently the barrier properties. Solidification of lipids (especially saturated) in a densely organized crystalline structure results in a very significant reduction in moisture permeability (Kamper and Fennema, 1984b; Kester and Fennema, 1989b; Landman et ai, 1960; Watters and Brekke, 1961). Water vapor permeability variations relative to aw reveal the non-Fickian behaviour of biological materials. At low aw, water diffusion, and especially the water solubility coefficient, remains relatively low: film permeability is minimal at this point. As expected in a hydrophilic film (Barrie, 1968; Biquet and Labuza, 1988; Kamper and Fennema, 1984a; Crank, 1975; De Leiris, 1985; Pascat, 1986; Schwartzberg, 1985), increasing aw leads to an
Water Vapor Permeability (i ltf2 mol. m . m*. s l . Pa *)
Lipid Materials Content (%w/w) Figure 5.10 Effect of reduced surface pH on the microbiological quality of an intermediate moisture cheese analog coated with a carrageenan and agarose film; challenged with Staphylococcus aureus S-6 (aw = 0.88 and 35°C) (after Torres and Karel, 1985).
increase in film moisture content (rise in the sorption isotherm) and so induces an increase in water vapor permeability. At high aw, extensive swelling of the protein network with water probably enhances water molecule diffusion and such films would clearly not be efficient water vapor barriers (Figure 5.3). Many authors have studied the effect of temperature on water vapor transfer using synthetic, simple edible films (protein or cellulose-based) or composite edible films (cellulose derivatives and lipids). A rise in temperature causes an increase in water vapor permeability (Figure 5.4). This is often characterized by Arrhenius-type representations (Donhowe and Fennema, 1993b; Higuchi and Aguiar, 1959; Kester and Fennema, 1989a, c). In films formed with hydrophilic materials, temperature-dependent variations in barrier properties are affected by the moisture level (see Figure 5.3 for gluten films). However, it was difficult to interpret the temperature dependence of the effect of hydration on water vapor permeability of gluten film in terms of disruptive water-polymer hydrogen bonding in a polymer hydrogen-bonded network. The critical role of water as plasticizer of gluten film appeared to be highly temperature dependent. According to Levine and Slade (1987) and Slade et al (1989), the structure/property relationships of hydrated proteins, and particularly of gluten, could be better understood through the theories of glass transition used in polymer science, in terms of critical variables of time, temperature and moisture content. Glass transition
Water Vapor Transmission Rate (WVT), (x 108 mol. m . iff2. r 1 )
Water activity Figure 5.3 Effect of average water activity on water vapor transmission rate of edible films composed of methylcellulose and palmitic-stearic acids (at 0.76 mg/cm2) at 25°C and 32% RH gradient (according to Kamper and Fennema, 1984a), and of edible wheat gluten films at 5, 30 and 500C, and 10% RH gradient (according to Gontard et al., 1993). •, MC and C 1 8 -C 1 6 at 25°C (WVT x 0.1); D, gluten at 500C; D, gluten at 300C; D, gluten at 5°C
hydrated proteins, and particularly of gluten, could be better understood through the theories of glass transition used in polymer science, in terms of critical variables of time, temperature and moisture content. Glass transition is typically described as a transition from a brittle glass to a highly viscous or rubbery solid. It is well established that plasticization by water affects the glass transition temperature of amorphous or partially crystalline proteins such as gluten (Gontard et al., 1993; Hoseney et al., 1986), gelatin (Marshall and Petrie, 1980; Yannas, 1972), collagen (Batzer and Kreibich, 1981) and elastin (Kakivaya and Hoeve, 1975; Scandola et al., 1981), thus resulting in a drop in the glass transition temperature. The anomalous diffusion behavior of glassy polymers may be directly related to the influence of the changing polymer structure on solubility of the penetrant and diffusional mobility of the penetrant (Crank, 1975). The water vapor barrier properties of edible active films and coatings have been used in several food systems. For example, meat coatings composed of corn starch and alginate were used by Allen et al. (1963a) to reduce moisture loss by 40-48%. Avena-Bustillos et al. (1993) found that sodium caseinate and stearic acid emulsion coatings improved the storage stability and reduced water loss of peeled carrots. The use of sucrose esters of fatty acids, mono- and diglycerides, and the sodium salt of carboxymethylcellulose coatings allowed the increase of water vapor resistance (up to 75%) in zucchini fruit (Avena-Bustillos et al., 1994). Wax is commercially applied to
Water Vapor Permeance (WVP), (x 10* mol.m . m 2 . s l )
Temperature (0C) Figure 5.4 Effect of temperature on water vapor permeance of edible wheat gluten films, at RH conditions 90-80% (according to Gontard et al., 1993); of edible glycerol monostearate films, at RH conditions 100-0% (according to Higuchi and Aguiar, 1959); and of edible films composed of methylcellulose and beeswax, at RH conditions 97-0% (according to Kester and Fennema, 1989b). a, Gluten (WVP X 0.1); A, MC and beeswax; •, monostearate.
many fruits and vegetables to reduce dehydration and improve consumer appeal (Hall, 1981), e.g. to oranges (Albrigo and Brown, 1970; Bursewitz and Singh, 1985; Eaks and Ludi, 1970), to apples (Hall, 1966; Trout et al, 1953), to prune plums (Bain and McBean, 1967), and to sweet cherries (Drake et aL, 1988; Lidster, 1981). Ukai et al (1976) used hydrophobic emulsions to coat fruits and vegetables such as orange, peas, apples, green beans, tomatoes, pears, and peaches. Lidster (1981) suggested the use of xanthan gum applied to cherries as a postharvest dip to prevent water loss. El Ghaouth et al. (1991) used edible chitosan coating to reduce water loss of cucumber and bell pepper fruits. The use of edible films to lessen internal migration of moisture in foods has been studied by several investigators, and some of these films appear promising for commercial use (Fennema et al, 1993; Greener and Fennema, 1989b; Kamper and Fennema, 1985; Kester and Fennema, 1989d; Rico-Pena and Torres, 1990). One standard example of moisture transfer control in heterogeneous food is the wafer-ice cream system of ice cream cones. In standard conditions, the wafer loses its crispness after three months' storage at -23°C. This qualitative degradation is due to moisture transfer between the ice cream and the wafer. Rico-Pena and Torres (1990) tested composite films (methylcellulose and chocolate and methylcellulose, chocolate and palmitic acid) to separate the two components. Films can limit moisture transfer during ice cream cone storage at -23 0 C or -12°C and preserve wafer crispness. Films based on methylcellulose, chocolate and palmitic acid are the most efficient. In addition, several studies have confirmed the efficiency of chocolate and cocoa butter coatings as moisture barriers (Biquet and Labuza, 1988; Kempf, 1967; Landman et al, 1960; Neidiek, 1981; Soboleva and Chizhikova, 1978; Tiemstra and Tiemstra, 1974). These examples show that water vapor transfer in food products can be controlled using edible films and coatings. The knowledge of the influence of parameters such as structure and composition of film-forming materials, allows modulation of the water barrier properties of these edible active layers either on the food surface or in multicomponent foods. 5.3 Use of edible active layers to control gas exchange The gas barrier properties of edible films and coatings are potentially of great interest. For instance, edible oxygen barrier films can be used to protect foods that are susceptible to oxidation (rancidity, loss of oxidizable vitamins, etc.). In contrast, a relatively high gas permeability is necessary for fresh fruit and vegetable coatings (especially carbon dioxide permeability). The development of edible films with selective gas permeability (oxygen, carbon dioxide, ethylene) allows the control of respiration exchange and microbial development and seems very promising for achieving a * modified
atmosphere' effect in fresh fruit and for improving the storage potential of these products (Smith et a/., 1987; Trout et al., 1953), as schematized in Figure 5.1. Gas permeability can be measured using air porosimeters and specific permeability cells. As in the case with moisture barrier properties, the formulation, manufacturing and environmental parameters have an impact on a film's response to gases. The oxygen and carbon dioxide permeability values of various edible films and synthetic films are given in Table 5.3. Films formed with hydrocolloids (proteins, polysaccharides) generally have good oxygen barrier properties, particularly under low-moisture conditions. The oxygen permeability of hydrocolloid-based films (at 0% relative humidity) is often lower than that of common synthetic films such as polyethylene and non-plasticized PVCs. For example, the oxygen permeability of wheat gluten film was 800 times lower than that of low-density polyethylene and twice as low as that of polyamide 6, a well-known high oxygen barrier polymer. Films formed with lipid derivatives have suitable oxygen barrier properties. For example, the oxygen permeability value for beeswax film lies between those of low-density and high-density polyethylene (Table 5.3). According to Blank (1962 and 1972), lipids with the best oxygen barrier properties are those formed with straight-chain and saturated fatty acids. Increased unsaturation (or branching) and a reduction in the length of the carbon chain result in decreased oxygen permeability. The following barrier efficiency order was observed by Kester and Fennema (1989c): stearic alcohol > tristearine > beeswax > acetylated monoglycerides > stearic acid > alkanes. These differences can be explained by the presence of pores or cracks, and by the homogeneity of the composition density of the network (Kester and Fennema, 1989b, c). The network density is dependent on the polymorphic shape and orientation of the chains and morphological differences in the lipid layers. As previously mentioned for water vapor permeability, formulation of composite films allows advantage to be taken of the complementary barrier properties of each component. At high aw, where hydrophilic materials are not effective as gas barriers (see below), the addition of lipidic compounds results in a decrease in the gas permeability of the film. For example, at aw = 0.91, the oxygen permeability is reduced by about 30% for a composite gluten and beeswax film (Table 5.3). The effect of temperature on gas permeability is similar to that reported for water vapor permeability (Donhowe and Fennema, 1993b; Gennadios et al.9 1993). These variations can be characterized by Arrhenius-type representations. But, as far as gas solubility decreases with temperature increase, the increase of gas permeability with temperature is lower than for water vapor permeability (Gontard et a/., 1994b). High aw conditions cause an increase in gas permeability in hydrophilic
Table 5.3 Oxygen and carbon dioxide permeabilities of various films Film _ _ _
O2 Permeability (X 1O18HIoImIn-2S-1Pa-1) __
HDPE (3) Rigid PVC (II) PET<3) Polyamide 6(3) PVDC (3) Cellophane(3) EVOH (H) Polybutadiene<3) Polypropylene(3) LDPE (l3) Flexible PVC film(3) Uncoated cellulose 0} HDPE (3) HDPE (I) Cellophane (H) Polyvinyl alcohol(1) PET (3) PET™ Nylon 6 (I) Cellophane (l4) EVOH (I3) MCandPEG ( I O ) MCandPEG ( 1 0 ) MC and beeswax<9) Beeswax<5) HPC and PEG(IO) HPC and PEG(IO) MC and palmitic acid (l2) Carnauba wax(5) Corn zein(2) Cornzein (2) Wheat gluten and glycerin(6) Cornzein ( l 0 ) Wheat gluten protein (l0) Wheat gluten protein (l0) Corn zein(10) Wheat gluten protein(2) Soy protein(6) Wheat gluten and mineral oil(7) Wheat gluten(8) Wheat gluten and soy protein(4) Wheat gluten protein(2) Pectin<8) Wheat gluten(8) Wheat gluten(8)
T (0C) __
0 0
972 37.6 19.5 7.65 -
23 23 23 23 23 23 23
0.0 0.0 0.0 0.0 0.0 0.0 0.0
703 -
23 23 23 23 23 23 23 23 23 23 23 23 23 23
0.50 0.50 0.50 0.50 i.00 0.50 1.00 0.95 1.00 0.50 1.00 1.00 0.50 0.95
29900 28900 -
30 21 25 25 30 21 24
0.0 0.0 0.0 0.0 0.0 0.0 0.0
81.1 34.8 19.6
216 -
25 38 23 23
0.0 0.0 0.0 0.0
16.1 8.92
-
30 30
0.0 0.0
119
21
0.0
285 16.0 11.9 11.6 1.86 1.34 0.155 19400 1170 961 682 406 227 224 130 50.8 24.3 12.2 10.2 8.27 6.20 522 499 480 470 190
-
CO2 Permeability (X 1018 mol m m 2 s 1 Pa 1 ) _ _
aw
3.50 2.30 1.70
95.0 -
21 38 23 23
0.0 0.0 0.0 0.0
1.24 1.19
-
25 25
0.0 0.0
1.20 21300 36700 24500
23 25 25 25
0.0 0.96 0.95 0.91
1340 1290 982
Table 5.3 Continued Film Wheat gluten and beeswax (8) Chitosan(8) MC and palmitic acid(12>
O 2 Permeability CO 2 Permeability T ( X 10 l 8 molm HT2S-1 Pa"1) ( X 1018 mol m m"2 s"1 Pa"1) (0C)
aw
687
6614
25
0.91
472 407
8010 -
25 24
0.93 0.79
(According to Ashley, 1985 (l) ; Aydt et al., 1991 (2) ; Bakker, 1986(3); Brandeburg et al, 1993(4); Donhowe and Fennema, 1993b(5); Gennadios et al, 1990(6); Gennadios et al, 1993a(7); Gontard et al., 1994b(8); Greener and Fennema, 1989a(9); Park et Chinnan, 1990(l0); Poyet, 1993 (n) ; Rico-Pena and Torres, 1990 (l2) ; Salame, 1986 (l3) ; Taylor, 1986(l4).) (aw = water activity; EVOH = ethylene-vinyl alcohol; HDPE = high-density polyethylene; LDPE = low-density polyethylene; MC = methylcellulose; PEG = polyethylene glycol; PET = polyester; PVDC = poly(vinylidene chloride); PVC = poly(vinyl chloride); T = temperature.)
films but generally not in synthetic hydrophobic films which are not water sensitive (Figure 5.5). In these hydrophilic films, increased aw promotes both gas diffusivity (due to the increased mobility of hydrophobic macromolecule chains) and gas solubility (due to the water swelling of the matrix), leading to a sharp increase in gas permeability (Kumins, 1965). The effects of aw on gas barrier properties of composite films (methylcellulose and palmitic acid) and of gluten protein-based films was studied by Rico-Pena and Torres (1990) and by Gontard et al. (1994b) respectively, and are compared in Figure 5.5 with oxygen permeabilities of synthetic hydrophilic films. Carbon dioxide permeability in hydrocolloid-based films is often much Oxygen Permeability (x 10* mol. m. m 2 . s 1 . P a ! )
Water activity Figure 5.5 Effect of water activity on oxygen permeability of edible and synthetic films at 25°C (according to Gontard et al, 1994b (o, gluten); Poyet, 1993 (A, EVOH; •, Nylon 6); Rico-Pena and Torres, 1990 (D, MC and palmitic acids, at weight ratio 3:1); Rigg, 1979 (•, cellophane)). (MC is methylcellulose; EVOH is ethylene-vinyl alcohol).
higher than oxygen permeability (Table 5.3). The effect of film aw on carbon dioxide permeability is similar to that on oxygen permeability, but the sharp increase of permeability is more important. This could be explained by the differences in water solubility of these gases (Schwartzberg, 1985), i.e. carbon dioxide is very soluble (carbon dioxide solubility in water = 34.5mmol/l at 25°C and 105 Pa; oxygen solubility = 1.25mmol/l at 25°C and 105 Pa). At high aw, the addition of lipidic components to gluten film results in a high decrease of carbon dioxide permeability. For example, at aw = 0.91, the carbon dioxide permeability is reduced by about 75% for a composite gluten and beeswax film (Table 5.3). This could be related to the hydrophobic characteristics of these components which for the same aw reduce the amount of water available for solubilisation of carbon dioxide. The selectivity coefficient between carbon dioxide and oxygen is defined as the ratio of the respective permeabilities of both gases. In hydrophilic materials, the effect of an aw increase on permeability is greater for carbon dioxide than for oxygen. The selectivity of these materials is thus sensitive to moisture variations (for example, the selective coefficient of edible gluten films varies from 4.0 at aw = 0.30, to 25 at aw = 0.95), (Gontard et aL, 1994b), whereas the selectivity coefficient for synthetic polymers remains relatively constant, at 4 to 5 (Table 5.4). Edible films with selective gas permeability can be applied to reduce degradation of some fresh fruits and vegetables. In fact, the diffusion of Table 5.4 Gas selectivity coefficient of various films (CO2/O2) Gas selectivity coefficient (CO2/O2)
T (0C)
aw
PP(2) PA(2) PVC(2) Wheat gluten(3) Corn zein(3) HPC and fatty acid(3) MC and PEG(3) HPC and PEG(3)
4.0 4.2 5.8 7.5 9.5 23.7 31.6 40.6
23 23 23 25 25 25 25 25
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Gluten(I) Gluten(1) MC and beeswax(l) Gluten and beeswax 0} Pectin(l) Chitosan(1) Gluten(l) Gluten*'>
4.0 4.5 6.7 9.6 16.0 17.0 25.0 2$A
25 25 25 25 25 25 25 25
0.35 0.62 0.56 0.91 0.96 0.93 0.91 0.95
Film
(According to Gontard et aL, 1994b(1); Lefaux, 1972(2); Park and Chinnan, 1990(3).) (aw = water activity; HPC = hydroxypropylcellulose; MC = methylcellulose; PA = polyamide; PEG = polyethylene glycol; PP = polypropylene; PVC = poly(vinyl chloride).)
oxygen and carbon dioxide between fruit or vegetables and the environment is a basic element in the post-harvest physiology of fruit (Burton, 1974, 1978; Cameron and Reid, 1982). The type of coating material applied alters the relative effects on the skin permeability to oxygen and carbon dioxide (Trout et a/., 1953). For example, waxing increased the carbon dioxide content and decreased the oxygen content of the orange's internal atmosphere (Eaks and Ludi, 1960). The coating of peach surfaces with beeswaxcoconut oil emulsion decreased oxygen gas transfer (Erbil and Muftugil, 1986). Chitosan coatings retarded ripening and prolonged storage life of tomatoes, cucumber and bell pepper fruit without affecting their ripening characteristics (El Ghaouth et a/., 1990, 1991). Lowings and Cutts (1982) reported that an edible composite coating (carboxymethylcellulose, sucrose fatty acid esters and mono- and diglycerides) would produce a semipermeable modified atmosphere within fresh fruit after application. The post-storage uses of these coatings were investigated on apples (Chu, 1985; Drake et a/., 1987; Elson et al., 1985; Smith and Stow, 1984), on pears (Elson et al.y 1985), on bananas (Banks, 1983, 1984; Lowings and Cutts, 1982), on limes (Motlagh and Quantick, 1988) and on mangoes (Dhalla and Hanson, 1988). For example, the selective gas permeability properties of these films applied on bananas can reduce oxygenfluxfivefold,while carbon dioxide flux is only decreased by about half (Banks, 1984). Other examples of gas permeability control by edible active layers are gelatin films used to protect frozen meats from rancidity (Klose et ah, 1952), to coat candies and dried products (Grouber, 1983) and to microencapsulate flavors (Anandaraman and Reineccius, 1980). The development of edible active films with selective gas permeability seems have considerable potential applications for achieving a modified atmosphere effect for fruits and vegetables. 5.4 Modification of surface conditions with edible active layers Edible active films and coatings can be applied on foods to modify and control surface conditions. They can be used as surface retention agents to limit food additive diffusion in the food core. The improvement of food microbial stability can also be obtained by using edible active layers which have specific antimicrobial and pH lowering properties. Surface microbial growth is a main cause of spoilage for many food products (Gill, 1979; Maxci, 1981; Olson et al, 1981; Vitkov, 1973, 1974). Edible films and coatings can be used in combination with treatments such as refrigeration and controlled atmosphere to improve the microbiological quality of certain foods. For example, calcium alginate-based films were tested to limit microorganism contamination on the surface of beef pieces (Williams et al., 1978). These films were found to have a significant effect
on microbial growth (natural microflora and coliform inocula). The specific antimicrobial activity of calcium alginate coatings has not yet been explained, but could be partially due to the presence of calcium chloride. El Ghaouth et aL (1990, 1991) used chitosan-based coatings to protect fresh vegetables (cucumbers, peppers). These coatings improved fruit appearance and reduced microbial degradation. This effect, which has also been obtained on strawberries, can be attributed to the specific antifungal properties of chitosan molecules (Allan and Hadwiger, 1979; Hirano and Nagao, 1989; Stossel and Leuba, 1984). Zein-based films have been found to successfully reduce microorganism penetration into chicken egg shells after surface inoculation (Tryhnew et aL, 1973). Development of processes that specifically enhance surface microbial stability is required; food processors have used preservative dips and sprays. Potassium sorbate (or sorbic acid), which has a wide range of bacteriostatic and mycostatic properties, can be used by dipping to reduce the total number of viable bacteria at both refrigeration and elevated temperatures (Cunningham, 1979; D'Aubert et aL, 1980; Lueck, 1984; Robach, 1979, Robach and Ivey, 1978; Robach and Sofos, 1982; To and Robach, 1980; Torres et aL, 1985b; Zamora and Zaritzky, 1987a, b). However, the shelf-life extension achieved by these surface treatments is limited by problems related to potassium sorbate (or sorbic acid) stability and diffusion. The stability of sorbic acid (in its active non-dissociated form) is dependent on application conditions; e.g. a lowered pH improves stability (Eklund, 1983). The diffusion into the core of the food results in a reduction in preservative concentration on the surface which allows microorganisms to overcome the sorbate-induced bacteriostasis (Greer, 1981; Torres, 1987). It is important to be able to predict and control surface preservative migration between phases during food treatments (e.g. absorption of sorbic acid during the processing of dried prunes, or its loss during cooking of fabricated foods), storage of composite foods (e.g. dairy products or cakes containing pretreated fruits), storage of foods in contact with wrapping materials or films containing sorbic acid (absorption by dairy products covered with paper saturated with sorbic acid) and storage of foods coated with an external edible layer highly concentrated in sorbic acid (Guilbert, 1988). Edible films and coatings can be used as food preservative media (particularly as antioxygen and antifungal agents) and as surface retention agents to limit preservative diffusion in the food core (Guilbert, 1986, 1988; Kester and Fennema, 1986; Torres et aL, 1985b; Vojdani and Torres, 1989a, b, 1990). Maintaining a local high and effective concentration of preservative may allow, to a considerable extent, a reduction of its total amount in the food for the same effect (as schematized in Figure 5.1), i.e. at the surface of the food to reduce aerobic contamination and/or oxygen influence.
% Retention
Time (days)
Figure 5.6 Tocopherol retention at 25°C in gelatin layers and in gelatin layers treated with tannic acid in contact with aqueous model food (aw = 0.95) or in contact with margarine (according to Guilbert, 1988). D, Gelatin layer - aqueous model food; •, gelatin layer treated - aqueous model food; o, gelatin layer - margarine; •, gelatin layer treated - margarine.
Guilbert (1988) determined the retention rate of a-tocopherol in gelatinbased films formed on intermediate moisture foods and margarine (Figure 5.6). The rapid decrease of tocopherol content in the gelatin layer in contact with margarine is due to the high apparent solubility of tocopherol in fatty material. The low solubility of tocopherol in water, and so its low diffusivity value, explains the retention effect exerted by the gelatin layer in contact with aqueous model food. Cross-linking treatment with tannic acid increases tocopherol retention by the gelatin layers in contact with margarine. These active layers enriched with tocopherol could be used to reduce the oxidative deterioration in some food products. Guilbert (1988) also investigated the retention of sorbic acid in gelatin and casein films treated respectively with tannic and lactic acid, and placed over an aqueous model food system (aw = 0.95). After 35 days at 25°C, a retention of 30% was observed with the treated casein film. Swelling and poor retention were observed with the gelatin film (30% retention after 10 days). Sorbic acid retention has been studied in zein-based (Torres et aL, 1985b) and casein-based films (Guilbert, 1986, 1988), gluten-based and pectinbased films (De Savoye et aL, 1994), and in composite polysaccharide-lipid derivative films (Vojdani and Torres, 1990). The sorbic acid permeability values determined for these films can also be compared with published apparent diffusion values for sorbic acid in food systems. In an intermediate
Table 5.5 Apparent coating diffusion coefficients for sorbic acid in intermediate moisture model food (agar model or cheese analog) at 24°C and aw = 0.88, in agar gel (with sucrose or glycerol) at 25°C and aw = 0.88, and in cheese analog coated with edible zein films Diffusion coefficient (m2 s~')
Experiment Agar gel and sucrose(1) Cheese analog(2) Agar model(2) Agar gel and glycerol(1) Cheese analog and zein Cheese analog and two zein
0.5 1.0 2.0 3.5 film(2) films(2)
X X X X
IO"10 IO"10 10"10 10"10
3.3 X 10~13 6.8 X 10~13
(According to Giannakopoulos and Guilbert, 1986a(l); Torres et al., 1985b(2).)
moisture agar model system, Guilbert et al. (1985) reported a value of 2.0 x 10~10m2/sec. Torres et al. (1985b) found with an intermediate moisture cheese analog a value of 1.0 x 10"10m2/sec (Table 5.5). The sorbic acid diffusivity values in edible films were found to be 150- to 300-fold lower than those determined for model intermediate moisture foods (Table 5.5). These few examples indicate that edible films could be used for additive retention on the surface of food products. Preservative diffusion through edible films is influenced by various parameters: film characteristics (type, manufacturing procedure), food characteristics (pH, aw), storage conditions (temperature, duration, etc.) and solute characteristics (hydrophilic properties, molar mass). The effect of the film-forming material on sorbic acid permeability has been studied for various edible films (Table 5.6). It appears that film composition (type of film forming agent, presence of lipids) affects even the sorbic acid permeability. For example, a 65% reduction of the sorbic acid permeability of a methylcellulose edible film is observed when palmitic acid is added to the hydrocolloid matrix, there is a 75% reduction for a hydroxypropyl methylcellulose edible film. Table 5.6 Sorbic acid permeability of various edible films at 24°C and aw = 0.77 Film Chitosan MC HPMC MC and palmitic acid (weight ratio 3:1) HPMC and palmitic acid (weight ratio 3:1) Zein
Sorbic acid permeability (x 108 g mm m"2 s'1 (g/1)"1) 0.865 0.334 0.830 0.120 0.205 Similar values
(According to Torres et al, 1985b; Vojdani and Torres, 1989a, b.) (HPMC = hydroxypropylmethylcellulose; MC = methylcellulose.)
According to Vojdani and Torres (1990), sorbic acid permeability decreases in composite films (hydroxypropyl methylcellulose or methylcellulose and fatty acids) as the lipid derivative concentration increases; it also decreases as the length of the carbon chain in fatty acids increases and with the presence of double bonds. This is consistent with published data on the solute permeability of synthetic lecithin liposomes. De Gier, et al. (1968) found that increasing the lecithin fatty acid chain length decreased the permeability of glycerol and erythritol through these artificial membranes. McElhaney et al. (1970) showed that the permeability of liposomes was altered by the geometrical configuration and the number of double bonds in the fatty acid component. The composite film-forming technique used to form composite films or coatings (emulsions or multilayers) also affects the sorbic acid barrier properties. According to Vojdani and Torres (1989a) the lowest permeability values are found with bilayer composite films. For example, the sorbic acid permeability for an emulsion composite film of methylcellulose and palmitic acid was 3 times lower than for bilayer composite film. As noted for water vapor and gas permeability, temperature and relative humidity affect the permeability of edible film to sorbic acid (Figure 5.7). At constant temperature, the sorbate permeation rate decreased as aw decreased (Vojdani and Torres, 1989a; Rico-Pena and Torres, 1991). This is consistent with studies in food model systems (Giannakopoulos and Guilbert, 1986a) where sorbic acid diffusivity rises at high aw. Sorbic Add Permeability (xlO f g.m.nr 2 .* 1 .**/!)- 1 )
Sorbic Add Diffusivity (xlO lf m ' . s 1 )
Water activity Figure 5.7 Effect of water activity on sorbic acid permeability of edible multicomponent films composed of methylcellulose and palmitic acid (weight ratio 3:1), at 24°C (•), (according to RicoPena and Torres, 1991); and on sorbic acid diffusivity in agar gels with sucrose (•) or glycerol (o) at 25°C (according to Giannakopoulos and Guilbert, 1986b).
Sorbic Add Permeability (SAP), (x 108 g . m . nr*. s ! . (g /1)')
Temperature (0C) Figure 5.8 Effect of temperature on sorbic acid permeability of edible multicomponent films composed of methylcellulose (MC) or hydroxypropylmethylcellulose (HPMC) and palmitic acid (weight ratio 3:1); of edible chitosan films, at aw = 0.77 (according to Vojdani and Torres, 1989b and 1990); and of edible pectin films, at aw = 1.0 (according to De Savoye et al., 1994). o, MC; •, MC and palmitic acid; a, HPMC; •, HPMC and palmitic acid; A, chitosan; A, pectin (SAP X 0.005).
An increase in temperature causes a decrease in film sorbic acid barrier properties (Figure 5.8). These variations can easily be analysed through Arrhenius-type representations. Vojdani and Torres (1989b) noted that activation energy is affected by the solvent embedding the film, which suggests that the diffusion process in the film occurs mainly through the aqueous phase. Consequently, the performance of edible coatings controlling surface preservative concentration will depend strongly on the composition of the aqueous phase of the coated food. The sorbic acid permeability of edible films varies according to pH (Figure 5.9). At constant aw and temperature, sorbic acid permeability decreases when pH increases. An increase in pH lowers permeability, possibly due to a change in the sorbic acid formed around the pKa (equal to 4.8), thus modifying the diffusion properties. Longer surface retentions of sorbic acid are possible at higher pH (and also at lower aw and lower temperature). It should be noted that the higher retention at higher pH would help balance the lowering of sorbic acid effectiveness as pH is increased (Eklund, 1983). The microbiological analyses carried out have confirmed the sorbic acid retention efficiency of zein-based edible films, formed on the surface of intermediate moisture cheese analogs, relative to microbial stability of the food product (Torres et al., 1985b; Torres and Karel, 1985). These edible
Sorbic Add Permeability (SAP), (x 10f g. m. m2 . s '. (g / W )
pH Figure 5.9 Effect of pH on sorbic acid permeability of edible multicomponent films (•) composed of methylcellulose and palmitic acid (weight ratio 3:1), at aw = 0.77 and 24°C (according to RicoPena and Torres, 1991) and of edible pectin (Psa X 0.01) films (D) at aw = 1.0 and 200C (according to De Savoye et ah, 1994).
barrier layers double the shelf-life of the food product before the appearance of microorganisms. Torres et al. (1985b) have assessed the potential sorbic acid retention efficiencies of edible films according to utilization conditions. For example, a composite film (methylcellulose-palmitic acid) applied to a food item (at pH = 5.0 and aw = 0.8) stored at 24°C reduces the amount of sorbic acid diffused in the food mass by more than 50%. The effect that edible active film (according to sorbic acid permeability) has on increased surface microbial stability has been estimated by Torres (1987). For example, with the use of a methylcellulose and palmitic acid (weight ratio = 3:1) edible film on food stored at 24°C, then the permeability = 0.042 g mm m"2 24 h"1 (g/1)"1; the surface protection can be predicted to last 30 and 120 days for a 0.1 mm film and 0.2 mm film respectively. If the item is stored under refrigeration (at 5°C), these values increase to 82 and 328 days respectively (Torres, 1987; Vojdani and Torres, 1989a). According to these authors, these estimations need to be confirmed using a specific food system and challenging microorganism. Guilbert (1988) conducted microbiological tests on intermediate moisture fruit pieces (Table 5.7). Significant improvement of the microbial stability was observed with coatings containing sorbic acid and no spoilage could be detected after 40 days' storage in the case of papaya cubes coated with carnauba wax containing sorbic acid. The coating efficiencies were in the following order: carnauba wax + sorbic acid > carnauba wax > casein + sorbic acid > casein > no coating. The fact that casein film with sorbic acid was less effective than the carnauba wax with sorbic acid may be explained
Table 5.7 Stability of coated intermediate moisture papaya cubes (as a function of a w ) at 30 0 C inoculated with Aspergillus niger(a) or Staphylococcus rouxii{h) Delay for first apparent spoilage (days)
aw 0.71
aw 0.75
aw 0.84
aw 0.90
Control (non coated)
(a) (b)
>40 >40
>40 >40
13 3
4 1
Coated with casein
(a) (b)
>40 >40
>40 >40
12 2
4 2
Coated with casein and sorbic acid
(a) (b)
> 40 >40
> 40 >40
22 >40
10 17
Coated with carnauba wax
(a) (b)
> 40 >40
> 40 >40
> 28 >40
10 14
Coated with carnauba wax and sorbic acid
(a) (b)
> 40 > 40
> 40 > 40
32 > 40
10 > 40
(According to Guilbert, 1988.)
by the poor retention properties and the high initial pH of the casein film (Guilbert, 1988). The improvement of food microbial stability can also be obtained by reducing surface pH. This can be achieved by using films or coatings that immobilize either specific acids or charged macromolecules. Torres and Karel (1985) succeeded in obtaining a temporary pH difference between the surface and the core of intermediate moisture foods by adding lactic acid to zein-based edible films. The development of edible films that immobilize Contamination: togN (cells / cm2)
(uncoated control)
(reduced pH surface)
Time (days) Figure 5.10 Effect of reduced surface pH on the microbiological quality of an intermediate moisture cheese analog coated with a carrageenan and agarose film; challenged with Staphylococcus aureus S-6 (aw = 0.88 and 35°C) (after Torres and Karel, 1985).
charged macromolecules has made it possible to obtain a pH difference between the surface and core of enveloped intermediate moisture food products. A Donnan equilibrium model for semipermeable membranes containing charged macromolecules can be used to assess the possibility of obtaining a permanent pH difference between two components separated by a membrane. The use of agarose- and carrageenan-based edible films (with charged macromolecules) decreases the surface pH by about 0.5. In addition to carrageenans, slightly esterified pectins could probably be used for similar applications (Guilbert and Biquet, 1989). A decrease in surface pH can also improve the stability and efficiency of antimicrobial agents such as sorbic acid. Microbiological analyses have confirmed the antimicrobial efficiency of this type of surface treatment (Torres and Karel, 1985). Studies of bacterial growth (Staphylococcus aureus) on intermediate moisture foods revealed improved microbiological stability with the use of carrageenan-based edible films (Figure 5.10). Edible films and coatings are a useful mean to reduce the rate of some deteriorative reactions such as surface microbial development or oxidation. They could also be used to achieve the slow release of flavor compounds during food storage or consumption. Initial choices of film-forming materials and of fabrication conditions allow modification of solute retention properties of these edible active layers, and allow selection of the efficient functional properties in relation to each specific application, according to specific needs. 5.5
Conclusion
Edible films and coatings can be used to control gas exchange (water vapor, oxygen, carbon dioxide, etc.) between the food product and the ambient atmosphere, or between components in a mixed food product, and to modify and control food surface conditions (pH, level of specific functional agents, etc.). It should be stressed that the characteristics of the film or coating and the application technique must be adapted to each specific utilization. Edible superficial layers provide supplementary and sometimes essential means to control physiological, microbiological and physicochemical changes in food products. The active edible layers concept can thus be extended to new fully adapted superficial or internal applications for food products. Among these new potential applications, layers enriched with susceptors for microwave treatments or with catalyst for specific reactions on the one hand, or with flavor, preservative, ethanol, etc., for slow-release systems on the other hand, can be mentioned. The development of an active edible layer is mainly limited by formulation constraints, i.e. the layer composition must be compatible with the product characteristics and regulation, and by industrial
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Lowings, P.H. and Cutts, D.F. (1982) The preservation of fresh fruits and vegetables. Proceedings Institute Food Sciences and Technology Annual Symposium, July, Nottingham, UK, pp. 52-67. Lueck, E. (1984) Sorbinsaeure und sorbate. Fleischwirtsch, 64, 727. Mannheim, C. and Passy, N. (1985) Choice of packages for foods with specific considerations of water activity. In: Properties of Water in Foods, D. Simatos and J.L. Multon (eds), Martinus Nijhoff Publishers, Dordrecht, the Netherlands, pp. 375-92. Marshall, A.S. and Petrie, S.E. (1980) Thermal transition in gelatin and aqueous gelatin solutions. Journal Photogr. Science, 28, 128. Marston, P.E. (1983) Moisture content and migration in bread incorporation dried fruit. Food Technology (Australia), 35(10), 463. Martin-Polo, M., Mauguin, C. and Voilley, A. (1992) Hydrophobic films and their efficiency against moisture transfer. Influence of film preparation techniques. Journal of Agricultural and Food Chemistry, 40, 407-12. Maxcy, R.B. (1981) Surface microenvironment and penetration of bacteria into meat. Journal of Food Protection, 44, 550. McElhaney, R.N., De Gier, J. and Van Deenen, L.L.M. (1970) The effect of alterations in fatty acid composition and cholesterol content on the permeability of Mycoplasma laidlawii B cells and derived liposomes. Preliminary Notes, Biochemistry and Biophysical Acta, 219, 245. Motlagh, H.F. and Quantick, P. (1988) Effect of permeable coatings on the storage life of fruits. I. Pro-Long treatment of limes (Citrus auratifolia cv. Persian). International Journal of Food Science and Technology, 23, 99-105. Murray, D.G., Mardtta, N.G. and Boettger, R.M. (1972) Amylose coating for deep fried potatoes. US Patent Reissue, 27, 531. Myers, A.W., Meyer, J.A., Rogers, CE., Stannett, V. and Szwarc, M. (1961) Studies in the gas and vapor permeability of plastic films and coated papers. The permeation of water vapor. TAPPI, 45, 58. Niediek, E.A. (1981) Problems of exchange reactions during conching (II). CCB Rev. Chocolate Confect. Bakery, 6(2), 17. Olson, V.M., Swaminathan, B., Pratt, D.E. and Stadelman, WJ. (1981) Marketing and products. Effect of five cycle rapid freeze-thaw treatment in conjunction with various chemicals for the reduction of Salmonella typhemurium. Poultry Science, 60, 1822. Pascat, B. (1985) Study of some factors affecting permeability. In: Food Packaging and Preservation, M. Mathlouthi (ed.), Elsevier Applied Science Publishers, New York, pp. 7-24. Park, H J . and Chinnan, M.S. (1990) Properties of Edible Coatings for Fruits and Vegetables. Paper No. 90-6510 presented at the American Society of Agricultural Engineers, St Joseph, MI. Park, HJ., Weller, CL., Vergano, PJ. and Testin, R.F. (1993) Permeability and mechanical properties of cellulose based edible films. Journal of Food Science, 58(6), 1361-4. Pavlath, A.E., Wond, D.S.W. and Kumosinski, T.F. (1993) New coatings for cut fruits and vegetables. Chemistry Technology, February, 36-40. Poyet, J. (1993) Les Polymeres 'Barrieres'. Presented at the Congres SAGE, 'L'Alimentarit6 dans les Matieres Plastiques et les Caoutchoucs', 1 et 2 Avril, Paris, France. Rico-Pena, D.C. and Torres, J.A. (1990) Edible methylcellulose-based films as moistureimpermeable barriers in sundae ice cream cones. Journal of Food Science, 55(5), 1990. Rico-Pena, D.C and Torres, J. A. (1991) Sorbic acid and potassium sorbate permeability of an edible methylcellulose-palmitic acid film: water activity and pH effects. Journal of Food Science, 56(2), 1991. Rigg, WJ. (1979) Measurement of permeability of chilled meat packaging film under conditions of high humidity. Journal of Food Technology, 14, 149-55. Robach, M.C (1979) Extension shelf life of fresh, whole broilers, using a potassium sorbate dip. Journal of Food Protection, 42, 855. Robach, M.C. and Ivey, FJ. (1978) Antimicrobial efficacy of a potassium sorbate dip on freshly processed poultry. Journal of Food Protection, 41(4), 284-8. Robach, M.C and Sofos, J.N. (1982) Use of sorbates in meat products, fresh poultry and poultry products: A review. Journal of Food Protection, 45(4), 374-83.
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6
Interactive packaging involving sachet technology J.P. SMITH, J. HOSHINO and Y. ABE
6.1 Introduction Over the past decade, there has been a tremendous growth in interactive packaging for shelf-life extension of food. Packaging can be defined as 'interactive' when it 'performs some role in the preservation of the food other than providing an inert barrier to outside influences' (Rooney, 1992), There are many examples of interactive packaging technologies including antimicrobial and antioxidant films, ethylene absorbing sachets and temperature control indicators, some of which have been discussed in previous chapters of this text. However, perhaps one of the best examples of interactive packaging, and one which fulfils the above definition in all aspects, is modified atmosphere packaging (MAP). MAP has been defined as 'the enclosure of food products in a high gas barrier film in which the gaseous environment has been changed or modified to slow respiration rates, reduce microbiological growth and retard enzymatic spoilage with the intent of extending shelf-life' (Young et aL, 1988). The growth in MAP technology has resulted from advances in packaging technology, the food industry's need for less energy-intensive forms of food preservation than drying, freezing or thermal processing, and consumer needs for convenience foods with extended shelf-life yet retaining their fresh characteristics. The atmosphere in MAP foods has traditionally been modified by vacuum or gas packaging, the latter involving various mixtures of CO2, either alone or in conjunction with nitrogen and sometimes oxygen depending on the product gas packaged, e.g., meat or fruits and vegetables (Smith et al., 1990). Although this form of interactive packaging can be used to extend the shelf life and keeping quality of food, aerobic spoilage can still occur in these packaged products depending on the level of residual oxygen in the package headspace. The level of residual oxygen in vacuum/gas packaged products could be due to a number of factors such as oxygen permeability of the packaging material; ability of the food to trap air; leakage of air through poor sealing; and inadequate evacuation and/or gas flushing (Smith et ai, 1986). In recent years, novel methods of oxygen control and atmosphere modification have been developed, primarily by the Japanese. These include
oxygen/carbon dioxide absorbents, oxygen absorbents/carbon dioxide generators, and ethanol vapor generators. This technology involves the use of sachets which can be placed alongside the food and actively modify the package headspace thereby extending product shelf-life. This chapter will review the various types of absorbent/generator sachets available in the marketplace, the methods by which these sachets actively modify the gas atmosphere in the packaged product, and their uses, advantages and disadvantages for shelf-life extension of food.
6.2 Oxygen absorbents One novel and innovative method of oxygen control and of atmosphere modification involves the use of oxygen absorbents. Oxygen absorbents can be defined as 'a range of chemical compounds introduced into the MAP package (not the product) to alter the atmosphere within the package' (AgriFood Canada, personal communication). Developed in Japan in 1976, oxygen absorbents were first marketed by the Mitsubishi Gas Chemical Co., under the trade name Ageless (Table 6.1). Several other Japanese companies also produce oxygen absorbents with the best known being the Toppan Printing Company which produces a range of oxygen absorbents under the label Freshilizer Series (Smith et al., 1990). In 1989, almost 7000 million sachets were sold in Japan with sales of absorbents growing at a rate of 20% annually. The Mitsubishi Gas Chemical Co. dominates the oxygen absorbent market (73%) while the Toppan Printing Co. has about 11% of the market with nine other companies sharing the remaining 16% of the market. Oxygen absorbent technology has been successful in Japan for a variety of reasons, including the hot and humid climate during the summer months which is conducive to mold spoilage of food products. Another important factor for their success is that Japanese consumers are willing to pay higher prices for preservative-free products with an increased shelf-life. Several years after the Mitsubishi Gas Chemical Co., successfully launched oxygen absorbent technology onto the marketplace, Multiform Desiccants in the United States introduced FreshPax, the first North American oxygen scavenger (Table 6.1). Like its predecessors, FreshPax is extremely effective in lowering package oxygen from 20% to less then 0.05% in about 36 hours. However, unlike the Japanese market, acceptance of oxygen scavengers in Table 6.1 Major companies producing oxygen absorbents Company
Product name
Mitsubishi Gas Chemical Co., Japan Toppan Printing Co., Japan Multiform Desiccants, US
Ageless Freshilizer Freshpax
North America and Europe has been slow, although several major companies in both continents are now using this technology. Oxygen absorbents comprise of easily oxidizable substances usually contained in sachets made of air-permeable material. These sachets come in a variety of sizes capable of absorbing 20-2000ml of headspace oxygen. When placed inside the packaged food, they actively modify the package headspace and reduce the oxygen levels to < 0.01% within 1-4 days at room temperature. However, some are designed to scavenge oxygen at refrigerated or frozen storage temperatures and are used to further extend the shelf-life and keeping quality of muscle foods. This oxygen-free environment protects the food from microbiological and chemical spoilage and is also effective in preventing damage by insects. A combination of some or all of these factors helps maintain the quality and freshness of food, which facilitates the marketing of oxygen absorbents. The classification and the main types of oxygen absorbents will now be briefly reviewed. 6.2.7
Classification of oxygen absorbents
Oxygen absorbents can be classified into several different categories as shown in Table 6.2. Each category will be briefly reviewed. 6.2.1.1 Classification according to material. In theory, any material that reacts easily with oxygen can be used as an oxygen scavenger. However, because the technology involved is used mainly for food preservation, the material used inside the sachet must meet the following criteria prior to approval by regulatory agencies. • • • • • • •
It It It It It It It
must must must must must must must
be safe. be handled easily. not produce toxic substances or offensive odors/gases. be compact in size. absorb a large amount of oxygen. have an appropriate oxygen absorption speed; and be economically priced (Harima, 1990).
Iron powder and ascorbic acid are commonly used in existing oxygen absorbers with powdered iron being most frequently used either alone or in conjunction with other specific chemical compounds in dual function absorbents (Table 6.2). 6.2.1.2 Classification according to reaction style. Water is essential for oxygen absorbents to function. In the self-reaction types, water required for the chemical reaction is added and the absorbents must be handled carefully as the oxygen absorbing reaction commences as soon as the self-reacting absorbent is exposed to air. In moisture dependent types, the oxygen
absorption reaction only takes place after moisture has been absorbed from the food; these type of absorbents are easier to handle as they do not react immediately upon exposure to air. However, they absorb oxygen quickly after sealing and oxygen can be absorbed within 0.5-1 day in certain products. Examples of self-reacting and moisture dependent types of oxygen absorbents are shown in Table 6.2. 62.13 Classification according to reaction speed.. Oxygen absorbents can be classified as immediate effect, general effect, and slow effect types (Harima, 1990). The average time for oxygen absorption is 0.5-1 day for the immediate type; 1-4 days for the general type; and 4-6 days for the slow Table 6.2 Classification of oxygen absorbents Function O2I
Reactant Iron
Catechol
Application
Absorption speed
Product
Dry aw < 0.3 Tea; nuts
4 to 7 days
Ageless Z-PK Vitalon T
Medium aw aw < 0.65 Dried beef
1 to 3 days
Ageless Z Keplon TS
High aw aw > 0.65 Cakes Bakeries
0.5 days
Ageless S Sequl CA
Frozen temp + 3 to - 25°C Raw fish
3 days at - 25°C
Ageless SS
Moisture dependent type
High aw aw > 0.85 Pastas
0.5 days
Ageless FX Vitalon LTM
Self-working type
Medium aw aw < 0.65 Nuts
Tamotsu A
High aw aw > 0.65 Cakes Roasted coffee
Tamotsu P
Self-working type
O2I & CO2I
Iron + Calcium
Self-working type
O2I & CO2T
Ascorbic acid
Self-working type
Medium aw 0.3 < aw < 0.5 Nuts
Ascorbic acid + Iron
Moisture dependent type
High aw aw > 0.85 Cakes
O2I & Iron + EthanolT Ethanol/Zeolite
Moisture dependent type
High aw aw > 0.85 Cakes
3 to 8 days
Ageless E
1 to 4 days
Ageless G Toppan C Vitalon GMA
Negamold
O2 concentration (%)
Time (day) Figure 6.1 Effect of storage temperature on oxygen absorbing speed of Ageless S-IOO, a self working type (fast working type) of oxygen absorber.
O2 concentration (%)
reacting type (Harima, 1990). The reaction time depends on storage temperature and water activity (aw) of the food. The effect of storage temperature on oxygen absorbing speed is shown in Figures 6.1 and 6.2. Most oxygen absorbents are used with foods stored at ambient temperature.
Time (day) Figure 6.2 Effect of storage temperature on oxygen absorbing speed of Ageless Z-100, a self working type (medium working type) of oxygen absorber.
However, if used with refrigerated or frozen products these absorbents react very slowly. To overcome this problem, some absorbents can now scavenge oxygen rapidly from the package headspace of food stored under lowtemperature conditions. A comparison of the oxygen absorbing speed of one such type of oxygen absorbent (Ageless SS) with absorbents which function mainly at ambient storage temperatures, is shown in Figure 6.3. 6.2.1.4 Classification according to use. Oxygen absorbents can be used for a variety of foods with different moisture contents ranging from very moist to very dry food products. In general, foods with a high moisture content are more susceptible to mold spoilage. Therefore, an immediate effect absorbent would be used with these products to rapidly absorb oxygen and extend the mold-free shelf-life of the product. General type absorbents are used with intermediate moisture food products where a moderate speed of oxygen absorption is required. For low moisture food products, which are not susceptible to microbial spoilage but to chemical spoilage, a slow effect oxygen absorbent can be used. Examples of the application of the various categories of oxygen absorbents are shown in Table 6.2.
02 concentration (%)
6.2.1.5 Classification according to function. The majority of absorbents have only one function - absorption of oxygen. However, dual functional absorbents have been developed for use in specific products. These include oxygen-carbon dioxide absorbents for use in coffee, and oxygen absor-
AGELESS FX-100
Time (day) Figure 6.3 Effect of frozen temperature (-250C) on oxygen absorbing speed of three types of oxygen absorbents.
bents-carbon dioxide generators. The latter type of sachets absorb O2 and generate the same amount of CO2 as that of absorbed oxygen. They are mainly used in products where package volume and package appearance is critical, e.g., packaged peanuts. These sachets contain iron carbonate and ascorbic acid as the reactants. Another dual functional absorbent scavenges oxygen and releases alcohol vapor. These absorbents could be used to control the growth of facultative bacteria and yeasts which grow under reduced oxygen tensions. However, this type of dual functional absorber is not widely used by the Japanese food industry (Harima, 1990). Examples of single function and dual function oxygen absorbents are shown in Table 6.2. 6.2.2 Main types of oxygen absorbents 6.2.2.1 Ageless. Ageless, made by the Mitsubishi Gas Chemical Co., consists of a range of gas scavenger products designed to reduce oxygen levels to less than 100 ppm in the package headspace. While both organic types (based on ascorbic acid) and inorganic types (based on iron powder) are available, the inorganic types are most commonly used in the Japanese market. The basic system is made up of finely divided powdered iron which, under appropriate humidity conditions, uses up residual oxygen to form nontoxic iron oxide, i.e., it rusts. The oxidation mechanism can be expressed as follows (Harima, 1990; Smith et ai, 1990; Smith, 1992): Fe -> Fe2+ + 2e ^O2H- H2O + 2e -> 2OHFe2+ + 2(OH)- -> Fe(OH)2 Fe(OH)2 + J^O2 +\ H2O -> Fe(OH3) To prevent the iron powder from imparting color to the food, the iron is contained in a sachet (like a desiccant). The sachet material is highly permeable to oxygen and, in some cases, to water vapor. Since Ageless relies on a chemical reaction and not on the physical displacement of oxygen as in gas packaging, it completely removes all traces of residual O2 and protects the packaged food from aerobic spoilage and quality changes. Several types and sizes of Ageless sachets are commercially available and are applicable to many types of foods including those with a high moisture content, intermediate moisture products, low moisture product foods and foods containing or treated with oil. The main types of absorbents are types Z, S, FX, E and G. Three new types of Ageless are commercially available in Japan. These are Ageless types SS, FM and SE (Table 6.3).
Table 6.3 Types and properties of Ageless oxygen absorbents Water activity
Absorption speed (day)
Type
Function
Moisture status
Z S SS
Decreases O2 Decreases O2 Decreases O2
Self-reacting Self-reacting Self-reacting
<0.65 > 0.65 > 0.85
FX FM E
Decreases O2 Decreases O2 Decreases O2 Decreases CO2 Decreases O2 Increases CO2 Decreases O2 Increases ethanol
Moisture dependent Moisture dependent Self-reacting
>0.85 > 0.85 < 0.3
1-3 0.5-2.0 2-3 (0-4 0 C) 10 ( - 2 5 0 C ) 0.5-1.0 0.5-1.0 3-8
Self-reacting
0.3-0.5
1-4
Moisture dependent
>0.85
1-2
G SE
Type Z is designed for food products with water activities of less than 0.65 and reduces residual headspace oxygen to 100 ppm in 1-3 days. It is available in sizes that can scavenge 20-2000 ml of oxygen (an air volume of 100-10 000 ml). Two other types of Ageless (FX and S) work best at higher water activities and have a faster reaction rate (0.5-2 days). They have the same oxygen scavenging capacity as above. Type FX is moisture dependent and does not absorb oxygen until it is exposed to an aw greater than 0.85. Thus, it can be easily handled if kept dry. Type S, on the other hand, contains moisture in the sachet and is a self-working type. This type of absorbent requires careful handling since it begins to react immediately on exposure to oxygen. Absorbent type SS is similar to type S. However, it has the ability to rapidly scavenge oxygen under refrigerated and frozen storage conditions. These absorbents (Ageless type SS) are widely used to extend the refrigerated shelf-life of muscle foods such as fresh meat, fish and poultry. Yet another new absorbent is type FM which can be used with microwaveable products (Table 6.3). A commonly used absorbent is type E which also contains Ca(OH)2 in addition to iron powder. Type E scavenges CO2 as well as O2. It is used for ground coffee, where CO2 removal reduces the chance of the package bursting. Marketed under the brand name Fresh Lock, it is used in Maxwell House ground coffee cans (Table 6.3). Two other types commonly used in the Japanese market are type G and type SE. Type G is a self-working type and absorbs oxygen and generates an equal volume of CO2. It is used mainly with snack food products, such as nuts, to maintain the package volume and hence appearance of the product. Another new innovation is Ageless type SE. This absorbent is moisture dependent and absorbs oxygen and generates ethanol vapor. It is used to extend the mold-free shelf-life of bakery products in Japan. The various types of Ageless and their characteristics are summarized in Table 6.3.
Table 6.4 Types and properties of Freshilizer oxygen absorbents Type
Function
F Series (ferrous metal) Decreases O2 FD Decreases O2 FH Decreases O2 FT C Series (Non-ferrous metal) C Decreases O2 Increases CO2 Decreases O2 CW Increases CO2 Decreases O2 CV Decreases CO2
Water activity
Absorption speed (day)
<0.8 0.6-0.9 > 0.8
1-3 0.5-1.0 0.5-1.0
Self-reacting
< 0.8
3-5
Self-reacting
0.8-0.9
2-3
Self-reacting
< 0.3
3-8
Moisture status Self-reacting Self-reacting Moisture dependent
Courtesy of Toppan Printing Co., Tokyo, Japan.
6.2.2.2 Freshilizer series. The Freshilizer Series of freshness keeping agents made by Toppan Printing Co., Japan, consists of the F series and the C series (Table 6.4). Series F Freshilizers use mainly ferrous metal and absorb only oxygen. Three types are commercially available - types FD, FH and FT. Type FD is designed for use in food products with aw values less than 0.8 (nuts, tea, chocolate) while type FH is suitable for use in products with a range of aw values ranging from 0.6 to 0.9 and is used mainly with beef jerky and salami to maintain the color of these products. Type FT works best in foods with water activities greater than 0.8 such as pizza crusts. Series F absorbents can absorb 20-300 ml O2 corresponding to a package volume of 100-1500 ml of air (Toppan Technical Information, 1989). The Freshilizer C series of absorbents comprises types C, CW and CV. These sachets consist of non-ferrous metal particles and can therefore be used in products which must pass through a metal detector. Types C and CW absorb oxygen and generate an equal volume of CO2 thereby preventing package collapse. Type C is used in foods with an aw of 0.8 or less (nuts) while type CW is suitable for foods with higher aw values (i.e. > 0.8). Type CW is commonly used to prevent mold growth in sponge cakes. Type CV absorbs both oxygen and carbon dioxide and was developed for use with roasted or ground coffee (Table 6.4; Toppan Technical Information, 1989). 6.2.2.3 FreshPax. FreshPax is a patented oxygen absorbing system developed by Multiform Desiccants, a leading manufacturer of desiccants and other protection products for more than 30 years. Manufactured in the United States, FreshPax, like other oxygen absorbent technology provides an alternative to gas/vacuum packaging to extend the shelf-life and keeping quality of products while simultaneously reducing costs and increasing
Table 6.5 Types and properties of Freshpax™ oxygen absorbents Type
Function
Moisture status
Type B Type D
Decreases O2 Decreases O2 at 2° to - 200C Decreases O2
Moisture dependent Self-reacting
Decreases O2 Increases CO2
Moisture dependent
Type R Type M
Self-reacting
Water activity
Absorption speed (day)
> 0.65 < 0.7
0.5-2.0 0.5-4.0
0.3-0.95
0.5-1.0 (depending on temp.) 0.5-2.0
> 0.65
Courtesy of Multiform Desiccants, Buffalo, NY.
profitability. Produced in sachet form, FreshPax absorbs headspace oxygen to < 0.1% using safe, non-toxic ingredients (mainly iron oxide) that rapidly absorb oxygen before the food deterioration process begins. Four main types of FreshPax are commonly available - type B, type D, type R and type M (Table 6.5). Type B is used with moist or semi-moist foods while type D can be used with dehydrated or dried foods. Type R can be used to scavenge oxygen at refrigerated or frozen storage temperature and is similar to Ageless type SS. It is mainly used to extend the shelf-life and keeping quality of muscle foods. Type M is used with most or semi-moist gas flushed products to maintain package volume and to remove all traces of residual oxygen (Table 6.5; FreshPax Technical Pamphlet, 1994). 6.2.3
Factors influencing the choice of oxygen absorbents
Oxygen absorbent sachets come in a range of sizes capable of absorbing 5 ml to 2000 ml of oxygen. Several interrelated factors influence the choice of the type and size of absorbent selected for shelf-life extension of food products (Harima, 1990; Smith et ai, 1990; Smith, 1992). These are: • • • • • •
The The The The The The
nature of the food, i.e., size, shape, weight. aw of the food. amount of dissolved oxygen in the food. desired shelf-life of the product. initial level of oxygen in the package headspace. oxygen permeability of the packaging material.
The latter parameter is critically important for the overall performance of the absorbent and shelf-life of the product. If films with high O2 permeabilities are used, e.g., > 100 cm3 mil m~2 day"1 atm"1, the oxygen concentration in the container will reach zero within a week but then returns to ambient air level after 10 days due to the fact that the absorbent is saturated. However, if films of low O2 permeability are used, e.g., < 10 cm3 mil m 2 day 1 atm 1 such as PVDC coated nylon/LDPE, the headspace O2 will be reduced to 100 ppm within 1-2 days and remain at this level for the duration of the storage
Table 6.6 Examples of films used with oxygen absorbents (Freshpax Technical Pamphlet, 1994) Film laminates including
OTR (ml/m2/d)
MVTR (g/m2/d)
Long-term preservation
Aluminium EVOH PVDC
< 0.6 < 3 < 15
< 0.6 < 4 < 8
Short-term preservation
Nylon PET
< 16 < 15
< 40 < 100
Not appropriate
Cellophane PP PE
< 200 < 2000 < 3000
<20 < 6 < 5
OTR = oxygen transmission rate; MVTR = moisture vapor transmission rate; EVOH = ethylene vinyl alcohol; PVDC = polyvinylidene chloride; PET = polyester; PP = polypropylene; PE = polyethylene.
period providing packaging integrity is maintained. Examples of appropriate packaging materials for use with oxygen absorbents and their permeability characteristics are shown in Table 6.6. A rapid and efficient method of monitoring package integrity throughout the storage period is through the incorporation of a redox indicator, e.g., Ageless Eye. When placed inside the package alongside an Ageless sachet, the color of the indicator changes from blue (oxidized state) to pink (reduced state) when the oxygen content of the container reaches < 0.005%. If the indicator reverts back to its blue color, it is indicative of poor packing integrity caused by poor sealing of the film or minute pin holes in die film. Similar redox indicators are made by the Toppan Printing Company. 6.2.4 Application of oxygen absorbents for shelf-life extension of food 6.2.4.1 In Japan. Several studies have reported significant extension in the chemical and microbiological shelf-life of low, intermediate and high moisture food products using oxygen absorbent technology (Nakamura and Hoshino, 1983; Abe and Kondoh, 1989; Harima, 1990; Minakuchi and Nakamura, 1990). As a result of these studies, oxygen absorbent technology is being used extensively in Japan to prevent chemical problems, e.g., discoloration problems in highly pigmented products, such as cured meat products and tea, and rancidity problems in high fat foods, such as peanuts, fried rice cake and chocolate covered almonds (Figures 6.4 and 6.5; Table 6.7). Oxygen absorbents are also used effectively to inhibit growth of aerobic spoilage microorganisms, especially molds, in intermediate moisture and high moisture bakery products. Examples of products found in Japanese supermarkets with extended chemical and microbiological shelf-life as a result of oxygen absorbent technology are summarized in Tables 6.8-6.11. More detailed information on the use of both Ageless and Freshilizer oxygen
Total Vitamin C (mg/100g)
AGELESS
AIR
(MONTHS) Figure 6.4 Effect of an oxygen absorbent on the vitamin C content of Green tea. Bag made from oriented nylon/polyethylene/foil/polyethylene. Conditions: Material, 100 g; air, 200 ml; storage temperature, 25°C; absorber, Ageless Z-50.
AIR POV (peroxide value : meq/kg)
N 2 gas
AGELESS
Time (day) Figure 6.5 Effect of oxygen absorbents on the peroxide value (POV) of packaged fried rice cakes by comparison under photo-irradiation. S, Sunlight; F,fluorescentlight.
Table 6.7 Effect of an oxygen absorbent on the shelf life quality of chocolate-coated almonds. Ageless
O2(%) POV almond POV chocolate Flavor score
Air
O2(%) POV almond POV chocolate Flavor score
40 days
3 months
6 months
0.01 2.3 1.1 5
0.01 1.2 1.0 5
0.01 1.5 1.0 4
18.2 7.8 0.9 4
13.3 16.4 1.2 3
9.5 23.5 2.1 2
(Almond without chocolate coating) Air O2(%) 14.3 POV almond 14.0 Flavor score 2
6.7 21.1 1
1.7 28.8 1
Material: 20Og in an ON/PVDC/PE bag; Water activity: 0.31; Moisture content: 2.1% (almond), 0.8% (chocolate); Stored: 25°C; Ageless: Z-50PK. Flavor score: 5 = no rancid smell, good aroma and good flavor; 4 = aroma slightly lost but good flavor; 3 = little rancid smell but good flavor; 2 = rancid smell and flavor weakened; 1 = obvious rancid smell and flavor loss.
absorbents for shelf life extension of food can be found in the excellent articles referenced at the beginning of this section. 62.42 In the USA. While oxygen absorbents are used extensively in Japan, their use in North America is still in its infancy. Examples of the known US companies currently using oxygen absorbents for shelf-life extension of products are shown in Table 6.12. The earliest use of this technology was with Maxwell House coffee which used a dual function absorbent (Ageless E). This absorbent, marketed under the trade name Fresh Lock, contains iron powder for absorption of oxygen and calcium hydroxide which scavenges carbon dioxide. The use of this absorbent in coffee delays oxidative flavor changes and absorbs the occluded carbon dioxide produced in the roasting process and, which if not removed, would cause the packages to burst. More recently, a mold-free shelf-life of 1 year has been achieved for a specialty therapeutic gluten-free bread, Ener-Getic. The bread is packaged in a copolymer film of Mylar/EVOH/Surlyn film with 100% carbon dioxide and a Freshilizer oxygen absorbent/carbon dioxide generator (type CW) supplied by the Toppan Printing Company, Japan. The bread remained mold-free for 1 year at room temperature and physico-chemical changes, i.e., staling, were minimal at the end of storage in the gluten-free bread (Anon., 1988). Studies by Powers and Berkowitz (1990) at the US Army Natick Research, showed that a FreshPax oxygen scavenger enclosed in a high gas barrier pouch made of polyester/aluminium foil/HDPE with baked, meal ready-to-eat (MRE) bread prevented mold growth on bread for 13 months at ambient storage temperature. Based on the results of this study, two companies, Sterling Foods
Table 6.8 Use of Ageless for shelf-life extension of bakery products Type of food Cheesecake Buttercake Plain sponge cake Egg bread Pumpernickel bread Pita bread Tortilla chips Custard cakes Blueberry cheesecake Soy bean pie Bean/jam cake Steamed bean/jam cake
Water activity of food
Packaging materials
Type and size of Ageless
0.88-0.92 0.9 0.86-0.88 0.9 0.9 0.9 <0.6 0.92-0.94 0.88-0.9 0.93 0.85 0.9
KON KON KON KON KON KOP KON/Box PET/PVDC/PE KON KON KON/PS Tray PP/EVAL/KON/PE
S200 SIOO S200 SlOO FXlOO SlOO Z200 S50 S200 S10-15 ZPTl 00-200 S200
Prevents Mold growth/oxidation Oxidation/mold growth Mold growth Mold growth Moid growth Mold growth Oxidation Mold growth Mold growth Mold growth Mold growth Mold growth
Shelf-life @ RT* 1 month 1 month 1 month 1 month 3-4 weeks 3-4 weeks 6 months 7-10 days 1 month 1 month 2 months 2-3 weeks
*RT = Room temperature PET = polyester; PE = low-density polyethylene; AC = acrylonitrile; KON = PVDC-coated nylon; KOP = PVDC-coated polypropylene; PVDC = polyvinylidene chloride; PP = polypropylene; EVAL = ethylene vinyl alcohol; PS = polystyrene.
Table 6.9 Use of Ageless for shelf-life extension of fish products Type of food
Water activity of food
Dried seaweed Dried salmon jerky Dried sardines Dried shark's fin Dried rose mackerel Dried cod Dried squid Fresh yellow taile Sliced salmon
<0.3 0.7-0.8 < 0.6-0.88 0.8 0.7-0.8 < 0.6 0.7-0.8 0.99 0.99
Dried/smoked salmon Dried octopus leg Dried bonito Salmon roe Dried squid/vinegar/ soybean sauce Sea urchin
0.8 < 0.85 0.6-0.7 0.8-0.9 0.7-0.8
* RT = Room temperature
0.98
Packaging materials
Type and size of Ageless
Prevents
PET/AC/PE KON KON KON KON/PET Tray KOP KON Nylon/PVDC/PP KON
ZlOO Z50 Z100-200 S200 S2000 ZlOO ZlOO SSlOO FXlOO
Oxidation/Discoloration Oxidation/Discoloration Oxidation Oxidation/Mold growth Oxidation Oxidation Oxidation/Mold growth Oxidation/Discoloration Discoloration
KON KON KON KON KOP
ZlOO FXlOO TypeZ200+100%N 2 SSlOO Z200
Mold growth/Discoloration/Oxidation Oxidation/Mold growth Oxidation/Discoloration Discoloration Oxidation/Mold growth
1 year 3 months 6 months 1-2 months 1 month 6 months 3 months 1 week at - 30C 6 months at -200C 1 month at 100C 1 month 3-6 months 6 months at - 3°C 3 months
Wood Tray/KON
SSlOO
Discoloration/Bacterial growth
2 weeks at - 50C
Shelf-life @ RT*
Table 6.10 Use of Ageless for shelf-life extension of meat and deli products Type of food Pizza crust Pizza Fresh carved sausage Pre-cooked hamburgers Pre-cooked chicken nuggets Salami slicks Sliced salami
Water activity of food ~ 0.94-0.95 0.99 0.99 0.99 0.99 0.8-0.85 0.8-0.85
Packaging materials
Type and size of Ageless
Prevents
KON KON KON PS Tray/KON KON KON KON
FX200 S200 FX50 SlOO FXlOO Z50 SlOO
Mold growth Mold growth/Discoloration Discoloration/Rancidity Discoloration/Rancidity Discoloration/Rancidity Discoloration Discoloration
Shelf-life @ RT 2 months at 50C 2 weeks at 50C 3 weeks at 0-40C 3 weeks at 0-40C 3 weeks at 0-40C 1-2 months 1 month
Table 6.11 Use of Ageless for shelf-life extension of miscellaneous products Type of food Rice crackers/fried bean Peanuts Peanut butter/chocolate coated Chocolate peanuts Candy-type cheese Smoked cheese Soy bean paste Bakery food (milk powder) Green tea Flavored tea EVA = ethylene vinyl acetate. RT = Room Temperature.
Water activity of food
Packaging materials
<0.3 <0.3 0.5-0.6 0.5-0.6 0.9-0.95 0.9 0.8-0.85 < 0.3 <0.3 0.5-0.6
Tin/Plastic/Aluminium lid KON KON/Paper box KON KON KON Paper/AC/EVA Tin can Tin can KON
Type and size of Ageless ZlOO GlOO Z200 Z30-100 FX50+100%N2 S30-50 FX20 ZlOO Z50 Z50
Prevents Oxidation/Rancidity Oxidation/Rancidity Oxidation/Rancidity Oxidation/Rancidity Oxidation/Rancidity Oxidation/Mold growth Discoloration Oxidation/Rancidity Oxidation Oxidation
Shelf-life @ RT 1 year 6 months 9 months 9 months 1 month at 0-40C 1 month at 0-40C 3 months 6-12 months 1 year 1 year
Table 6.12 Use of oxygen absorbent technology in USA _ _ Product
(months @ RT1)
Maxwell House coffee Ener-Getic gluten-free bread MRE2 bread MRE bread/buns
12-36 12 36 6
Company General Foods Ener-G Foods Sterling Foods Franz Bakery The Famous Pacific Dessert Company
Fruit tortes
6-9
1 2
RT = room temperature MRE = meals, read-to-eat
uses Ageless oxygen absorbents to prevent rancidity and mold problems in packaged tortes (Table 6.12). Clearly, there is an increasing interest by many companies, particularly baking companies, in the use of oxygen absorbent technology as an alternative or adjunct to gas packaging to prevent the perennial problem of mold growth. 6.2.4.3 Research activities in oxygen absorbent technology. Although the use of oxygen absorbent technology has not been accepted commercially with the same enthusiasm in North America compared to Japan, there is, however, tremendous interest and ongoing research in this innovative method of atmosphere modification. Studies in our laboratory have shown oxygen absorbents to be three times more effective than gas packaging for increasing the mold-free shelf-life of crusty rolls (Smith et al., 1986). While the mold-free shelf-life of crusty rolls can be extended by packaging in 60% CO2 and 40% N2, mold growth still occurred after 19 days due to the oxygen permeability of the packaging film. Studies have shown that Aspergillus and Penicillium species, the major spoilage isolates of crusty rolls, can tolerate and grow in < 1% (v/v) headspace oxygen even in the presence of elevated levels of CO2. However, when Ageless (Type S or Type FX) was packaged alongside crusty rolls, either alone or in conjunction with gas packaging, headspace O2 never increased beyond 0.05% and the product remained mold-free for > 60 days at ambient storage temperature (Smith et al., 1986). While a longer extension in the mold-free shelf-life was possible using oxygen absorbents, mold problems occasionally arose in the Ageless packaged product. This was due to absorption of headspace gas and package collapse resulting in some products being tightly wrapped with the packaging film. This created pockets of localized environments between the product surface and the film where the oxygen concentration may have increased to a level sufficient to permit mold growth. This clearly demonstrates the need for a free flow of gas around the product if Ageless is to be totally effective as an oxygen scavenger (Smith et al, 1986). More recently, Ageless oxygen absorbents have been used in our laboratory to extend the mold-free shelf-life of fresh bagels and pizza crusts.
with the packaging film. This created pockets of localized environments between the product surface and the film where the oxygen concentration may have increased to a level sufficient to permit mold growth. This clearly demonstrates the need for a free flow of gas around the product if Ageless is to be totally effective as an oxygen scavenger (Smith et a/., 1986). More recently, Ageless oxygen absorbents have been used in our laboratory to extend the mold-free shelf-life of fresh bagels and pizza crusts. Furthermore, the use of specific types of absorbents appeared to have an anti-staling effect on bagels. Further studies are now underway to compare the anti-staling effects of oxygen absorbents with other methods of atmosphere modification for shelf-life extension of bakery products. The use of absorbent technology for shelf-life extension of food has also been extensively studied by Cryovac, a world leader in vacuum packaging technology. Cryovac in the US has been given the distribution rights to Ageless in North America by the Mitsubishi Gas Chemical Company. Researchers at Cryovac have found that oxygen absorbent technology can be used to prevent mold growth, discoloration and flavor changes in cooked cured sausage, cooked poultry and smoked poultry chops. Using a combination of oxygen absorbent technology and Cryovac's high barrier shrink film, headspace oxygen (which can reach 4-7% due to entrapped oxygen in the foam tray) is reduced to < 0.3% before products are exposed to retail display lighting. Thus, oxygen absorbent technology can be used to inhibit photodegradation of meat pigments in cured meat products thereby enhancing consumer appeal for such products (Cryovac, personal communication). Oxygen absorbent technology has also been investigated to inhibit mold growth and rancidity problems in fresh pasta and pizza crusts. Other areas of applied research by Cryovac involving oxygen absorbents include prevention of rancidity problems in snack food; prevention of flavor changes in wine stored in oxygen permeable PET bottles using a barrier pouch overwrap with an oxygen absorbent; and master packaging of fresh meat and poultry. This latter concept is in response to consumer concerns about absorbents being visible inside the packaged food product. In the MasterPak, retail cuts of meat/poultry are packaged in trays with an oxygen permeable overwrap. These are placed inside a cardboard carton with a high gas barrier film and absorbents added. The system is then evacuated, gas flushed and heat sealed and distributed under refrigeration to the retailer where the individual packs are removed from the MasterPak and displayed on retail shelves. Color or 'bloom' returns to the meat/poultry due to the low barrier characteristics of the overwrap. Extensive research on Freshilizer absorbents has been conducted by Multiform Desiccants in conjunction with Dr Joe Hotchkiss at Cornell University. Studies by Alarcon and Hotchkiss (1993) have shown that FreshPax oxygen absorbents can prevent mold growth in preservative-free white bread and low moisture, low fat mozzarella cheese for 8 weeks at
ambient/refrigerated temperature. Furthermore, oxygen absorbents were also effective in reducing the oxidative formation of rc-hexanal (an indicator of off-flavor development) in both sunflower seeds and cornchips. Sensory panel analysis also showed that absorbents inhibited the formation of undesirable rancid odors during accelerated storage tests (Alarcon and Hotchkiss, 1993). These authors also reported that FreshPax oxygen absorbents were also effective in controlling permeated oxygen, and hence iiiold growth ai/id rancidity problems, in bread and cheese packaged in jnedium barrieij films with an OTR of about 32 ml/m2/day. Therefore, medium barrier] films could be substituted for a high barrier and more expensive film if an oxygen absorber pack was included in the package (Alarcon and Hotchkiss, 1993). 6.2.5 Advantages and disadvantages of oxygen absorbents Oxygen absorbents have several advantages for the food processor, both from a marketing and food quality viewpoint (Harima, 1990; Smith et ai, 1990; Smith, 1992). These are: • •
Inexpensive and simple to use. Approved by the US Food and Drug Administration as non-toxic and safe to use. • Prevent aerobic microbial growth and extend shelf-life of product. • Arrest the development of rancid off-flavor in fats and oils. • Maintain flavor quality by preventing oxidation of the flavor compounds. • Maintain product quality without additives. • Increase product shelf-life and distribution radius. • Allow a long time between deliveries and allow fewer, longer deliveries. • Increase length of time product can stay in the distribution pipeline. • Reduce distribution losses. • Replace chemical pesticides to prevent insect damage of foods. • Reduce evacuation/gas flushing times in gas packaged products thereby increasing product throughput. • Reduce costs required for gas flushing equipment. A few of the disadvantages of oxygen absorbents are (Harima, 1990; Smith et aL, 1990; Smith, 1992): • •
There needs to be a free flow of air surrounding the sachet in order to scavenge headspace oxygen if an O2 absorbent is used alone. May cause package collapse; this can be overcome by using an O2 absorbent/CO2 generator.
• • • •
O2 absorbents/CO2 generators may cause flavor changes in high moisture/fat foods due to the dissolution of CO2 in the aqueous/fat phase of product. Cost - approximately 2.5-10 cents per sachet depending on size of sachet and volume ordered. Many processors regard oxygen absorbent sachets too expensive. Consumer concerns about sachets inside the packages and possible consumer misuse of sachets. May promote growth of potentially harmful anaerobic bacteria.
The two latter concerns will be briefly discussed. 6.2.5.1 Consumer resistance to sachets. The two main consumer concerns about sachets being placed inside the packaged food product are: fear of ingestion even although the contents are safe and the label clearly says 'Do not eat'; and spillage of sachet contents into food and consequent adulteration of the food product. These concerns have been addressed by the Mitsubishi Gas Chemical Company by producing the deoxidizer in a tablet form or in a board form and affixing it to the lid or to the bottom of packages. This approach has been successfully used for health foods sold in bottles and with individually wrapped teacakes in the Japanese marketplace. An alternative method is to package the primary packages inside a secondary packaging containing the absorbent. This is the principle behind the MasterPak system, i.e., bag-in-box system, designed to hold 12-48 consumer-size primary packages made of material with a higher oxygen transmission rate than for secondary containers, as discussed in the previous section. The main advantage of this approach is that the consumer never sees the absorbents, thus eliminating the possibility of consumers misusing the packets or reacting negatively to the absorbents (Smith, 1992). Another approach to overcome consumer resistance to sachets is to incorporate the oxygen scavenging capacity in the form of a package label. This approach has been developed by both Mitsubishi Gas Chemical Company and Multiform Desiccants. This latter company currently markets both Type B and Type M absorbents in the form of labels marketed under the trade name FreshMax. This label can be backed with various adhesives for different product applications. The technology has a printed surface and is contact acceptable (oil/grease resistant). The label can be applied at conventional line speeds and is compatible with both moist and dry food products. The prototype FreshMax is 2.5" x 2.5" and 15 mil thick. It is capable of absorbing 100 ml of oxygen in 24 h and can be made into various shapes. The labels can reduce headspace oxygen to less than 0.01% (100 ppm) and can be used to retard both chemical and microbiological spoilage in stored products. Marks & Spencer Ltd., London are now using FreshMax labels to extend the shelf-life and keeping quality of deli meats. The new
labels cover only one third of the lid surface, providing consumers with a clearer view of the product yet absorb oxygen and protect products from the adverse oxidizing effect of light and oxygen on meat pigments (Multiform Desiccants, personal communication). 6.2.5.2 Public health concerns. Another consumer and regulatory concern about oxygen absorbent technology is that the anaerobic environment created inside the package may be conducive to the growth of facultative anaerobic or strictly anaerobic food pathogens. These concerns are justified in view of the ability of many food pathogens to grow at refrigerator temperatures, e.g. Listeria monocytogenes and Clostridium botulinum type E; the inhibition of aerobic microorganisms as indicators of spoilage; and the potential for temperature abuse. Recent studies have shown that average temperatures of retail cases in supermarkets were about 12°C while temperatures of domestic refrigerators ranged from 2 to 200C (Palumbo, 1986). Nakamura and Hoshino (1983) reported that an oxygen-free environment alone was insufficient to inhibit the growth of Staphylococcus aureus, Vibrio species, Escherichia colif Bacillus cereus and Enterococcus faecalis at ambient storage temperature. For complete inhibition of these microorganisms they recommended combination treatments involving oxygen absorbents with thermally processed food or refrigerated storage or a CO2 enriched atmosphere. Using this latter combination, i.e., low O2-high CO2 gas atmosphere, they found that this modified atmosphere inhibited the growth of S. aureus and E. coli but promoted the growth of E. faecalis. While low levels of CO2 (20%) failed to inhibit the growth of B. cereus, higher concentrations (100%) completely inhibited the growth of this microorganism in an oxygen-free environment. When similar studies were done with Clostridium sporogenes, some interesting results were obtained, as shown in Table 6.13. These studies demonstrated that an O2/CO2 absorbent will inhibit the growth of C. sporogenes while an O2 absorbent-CO2 generator will enhance its growth, indicating the importance of selecting the correct absorbent to control the growth of Clostridium species in MAP food. In challenge studies with C. botulinum type A and B spores, Lambert et al. (1991a) observed that Table 6.13 Effect of various gas atmospheres on the growth of C. sporogenes (adapted from Nakamura and Hoshino, 1983) Packaging conditions _ Ageless oxygen absorbent generating carbon dioxide Ageless oxygen absorbent not absorbing carbon dioxide Ageless oxygen absorbent absorbing carbon dioxide
Growth of C. sporogenes _ 4+ 2+ 1+
- = no growth; 1+ = slight growth; 2+ = medium growth; 3+ = heavy growth; 4+ = extensive growth.
package headspace was not significant in affecting the time until toxin production (Lambert et al, 1991b,c). In more recent agar plate studies with Listeria monocytogenes, high CO2 levels (> 60%) promoted the growth of this pathogen at 10-150C (Morris et al, 1994). However, when an oxygen-free environment was achieved using Ageless SS, growth of L. monocytogenes was completely inhibited even at mild temperature abuse storage conditions. Further studies are now underway to determine the antimicrobial efficacy of Ageless SS and gas packaging on L. monocytogenes in packaged pork. 6.2.6 Effect of oxygen absorbents on afiatoxigenic mold species While the use of oxygen absorbent technology may fail to completely inhibit the growth of facultative or strictly anaerobic pathogenic bacteria, it is very effective in controlling the growth of Aspergillus flavus and Aspergillus parasiticus. In inoculation studies with these aflatoxigenic molds in peanuts packed in air alone and with an oxygen absorbent, mold growth and aflatoxin were completely inhibited in absorbent packaged peanuts while ~1000 ppb (1000 ng) of aflatoxin B1 were detected in all air-packaged samples after only 6 days at room temperature (Ellis et al, 1993). Similar control of A. parasiticus has also been reported in inoculation studies with peanuts using oxygen absorbent technology (Ellis et al, 1994). However, this control was dependent on both the OTR of the packaging films used and storage temperature. While packaging in high-medium barrier films inhibited aflatoxin B1 production, aflatoxin was detected in all absorbent packaged peanuts using a low barrier film (OTR -4000 ml/m2/day). However, when absorbent packaged peanuts were packaged in medium barrier films (OTR - 5 0 ml/m2/day), aflatoxin production occurred in all peanuts stored at 300C (Ellis et al, 1994). This was attributed to the greater permeance of the film to oxygen at higher storage temperatures, resulting in saturation of the absorbents, a concomitant increase in headspace oxygen, and subsequent mold growth and aflatoxin production. Similar results were obtained with a CO2 generating oxygen absorbent (Ageless G), indicating the importance of packaging film permeability to ensure the efficacy of oxygen absorbents and the public health safety of absorbent packaged peanuts. 6.3 Ethanol vapor The use of ethanol (ethyl alcohol) as an antimicrobial agent is well documented. Alcohol was used by the Arabs over 1000 years ago to preserve fruit from mold spoilage. However, alcohol is most commonly recognized as a surface sterilant or disinfectant. In high concentrations (60-75% v/v),
ethanol acts against vegetative cells of microorganisms by denaturing the protein of the protoplast (Seiler and Russell, 1993). Lower concentrations of alcohol (5-20% v/v) have also been shown to have a preserving action against food spoilage and pathogenic microorganisms in agar model systems. In tests with surface inoculated agar medium containing concentrations of ethanol ranging from 4 to 12% (v/v) ethanol was shown to be effective in controlling 10 species of mold including Aspergillus and Penicillium species; 15 species of bacteria including S. aureus and E. coli; and 3 species of spoilage yeasts (Freund Technical Information, 1985). Most molds were inhibited by 4% ethanol while yeasts were more resistant and required 8% ethanol for complete inhibition. However, S. aureus required 12% ethanol for complete inhibition (Freund Technical Information, 1985). Shapero et al. (1978) reported that the effectiveness of ethanol against S. aureus was a function of water activity (aw). They reported that in broth media adjusted to aw 0.99, 0.95 and 0.9 with glycerol, inhibition of S. aureus occurred at 9, 7 and 4% of ethanol respectively. A similar synergism between aw and the antimicrobial efficacy of ethanol against A. niger and R notatum has been observed by Smith et al. (1994). In studies with unadjusted Potato Dextrose Agar (PDA) plates a concentration of 6% ethanol was required for complete inhibition of these common mold contaminants of bakery products. However, complete inhibition of A. niger and P. notatum was observed with 4 and 2% of ethanol respectively in plates adjusted to aw 0.95 and 0.9 with glycerol. These results confirm previous observations that lower concentrations of ethanol can be used to extend the mold-free shelf-life of food products with a low aw. The antimicrobial effect of low concentrations of ethanol for shelf-life extension of bakery products has also been demonstrated. Plemons et al. (1976) showed that the mold-free shelf-life of pizza crusts could be extended for about 20 weeks at ambient temperature by spraying the crusts with 95% ethanol and overwrapping in a large unsealed polyethylene bag (Seiler, 1978; Seiler, 1988). Seiler and Russell (1993) also reported a 50-250% increase in the mold-free shelf-life of cake and bread sprayed with 95% ethanol to give concentrations ranging from 0.5 to 1.5% by weight of product. Both these studies indicate the potential of ethanol as a vapor phase inhibitor. However, a more practical and safer method of generating ethanol vapor is through the use of ethanol vapor generating sachets, which will now be discussed in greater detail. 6.3.1 Ethanol vapor generators A novel and innovative method of generating ethanol vapor, again developed by the Japanese, is the use of ethanol vapor generating sachets or strips. These contain absorbed or encapsulated ethanol in a carrier material and enclosed in packaging films of selective permeabilities which allow the
Table 6.14 Manufacturers of ethyl alcohol generators
6J.I
Trade name
Manufacturer
Antimold (Ethicap) Negamold Oitech ET Pack
Freund
Sachets/yr 60m
Entered 1978
Freund Nippon Kayaku Ueno Seiyaku
9m 1m
1982 1980 1988
Ethanol vapor generators
A novel and innovative method of generating ethanol vapor, again developed by the Japanese, is the use of ethanol vapor generating sachets or strips. These contain absorbed or encapsulated ethanol in a carrier material and enclosed in packaging films of selective permeabilities which allow the slow or rapid release of ethanol vapor. Several Japanese companies produce ethanol vapor generators, with the most commercially used system being Ethicap or Antimold 102 produced by the Freund Industrial Co., Ltd., Japan (Table 6.14). Ethicap consists of food grade alcohol and water (55% and 10% by weight respectively) adsorbed on to silicon dioxide powder (35%) and contained in a sachet made of a laminate of paper/ethyl vinyl acetate copolymer. To mask the odor of alcohol, some sachets contain traces of vanilla or other flavors as masking compounds. The sachets are labelled 'Do not eat contents' and include a diagram demonstrating this warning. Another type of ethanol vapor generator is Negamold (Table 6.15), which scavenges oxygen as well as generating ethanol vapor. Negamold, like its competitor, Ageless type SE, can be used to extend the shelf-life and keeping quality of bakery products. However, Negamold is not widely used since it does not generate as much ethanol vapor as Ethicap (Freund Technical Information, 1985). Sachet sizes of Ethicap range from 0.6 to 6 G or 0.33 to 3.3 g of ethanol, which can be evaporated. The actual size of the sachet used depends on the weight of food; aw of food; and the desired shelf-life of product. This interrelationship is shown in Figure 6.6. For example, if the aw of the product is 0.95 and a 1-2 week shelf-life is desired, then a 2 G size of Ethicap Table 6.15 Types of alcohol generators _ Type
Function
Application
Ethicap (Antimold 102)
Generates EtOH vapor
Negamold
Absorbs O2 Generates EtOH vapor
Moisture dependent Cakes/ Bread Moisture dependent Cakes/ Bread
_
products > 0.85 > 0.85
Required sachet size of ETHICAP® per 10Og of food
(~ 1.1 g ethanol) should be used per 100 g of product. However, if a longer shelf-life is desired (2-3 months), a 4 G size of Ethicap should be used for the same product (Freund Technical Information, 1985). When food is packed with a sachet of Ethicap, moisture is absorbed from the food, and ethanol vapor is released from encapsulation and permeates the package headspace. However, both the initial and final level of ethanol vapor in the package headspace is a function of sachet size and water activity, as shown in Figure 6.7. Smith et al. (1987) examined this relationship in PDA plates adjusted to aw 0.95 and 0.85 with glycerol and packaged in a film of low ethanol vapor permeability with a 1 G and 4 G (E1 and E4) size of Ethicap respectively. After day 1, the level of headspace ethanol generated from a 1 G sachet of Ethicap (E1), ranged from 0.7% v/v at a water activity of 0.99 to 0.5% v/v at a water activity of 0.85 (Smith et al, 1987). However, after 7-10 days' storage, headspace ethanol was approximately 0.4% v/v at a water activity of 0.85 compared to 0.2% v/v at the highest water activity under investigation and remained at these levels for the duration of the storage period (Smith et al., 1987). A similar, higher trend was noted for headspace ethanol generated from 4 G of Ethicap, E4 (Smith et al., 1987). These studies indicated the importance of product aw on the vaporization of ethanol into, and absorption of ethanol from, the package headspace.
for long term for short term
Water activity of food
Figure 6.6 Relationship between aw of food and required size of Ethicap sachet. Short term, 1-2 weeks; long term, 8-13 weeks. (Reproduced with permission from the Freund Technical Co., Ltd., Japan.)
-I*
O Z < Z Ul Ul O (0
S
Ul X
DAYS STORAGE
25 *C
Figure 6.7 Effect of water activity (aw) on generation and absorption of ethanol vapor. •, 0.85 + E4; A 0.95 + E4; •, 0.99 + E4; o, 0.85 + E1;*, 0.95 + E1; o, 0.99 + E1.
Another important factor in using Ethicap as a food preservative is to package the product in a film with a high or medium barrier to ethanol vapor. The ethanol vapor permeabilities of appropriate packaging materials are shown in Table 6.16. Generally, a film with an ethanol vapor permeability of < 2 g/m2/day @ 300C is recommended (Freund Technical Information, 1985). 6.3.2 Uses of Ethicap for shelf-life extension of food Ethicap is used extensively in Japan to extend the mold-free shelf-life of high ratio cakes and other high moisture bakery products. A 5-20 times extension in mold-free shelf-life has been observed for high ratio cakes depending on the size of Ethicap used (Freund Technical Information, 1985). Results also showed that products with sachets did not get as hard as the controls and results were better than those using an oxygen scavenger to
Table 6.16 Ethanol vapor permeability of certain plastic films (adapted from Freund Technical Information, 1985) Film _ _ _ _ _
Ethanol vapor permeability (g/m2/day) @ 300C ___
OPP/EVOH/LDPE PVDC/PET/PP PVDC/OPP/PP AL/PET/LDPE PP/PVDC/PP PET/PP OPP/PP HDPE OPP PP LDPE EVA/LDPE
0.8 0.9 1.0 1.2 1.5 1.8 2.0 4.1 4.7 8.0 19.0 56.1
PVDC = polyvinylidene chloride; PA = nylon; OPP = oriented polypropylene; PP = cast polypropylene; PET = polyester; AL = aluminium; HDPE = high-density polyethylene; LDPE = low-density polyethylene; EVA = ethylene vinyl acetate.
inhibit mold growth, indicating that the ethanol vapor also exerts an antistaling effect (Freund Technical Information, 1985). Ethicap is also widely used in Japan to extend the shelf-life of semi-moist and dry fish products. Examples of bakery and fish products preserved by Ethicap in the Japanese market are shown in Table 6.17. Pafumi and Durham (1987) also found that the mold-free shelf-life of 200 g Madeira cake could be extended for about 6 weeks at room temperature using a 3 G sachet of Ethicap. However, there was a significant change in Table 6.17 Use of Ethicap for shelf-life extension of food (adapted from Freund Technical Information. 1985") Food
Size of Ethicap
Packaging material
0.92 0.85 0.83 0.80 0.76 0.72
IG 3G 3G 2G 4G 2G
OPP OPP/PP OPP OPP/PP OPP/PP OPP/PP
1 week 2 months 20 days 6 months 2 months 6 months
0.85 0.68 0.69 0.63
IG 0.6G 0.6G IG
OPP OPP OPP/PE OPP
3 3 2 3
aw of food
Shelf-life @ RT
Bakery products Bread Cupcake Jam doughnut American cake Rice cake Chocolate sponge cake Fish products Smoked squid Boiled squid Boiled & dried squid Boiled & dried small fish
months months months months
quality after 3 weeks, characterized by a loss of moistness and firmness of the texture and development of soapy and rancid flavors. Therefore, while Ethicap could be used to extend the mold-free shelf-life of the product, the sensory quality of the product was not significantly extended (Pafumi and Durham, 1987). Black et al (1993) examined the combined effect of gas packaging and Ethicap (size unknown) to extend the shelf-life of pita bread. They reported a 14 day mold-free shelf-life for pita bread packaged in 40% CO2 (balance N2) or 100% CO2. However, the shelf-life was doubled when an Ethicap sachet was incorporated into the gas packaged products (Black et al, 1993). However, these authors reported that ethanol vapor had little anti-staling effect on pita bread. While the effect of increased firmness due to staling was reversed by microwave heating, it failed to eliminate the stale flavors in the pita bread (Black et al, 1993). The authors also observed a slight increase in headspace oxygen in all gas packaged products stored with Ethicap. They hypothesized that the ethanol vapor may have dissolved in the film and acted as a plasticizer, thereby affecting the permeability of the film to both oxygen and carbon dioxide (Black et al, 1993). Ethicap has also been used as a means of further extending the shelf-life of gas packaged apple turnovers (Smith et al., 1987). Apple turnovers, with a water activity of 0.93, had a shelf-life of 14 days when packaged in a CO2: N2 (60:40) gas mixture at ambient temperature due to growth of and CO2 production by Saccharomyces cerevisiae. However, when Ethicap (E4) was incorporated into the packaged product either alone or in conjunction with gas packaging, yeast growth was completely suppressed and all packages appeared normal at the end of the 21-day storage period (Smith et al, 1987). Ethicap has also proved effective in controlling secondary spoilage problems by 5. cerevisiae in gas packaged strawberry and vanilla layer cakes and cherry cream cheese cake. Ooraikul (1993) reported that 5 G sachets of Ethicap inhibited growth of, and CO2 production by, 5. cerevisiae in both strawberry and vanilla layer cakes. This size of the Ethicap sachet had no adverse effect on the organoleptic quality of cakes. Indeed, Ooraikul (1993) reported that the aroma of the cake packaged with Ethicap was more pleasant than that of the fresh cake and that the taste also remained excellent. However, a 6 G Sachet of Ethicap failed to inhibit yeast spoilage in cherry cream cheese cake (Ooraikul, 1993). He recommended a 7-8 G sachet of Ethicap in conjunction with a preservative, such as ethyl paraben, to control secondary yeast fermentation problems in cherry cream cheese cake (Ooraikul, 1993). These studies show that larger sizes of Ethicap, and hence higher levels of ethanol vapor, are required to inhibit yeast growth compared to mold growth. Indeed, agar model studies have shown that yeasts can grow in media containing 8% (v/v) ethanol while most molds were inhibited by 4% ethanol (Freund Technical Information, 1985).
633
Effect of ethanol vapor on food spoilage/food poisoning bacteria
While most studies to date have focused on the use of Ethicap as an antimycotic agent, few studies have evaluated its potential to control food spoilage and food poisoning bacteria. Ethanol, when incorporated into agar media, has proved to be effective against several spoilage and pathogenic bacteria (Freund Technical Information, 1985; Shapero et al., 1978). Seiler and Russell (1993) also reported that ethanol at a level of 1% by product wt., delayed the onset of 'rope' caused by the growth of Bacillus subtilis. They also reported that low concentrations of ethanol (0.5-1% by product wt.) inhibited bacterial growth in both whipping cream and custard, two wellknown vectors of food poisoning bacteria in filled bakery products. These studies clearly illustrate the antibacterial properties of ethanol when incorporated directly into media or a food product. More recently, Morris et al. (1994) examined the effect of Ethicap on the growth of Listeria monocytogenes, a psychrotrophic food-borne pathogen of public health significance. They observed that a 4 G sachet of Ethicap could control the growth of L. monocytogenes (Scott A) on agar media at 5, 10 and 15°C, the latter storage conditions representing mild temperature abuse. Further studies are now underway to determine the volume of ethanol vapor generated at these storage temperatures and the effect of these concentrations on the microbiological and chemical shelf-life of fresh packaged pork. 63 A Advantages and disadvantages of ethanol vapor generators According to Smith et al. (1987) the advantages of Ethicap are: •
Ethanol vapor can be generated from sachets without having to spray ethanol directly onto product surface prior to packaging. • Sachets can be conveniently removed from packages and discarded at the end of the storage period. • It eliminates the need for preservatives such as benzoic acid or sorbic acid to control yeast spoilage. • It can control mold spoilage and delay staling in bakery products. A disadvantage of using ethanol vapor for shelf-life extension is its absorption from the package headspace by product. However, the concentration of ethanol found in apple turnovers (1.45-1.52%) was within the maximum level of 2% by product weight when ethanol is sprayed onto pizza crusts prior to final baking (Smith et al., 1987). However, the level of ethanol in apple turnovers can be reduced to < 0.1% by heating product at 375° F prior to consumption. Therefore, while a longer shelf-life may be possible by packaging product with Ethicap, its use as a food preservative may be limited to 'brown & serve' or microwaveable products. Another disadvantage is cost, which limits its use to products with higher profit
margins. Nevertheless, Ethicap is a viable alternative or supplement to gas packaging to extend the shelf-life of baked bakery products in relation to both mold and yeast spoilage and to prevent staling. 6.4
Conclusion
In conclusion, the use of gas absorbents and ethanol vapor generators is, without doubt, one of the most exciting interactive packaging technologies available to the food industry. While both oxygen absorbent technology and ethanol vapor generators are used extensively in Japan to extend the shelflife and keeping quality of a variety of products, their use to date in the North American market is limited due to the cost of the sachets, consumer resistance to the inclusion of sachets in packaged products and lack of regulatory approval for Ethicap. Nevertheless, the use of gas absorbents/ ethanol vapor generator sachets or labels offers the food industry a more viable alternative method of interactive packaging than vacuum/gas flushing for shelf-life extension of its products. References Abe, Y. and Kondoh, Y. (1989) Controlled/Modified Atmosphere/Vacuum Packaging of Foods, Food & Nutrition Press PubL, Trumbull, CT, pp. 149-58. Alarcon, B. and Hotchkiss, J.H. (1993) The effect of FreshPax oxygen-absorbing packets on the shelf-life of foods. Technical Report, Dept. of Food Science, Cornell University, NY, 1-7. Anon. (1988) Ener-Getic all year long. Packaging Digest, 8, 70, 72, 75. Black, R.G., Quail, K.J., Reyes, M., Kuzyk, M. and Ruddick, L. (1993) Shelf-life extension of pita bread by modified atmosphere packaging. Food Australia, 45, 387-91. Ellis, W.O., Smith, J.P., Simpson, B.K., Khanizadeh, S. and Oldham, J.H. (1993) Control of growth and aflatoxin production by Aspergillus flavus under modified atmosphere packaging (MAP) conditions. Food Microbiology, 10, 9-21. Ellis, W.O., Smith, J.P., Simpson, B.K. and Doyon, G. (1994) Effect of films of different gas transmission rates on aflatoxin production by Aspergillus flavus in peanuts packaged under modified atmosphere packaging (MAP) conditions. Food Research International, 27, 505-12. FreshPax Technical Pamphlet (1994) Protect and preserve your products and profits with FreshPax. Multiform Desiccants, Buffalo, NY. Freund Technical Information (1985) No-mix-type mould inhibitor Ethicap. Freund Industrial Co., Ltd., Tokyo, Japan, pp. 1-14. Harima, Y. (1990) Food Packaging, Academic Press PubL, London, UK, pp. 229-52. Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991a) Combined effect of modified atmosphere packaging and low-dose irradiation on toxin production by Clostridium botulinum in fresh pork. / . Food Protection, 54, 97-104. Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991b) Effect of headspace CO2 on toxin production by Clostridium botulinum in MAP, irradiated fresh pork. / . Food Protection, 54, 588-92. Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991c) Effect of initial O 2 and CO2 and low-dose irradiation on toxin production by Clostridium botulinum in MAP fresh pork. / . Food
Protection, 54, 939-44. Minakuchi, S. and Nakamura, H. (1990) Food Packaging, Academic Press PubL, London, UK, pp. 357-78.
Morris, J., Smith, J.P., Tarte, I. and Farber, J. (1994) Combined effect of chitosan and MAP on the growth of Listeria monocy togenes. Food Microbiology; (Submitted for publication). Nakamura, H. and Hoshino, J. (1983) Techniques for the preservation
of food by the
employment of an oxygen absorber. Mitsubishi Gas Chemical Co., Tokyo, Japan, 1-45. Ooraikul, B. (ed.) (1993) Modified Atmosphere Packaging of Food, Ellis Horwood Publ, New York, NY, pp. 49-117. Pafumi, J. and Durham, R. (1987) Cake shelf life extension. Food Technology in Australia, 39, 286-7. Palumbo, S.A. (1986) Is refrigeration enough to restrain food borne pathogens? /. Food Protection, 49, 1003-9. Plemons, R.F., Staff, CH. and Cameron, F.R. (1976) Process for retarding mold growth in partially baked pizza crusts and articles produced thereby. US Patent 3979525. Powers, E.M. and Berkowitz, D. (1990) Efficacy of an oxygen scavenger to modify the atmosphere and prevent mold growth in meal, ready-to-eat pouched bread. /. Food Protection, 53, 767-71. Rooney, M. (1992) Reactive Packaging Materials for Food Preservation. In: Proceedings of the First Japan-Australia Workshop on Food Processing, Tsukuba, Japan, pp. 78-82. Seiler, D.A.L. (1978) The microbiology of cake and its ingredients. Food Trade Review, 48, 339-44. Seiler, D.A.L. (1988) Microbiological problems associated with cereal based foods. Food Science and Technology Today, 2, 37-41. Seiler, D.A.L. and Russell, NJ. (1993) Food Preservatives, Blackie Academic & Professional, Glasgow, UK, pp. 153-71. Shapero, M., Nelson, D.A. and Labuza, T.P. (1978) Ethanol inhibition of Staphylococcus aureus at limited water activity. /. Food Science, 43, 1467-9. Smith, J.P., Ooraikul, B., Koersen, WJ. and Jackson, E.D. (1986) Novel approach to oxygen control in modified atmosphere packaging of bakery products. Food Microbiology, 3, 315-20. Smith, J.P., Ooraikul, B., Koersen, WJ., van de Voort, F.R., Jackson, E.D. and Lawrence, R.A. (1987) Shelf life extension of a bakery product using ethanol vapor. Food Microbiology, 4, 329-37. Smith, J.P., Ramaswamy, H. and Simpson, B.K. (1990) Developments in food packaging technology. Part 2: Storage aspects. Trends in Food Science and Technology, 1, 112-19. Smith, J.P. (1992) MAP Packaging of Food - Principles and Applications, Academic and
Professional Publ., London, UK, pp. 134-69. Smith, J.P., Lyver, A. and Morris, J. (1994) Effect of ethanol vapor on the growth of common mold contaminants of bakery products. Food Microbiology; (Submitted for publication). Toppan Technical Information (1989) Freshness keeping agents. Toppan Printing Co., Ltd., Tokyo, Japan, pp. 1-8. Young, L.L., Reviere, R.D. and Cole, A.B. (1988) Fresh red meats: a place to apply Modified Atmospheres. Food Technology, 42, 65-9.
7 Enzymes as active packaging agents A.L. BRODY and J.A. BUDNY
This chapter discusses the role of enzymes in active packaging, especially as oxygen scavengers. Almost all mechanisms in which packaging structures function in response to a stimulus involve physical, chemical or physiochemical actions. In a physical action, the active element of the material, which is usually external, opens, expands, closes, contracts, etc. as one or more variables are changed. In chemical or physiochemical reactions, a component of the total package reacts with the package structure or component, usually in an irreversible manner, with the result that the active component is effectively consumed or changed as the internal package environment is changed. However, in catalytic processes physiochemical or chemical reactions occur in which the catalyst remains effective and intact. Enzymes, which are biological catalysts, accelerate chemical reactions but are not consumed as a result of the reactions. Within limits, for as long as reactants or substrates are present, enzymes will function to catalyze chemical, or more specifically biochemical, reactions. When the proper enzymes are introduced under the proper conditions, they are capable of catalyzing reactions which can either prevent the product from being changed or extend the function of packaging beyond its accepted or previously understood functions by actively serving as a processing unit. 7.1
Enzymes
Enzymes are biological catalysts which are found in all living cells, whether plant or animal. These macromolecular proteins exhibit two outstanding characteristics in addition to the fact that they occur naturally and are found in living systems. The first characteristic is their catalytic power. Enzymes accelerate chemical reactions that occur in biological systems by factors that exceed a million over their uncatalyzed rate. In essence, enzymes allow living systems to carry out reactions that would not ordinarily occur or occur so slowly that the rates would not be of any practical significance. A simple reaction of the formation of carbonic acid from carbon dioxide and water occurs 107 times faster with the enzyme carbonic anhydrase than the nonenzymatic or chemical reaction.
The second important characteristic of enzymes is their specificity. Enzymatic specificity takes on two distinct forms: the type of chemical reaction; and for any type of chemical reaction, a specificity for the reactant or substrate. Consequently, for each chemical reaction that occurs in a biological system, there is a unique enzyme required for the optimal production of reaction products. With so many different biological reactions, it follows that there are many different enzymes. As with all catalysts, enzymes do not alter equilibrium conditions. An enzyme increases a forward reaction in the same way and to the same extent that it increases the reverse reaction, i.e., enzymes accelerate the rate at which a chemical equilibrium is reached but an enzyme does not distort the ratio of the equilibrium concentrations of the products to reactants. The catalytic potential of enzymes and the speed at which they facilitate chemical reactions lies in their ability to reduce the Gibbs Free Energy of Activation (Ea). Enzymes accomplish their catalytic objectives, not by reducing the Ea of the uncatalyzed reaction but by creating a new and different transition state and hence a different reaction path or mechanism. This new or different transition state is the enzyme-substrate complex (ESC) where the reactant becomes associated or bound to the free enzyme at the reactive center or site, followed by the release of the product which generates the free enzyme again. The now free enzyme is once again available to combine with another molecule of reactant to repeat the process. The net effect of this sequence is that reactant or substrate becomes product and the enzyme is unchanged. The active site is an area or region of an enzyme where the bond-breaking and bond-forming of the reactants and products occur. The participation, and hence the reactivity, of an enzyme for a particular substrate-product pair is determined by the amino acid sequence and the geometric or spatial arrangement of the enzyme. Because enzymes are high molecular weight polymers which are made up of amino acids, it is not surprising that the active site represents only a small percentage of the total enzyme. It is also not surprising that the polymeric catalysts are three dimensional, and consequently the active site has a size (volume) shape to it. This spatial characteristic of the active site defines the size, shape and type of substrates or reactants which can be catalyzed by the enzyme. The kinetics of enzyme reactions are obviously of great importance in considering their potential commercial applications. At the outset, enzymatic reaction rates are linear with time until all of the free enzyme is used to form the ESC. When all of the enzyme exists as ESC, or as soon as the product is formed, the enzyme reacts with another reactant or substrate molecule, and the rate of conversion of reactant to product plateaus at the maximal reaction rate or velocity. Once the initial velocity has been achieved, all of the enzyme exists as the ESC.
Although enzymes may be classified according to the substrates they affect, as, for example, proteases for proteins, lipases for lipids, etc., in reality, these are designations for broad families to break proteins of entities that are specific to a single protein or lipid under a particular set of circumstances. An enzyme suitable for a single 16 carbon fatty acid oxidation reaction will not catalyze an 18 carbon fatty acid oxidation even though the actual reactions at the sites may be identical. This characteristic may be viewed as beneficial in that only the specific reaction and no other is catalyzed by the enzyme. On the other hand, this attribute may be regarded as undesirable since a specific enzyme is required for a specific reaction, and no single enzyme can effect a series of related reactions. Enzymes are proteins whose reactivity is quite sensitive to temperature. At temperatures as low as 1400F (680C), the catalytic reactivity of the enzymes may be temporarily or permanently disrupted, thus rendering enzymes among the most vulnerable of all biological matter. This temperature sensitivity is an important consideration in the commercial application of enzymes in processing operations. Among the many enzymes functioning in reactions that have been and are being used commercially are rennin (chymosin) to precipitate the casein of milk in cheese making; proteases in laundry detergents to assist in protein stain removal; amylase to convert starch to sugar for brewing; lactase to break down lactose in milk; various oxidases to accelerate oxidative reactions; and catalase to remove hydrogen peroxide that might be formed during prior oxidative reactions. Other, more generic applications of enzymes include stereospecific amino acid production, high fructose sugar production, beer and wine fermentation, tenderizing meats, milling and baking, juice and wine clarification, juice extraction from fruits and production of flavor enhancers, to cite a few. 7.2 Potential roles of enzymes in active packaging In many commercial situations, enzymes may be viewed as chemicals to be added to the product to catalyze a reaction as one way to affect batch processing. The addition can occur to the in-plant batch or individual package. For in-package situations, the enzyme may be added directly to the product to effect a reaction or may be incorporated into the package structure. To function within a package material, the enzyme must be immobilized and the substrate, reactant or a constituent circulated past the site to initiate a reaction. Immobilization of an enzyme, or placing it in a static position where it may function for an indefinite period, may be accomplished by making the enzyme an integral part of the packaging material. Active packaging in general often involves the incorporation of a
chemical into the package material. Active packaging employing one or more enzymes involves the incorporation into the package material of the specific enzymes in much the same manner as the incorporation of a more conventional chemical to create the active package. The key differences are that the enzyme is not changed by the reaction and can continue to function indefinitely; the enzyme is vulnerable to variations in temperature, pH, etc.; and the range of environmental conditions for the functioning of the enzyme is a relatively narrow band. These key considerations which affect the ability of the enzyme to function require special processes and techniques for incorporating enzymes into packaging materials. Often, harsh manufacturing processes, geometric configurations, etc. that are adequate and even appropriate for non-enzyme packaging components render the use of enzymes inappropriate. Consequently, new and innovative methods are likely to be required for the incorporation of enzymes into packaging materials. Although a broad range of enzymatic reactions stemming from enzyme incorporation into package materials can be conceived, only a relatively small number have actually been attempted on a practical basis. Examples of those that have been actively pursued include: • Oxygen removed by means of glucose oxidase plus catalase. • Removal of products of microbiological degradation by glucose oxidase/ catalase. • Incorporation of lactase to remove lactose from milk. • Incorporation of cholesterol-changing enzymes to remove cholesterol from liquid egg or milk. • Time-temperature integrator indicators which are triggered enzymatically. Examples of enzymatic reactions that have not found general use but which might have some future potential and require development include: • Conversion of sugar into alcohol and carbon dioxide in secondary fermentations of wine to produce champagne-like products. • A United Kingdom patent application (Thomas and Harrison, 1983) which describes an in-package secondary fermentation system using immobilized yeast within a liquid porous container immersed in an alcoholic beverage. In one manifestation, the container was a flexible pouch. Further, the inventors refer to isolated enzyme complexes as being useful as yeast substitutes. The objective here was to consume the residual fermentable sugars, converting them into carbon dioxide and water. • In-package production of lactic acid for pickles, sauerkraut or sour dairy products. • Production of 'natural' antimicrobial agents such as benzoic or propionic acids to help preserve the product contents.
• • • •
Destruction of natural toxins in foods. Removal of undesirable respiratory end products such as ethylene that accelerate the respiratory processes of fresh fruits and vegetables. Removal of undesirable end products of microbiological or endogenous enzymatic reactions such as polypeptides, carbonyls, ketones, volatile acids, etc. Tenderizing of fresh meat such as beef by proteases such as papain.
7.3 History Although many enzymes and their roles have been known for several decades, the notion of incorporating them into package materials to achieve a desirable result dates back only to the 1940s. Almost simultaneously with the idea of protecting against browning of dry foods such as eggs by removing residual oxygen, the notion of in-package glucose oxidase/catalase reactions was born. In reality, the initial action of glucose oxidase is with residual quantities of glucose, a reducing sugar active in the non-enzymatic, non-oxidative Maillard browning reactions. Highly reactive hydrogen peroxide is produced by glucose oxidase, and is removed by catalase which breaks it into water and oxygen. This concept was put into practice by employing porous packets of the enzyme mix in which the enzymes slowly reacted with minute quantities of residual oxygen, an analogue of the commercial incorporation of sachets of desiccants to reduce the in-package relative humidity. The applications during the 1940s and 1950s appear to have been largely confined to very long term storage of military foods. The concern for the adverse effects of temperature abuse on frozen foods led to numerous ventures into development of time-temperature indicators, among which have been enzymatically actuated versions, beginning in the 1970s. The exponential growth of modified atmosphere packaging in the 1980s led to the notions of oxygen and carbon dioxide and moisture control using in-package sachets of chemicals. Some enzymatic agents were included in these chemicals. Towards the end of the 1980s, interest increased with the formation of PharmaCal, Ltd. whose objective was to develop the application of enzymes in unit size situations. This company and its principal, the co-author of this article, suggested and, in some instances, physically evaluated three areas in which immobilized enzymes within package structures would catalyze reactions of products contained within packages. • Lactase to remove lactose. • Cholesterol reductase to remove cholesterol. • Glucose oxidase/catalase to remove oxygen. Whether or not the communications emanating from PharmaCal were
directly responsible, several other enzymatically based active packaging oxygen control devices have been proposed since that time. 7.4 Oxygen removal As is documented elsewhere in this book, oxygen is a highly reactive gas which can cause deterioration of almost all food products in terms of flavor, color, nutritional value and safety. Further, the presence of oxygen is essential to the growth and potential deteriorative effects of aerobic microorganisms including most bacteria, yeasts and molds. Thus, minimization or removal of oxygen is an important factor in prolonging the quality retention of many food products. According to Baker (1949, but evidently conceived in 1944) the addition of an oxidase to liquid-containing food products such as beer, peas, corn, milk, apple cider or orange juice, protects them from oxidation. 'In some instances, it is better to produce in the product a substrate for the oxidase that is to be introduced rather than to use a substrate already present.' For example, glucose originally present or added can be oxidized to gluconic acid. Baker's patent indicates that if the oxidase produces an objectionable end product such as hydrogen peroxide, then an additional enzyme might be introduced to remove the undesirable end product. Among the interesting aspects of this early patent is the notion that as the oxygen in the product is removed, free oxygen in the headspace is further dissolved by equilibrium dynamics, thus removing oxygen from the headspace. The reaction, now very well known, is 2G + 2O2 + 2H2O -> GO + 2H2O2 where G is the substrate. Since hydrogen peroxide is a very good oxidizing agent, it is 'just as objectionable, or even more so, than is the original molecular oxygen.' Thus, catalase is introduced to break down the hydrogen peroxide 2H2O2 + catalase -» 2H2O2 + catalase The sum of these two reactions yields half the oxygen originally present and therefore ultimately the free oxygen approaches zero. Baker's invention was implemented by introducing one or more pellets of the enzyme into the product such as beer or orange juice. The patent also mentions the incorporation of lactase to hydrolyze lactose into glucose and galactose which are then oxidized in the presence of oxidases. Perhaps without realizing the significance of this assertion, the patent suggested the use of enzymes to reduce the lactose content of milk. The patent does not explicitly describe precisely how the enzymes are incorporated. The inference is that the enzymes are introduced directly into
the product. Expressed differently, this patent does not indicate that the enzymes are either part of the package structure or in an independent packet within the primary package. Thus, although the 1949 patent described perhaps for the first time the employment of enzymes to eliminate inpackage oxygen, it did not indicate that the enzymes were part of the package material or structure. This concept of enzyme incorporation into a package material was first overtly described in a 1956 patent (Sarett, 1956). (Sarett, incidentally, was the assignee for the Baker patent.) In this patent, the same basic enzymatic reactions as in the Baker patent were reiterated as a reference, but the enzymes glucose oxidase and catalase in a solution were impregnated into or on a moistureproof or fabric sheet. The enzyme was bound to the sheet with a water-dispersible adhesive such as polyvinyl alcohol, starch, casein or carboxymethyl cellulose. The enzyme-coating face must contact the moist product to ensure that the requisite oxygen reduction reactions take place. The enzyme system was indicated to serve as a barrier to oxygen which would otherwise be transmitted through the sheet. Products described as being benefited by this system of oxygen reduction include cheese, butter, frozen foods subject to browning, etc. Although during the period of the patent a Kraft packaging paper called moistureproof (which, as it happens, was not actually moistureproof) was often used to package butter and cheese, the patent does not indicate the use of this material. Rather, the package material is described as having \ .. an exposed surface covered with a gas-permeable packaging material and having an inter layer between and in contact with packaging material and . . . food . .. inter layer providing an oxygen barrier. . . . ' The specific package materials identified were moistureproof cellophane, paper, rubber hydrochloride with impregnation employed for the papers and coating for the plastic and cellulose films. Also cited as being suitable substrates were wax paper, styrene, polyethylene and vinyls. Experiments discussed in the body of the patent indicated results in which oxidation of cheese surfaces was retarded by the presence of the enzymecontaining package material. In 1958, Scott (co-inventor on the 1956 Sarett patent) of Fermco Laboratories, published a paper on Enzymatic Oxygen Removal from Packaged Foods in which enzymes were incorporated into packaging materials or introduced into packets. Fermco Laboratories was a manufacturer of enzymes, one category of which was labeled Fermcozyme antioxidants, and of the packets which were named Oxyban. This paper marked the first publication to our knowledge on the use of packets of chemicals in packages. The glucose oxidase/catalase systems were derived from mold mycelia which were disrupted, filtered and further purified. To be effective in
reducing oxygen, glucose oxidase/catalase systems must be used in gas-tight packages. Among the applications indicated were: Aqueous foods • • •
direct incorporation, in mayonnaise or carbonated beverages; surface treatment in canned dog food; in packets in situations in which the enzyme and the product should be kept separate. Non-aqueous foods • direct incorporation; • in packets, as for chow mein noodles. The mayonnaise and carbonated beverage examples involved incorporation of the enzyme system directly into the products, with oxidative rancidity delayed in the former class of products and color fading (e.g., grape-flavor carbonated beverages) as well as flavor oxidations delayed in the latter. The dog food example was also a direct addition to retard surface discoloration on the top of the dog food in retorted cans. As a coating, the dried enzyme system was coated on the surfaces of package materials for processed cheese. Deposition of the enzymes was in solution form or via incorporation into a dry starch mixture prior to 'dusting' the package material surface. When the dry and therefore inactive enzyme picked up moisture from the product, it was activated and was a sufficiently good oxygen interceptor to control the formation of brown ring. Another series of experiments focused on obviating oxidative gray coloration on the surfaces of luncheon meats. Fermco's Oxyban product was a dry glucose oxidase/catalase/glucose/ buffers blend to be incorporated into products to reduce headspace and occluded and dissolved oxygen in dry foods such as coffee or soup. In another manifestation, the Oxyban was placed in small packets in which it reacted with oxygen in packages of roasted and ground coffee, smoked yeast or egg solids. Exactly how the enzyme was activated without moisture was not indicated, but clearly some moisture from the product was required. The author noted that this in-package packet was analogous to the desiccant packet. Three years later, Scott, then with Hammer (Scott and Hammer, 1958), elaborated on the oxygen-scavenging packet for in-package deoxygenation. Using the same glucose oxidase/catalase packet system described earlier from their laboratory experiments, they proceeded forward to a more commercially viable mechanism. Among the problems they enumerated were: • • •
Oxygen-scavenger surface area owing to the gas phase reaction. The need for moisture (cited above). Necessity to neutralize gluconic acid to avoid enzyme deactivation.
•
Package material structure allowing passage of oxygen but not moisture.
The gluconic acid problem was obviated using phosphate buffers. As little as 15 g of Oxyban enzyme mix in a packet was capable of removing all measurable oxygen from a sealed No. 2 size can held at ambient temperature. Once again, the type of package material used for the packet was not indicated. An interesting side note was an exploration of the use of glucose oxidase alone which, of course, led to an increase in the amount of hydrogen peroxide which would, in turn, slow the subsequent rate of oxygen uptake. The products benefited by the total system were primarily dry milk, potato granules and ice cream mix. An international patent application (Lehtonen et ai, 1991) described a package material containing an enzyme system to remove oxygen from the interior of the package by enzymatic reaction. By removing the oxygen, the growth of aerobic microorganisms was significantly retarded, and so this technology was favorable to shelf-life from both microbiological and chemical standpoints. The enzyme, for example, glucose oxidase, was incorporated into a package material with a gas-impermeable layer on the exterior and a gas permeable layer on the interior, i.e., the layer containing the oxygen-consuming enzyme was sandwiched between two plastic film layers. The background of this patent cited a 1969 German publication describing the use of glucose oxidase in package materials for the surface protection of meats, fish and cheese products but without elaboration. And, of course, the classical review paper by Labuza et al. (1989) described a similar technology of coating plastic film with glucose oxidase catalase, with the enzyme system activated by moisture from the food as Scott had previously cited. This patent application from Cultor Ltd. of Helsinki, Finland, details a flexible package structure containing an enzyme system in the liquid phase trapped between films, the outer of which might be polyamide or polyvinylidene-coated polyester. The inner film would be polyethylene which is generally not a good gas barrier. The enzymes of choice were oxidases of the oxidoreductase family using oxygenases and hydroxylases which bind oxygen to oxdizable molecules. The enzyme solution contains a buffer and a stabilizer, and may also be mixed with a filler. The enzyme layer was applied on the film by gravure or screen technique with the layer thickness being about 12 fim. The enzyme is not directly in contact with the contents. The film produced was employed either as the cover film layer or as the thermoformable bottom layer for tray-type packages.
The inventors noted that with increasing temperature, the gas permeability of package materials increases and so also does the ability of the enzyme system to reduce the oxygen content from the 20.9% of air to about 1% at ambient temperature within 24 hours. From technological and potential commercial perspectives, this Finnish work is so precise as to imply a major advance in the ability to implement the principles of enzymes as active package components. Co-author Budny and his company PharmaCal, Ltd. have been actively researching enzymes for active packaging since the 1980s. The contribution of PharmaCal, Ltd. to enzymes in active packaging was to expand the concept of packaging beyond the two long-regarded functions of packaging: containment of the product; and protection of the contents. These requirements originally were embodied in wine skins that ancient goat- and sheepherders used for their sustenance beverages. Throughout history, while there have been advancements in materials and approaches, there have not been any fundamental changes or additions to the necessary requirements for containers or packages. Whether they are animal skins, a lid or a multi-layer stock, they should protect the contents and not leak. PharmaCal, Ltd. added a third dimension to packaging by allowing an individual package to become a processing unit or to perform a process step or function that previously was limited to in-plant operations. With a combination of patent applications and proprietary technology, PharmaCal, Ltd. has been able to expand the concept of packaging to include processing steps, value-addition to packaged products and increased processing efficiencies. PharmaCal, Ltd. has developed a two-enzyme system involving glucose oxidase and catalase to intercept oxygen and has applied the technology for enzymes in active packaging to improve the proven concept of oxygen removal with the dual enzyme system of glucose oxidase and catalase. The use of the enzymes to remove oxygen has been acknowledged as not new, but their role in enzyme-based active packaging has been regarded as a more advanced application. Figure 7.1 illustrates the mechanism in which packaged liquid reacts enzymatically with glucose in the package wall to form gluconate. The resulting hydrogen peroxide is enzymatically reacted with catalase to produce oxygen and water that re-enter the contained product liquid. A container with an internal reactor, in reality an integral section of the package wall through which the liquid contents may flow, permits the enzymes to be retained for a reaction described in a 1989 patent application (Budny, 1989). A 1991 patent (Ernst, 1991), described a glucose/glucose oxidase enzyme mixture in a porous precipitated silica acid carrier. Calcium carbonate, calcium hydrogen phosphate, magnesium carbonate or disodium hydrogen
Head space
Gluconate
Glucose oxidase enzyme
Glucose Packaged liquid
Catalase
Outside of container
enzyme
Inside of container Container wall
Figure 7.1 Oxygen removal from liquid products.
carbonate may also be employed as carriers or reaction accelerators. The oxygen scavenger may be in the interior of in-package sachets. A 1991 patent (Copeland et al, 1991) describes the incorporation of oxygen scavenging cell membrane fragments which contain an electron transfer system in solutions containing alcohol or acids to reduce oxygen to water. Although neither purified nor crude enzymes, the active component of membrane fragments in this technology must constitute the enzyme system. The inventors note that the major mechanism to effect the reaction is incorporation of the membrane fragments into the product, and that these active components may also be made part of the package structure. Sources of the membrane fragments were cell membrane of bacteria such as Escherichia coli and/or mitochondrial membranes. Examples of products from which oxygen might be removed by the system include beer, wine, fruits, juices and a variety of non-food products. Both red and white wines were treated with materials supplied by Oxyrase, Inc., which is also the patent assignee. Dissolved oxygen was removed within 16 minutes at 37°C. Less than 12 minutes was required to remove 100% of the oxygen from beer or tomato juice. A five-fold increase in the time to the onset of browning of cut surfaces of bananas and apples was observed at ambient temperature. Developers from chewing gum producer, William Wrigley, Jr., have described the use of porous polymeric beads containing glucose oxidase in multilayer flexible package materials (Courtright et al, 1992). The porous particles are made from styrene divinyl benzene, with the enzyme incorporated mechanically. The beads are then blended into a thermoplastic coating in the multilayer film.
Labuza and Breen (1989) have analyzed the issues involved in the incorporation of glucose oxidase into package materials. To counteract the quantity of oxygen passing through an aluminum foil lamination an enzyme surface will have to react with oxygen in the following manner: Rate = permeability X area X oxygen pressure difference between the outside and inside Rate = 0.1 X 1 [0.21-0.01] = 0.2 ml per day per m2 = 20 jxl/day The calculation above assumes air outside and < 1% oxygen inside. For the worst case and with a pinhole or cracked score, there would be the need to scavenge 1 ml/day. A film could be made equivalent to a barrier by binding the oxygen scavenging enzyme to the inside surface of the film to react with the excess oxygen. Glucose oxidase transfers two hydrogens from the -CHOH group of glucose to oxygen with the formation of glucono-delta-lactone and hydrogen peroxide. The lactone then spontaneously reacts with water to form gluconic acid. One mole of glucose will consume one mole of oxygen and so a package with 500 ml headspace is required, to reach zero oxygen, with only 0.0043 mole of glucose needed as a substrate. The major factors are the speed at which the enzyme works, the amount of glucose available, and the rate at which oxygen permeates into the package. In the presence of catalase, a normal contaminant of commercial glucose oxidase, the hydrogen peroxide is broken down, and so with catalase one mole of glucose will react with only a half mole of oxygen, decreasing the overall effectiveness of the system. Pure glucose oxidase without catalase is reportedly expensive. If no surface exists for the peroxide for diffusion, the glucose oxidase will be inactivated, precluding this application. Since many foods may have minimal contact with the package surface, except on the sides and bottom, this may not be the best approach for oxygen scavenging. At 30-400C, pure glucose oxidase has a rate of oxygen consumption of about 150 000 (il/h/mg. Based on this, and spreading 1 mg per m2 on a film, this would be equivalent to reacting with all the oxygen passing through a film with an oxygen permeability of about 18 000 ml/day m2 atm. Thus at room temperature, a i m square surface with 1 mg of enzyme spread out on it should be able to handle all the oxygen passing through any package film. One advantage is that both polypropylene and polyethylene are good substrates for immobilizing enzymes. One factor to take into account is the stability of the enzyme when bound to the film. An unknown factor is how stable the enzyme will be on the film over time. Glucose oxidase bound to a plastic surface has been shown to undergo a 50% drop in activity in 2-3 weeks followed by little loss over the next four weeks.
The Japanese have worked on binding of enzymes to chitosan, which is an insoluble polymeric carbohydrate from shellfish shells, but a 70% loss in activity for bound glucose oxidase has been reported. Glucose oxidase immobilized on polyethylenimine-coated glass beads retained 78-87% of its activity and was more stable to heat inactivation. Since the enzyme is a protein and can serve as a nutrient for microbes along with the glucose substrate, a microbial inhibitor may be needed in the film. Besides glucose oxidase mentioned previously, other enzymes have potential. One such enzyme is ethanol oxidase which oxidizes ethanol to acetaldehyde. The reaction is extremely rapid. Hopkins et a/. (1991) describe a package in which alcohol or oxidase or cellular extracts of Pichia pastoris cells containing alcohol oxidase are the enzymes used for oxygen scavenging in dry foods. An alcohol substrate either from the product or introduced into the package from the exterior is required to remove the oxygen from the package headspace. 7.5 Antimicrobial effects The use of enzymes in active packaging to control microbial growth and subsequent packaged-product degradation can be achieved by two independent approaches. By controlling the amount of available oxygen, selective control of aerobic bacteria can occur. However, this method of bacterial control can, under certain circumstances, allow the overgrowth of pathogenic anaerobic bacteria which, from a human view, may be worse than aerobic bacterial overgrowth. A second approach that has been implemented by several investigators is non-specific relative to oxygen requirements and is a direct attack on the organisms present, independent of whether the organisms are aerobic or anaerobic. This second approach can be either by a direct attack on bacteria (both aerobic and anaerobic) or by the production of broad-spectrum antimicrobial agents. Neither the literature nor the memories of the authors indicates the commercial implementation of the Fermco products. Meanwhile, the use of immobilized enzymes in commerce has increased significantly. During the 1970s, Scott (1975), in his continuing research on the technology of glucose oxidase, noted that catalase-free glucose oxidase might exert antimicrobial effects due to the production of hydrogen peroxide. At the University of Rhode Island, Rand and his co-workers conducted research and development on catalase-free glucose oxidase as a food preservative, especially with regard to fish (Field et aL, 1986). The enzymes (not coincidentally, supplied by Fermco) were applied to fresh flounder fillets or whole fish by dips, immersion in ice or by enzyme/ algin blankets. In some experiments, the enzyme system included catalase and/or glucose. The university researchers' experiments (which had begun
during the early 1980s) demonstrated that the enzyme treatments retarded the onset and magnitude of adverse microbiologically triggered spoilage odors. The researchers explained the result as due to reductions in surface pH under the refrigerated conditions of the test. These changes influenced the metabolism of putrefactive microorganisms. They also suggested that the generation of hydrogen peroxide might inhibit the growth of psychrotropic microorganisms which are reported to be sensitive to the chemicals used. Other possible microbistatic agents include gluconic acid, reportedly a metal complexing agent, and gluconolactone, reported to be a binding agent for water and metal ions. Another factor reported by the group was an altered gaseous microenvironment in which oxygen in the muscle interstices was depleted by the enzymatic action thus retarding the growth of aerobic psychrophiles. This last, of course, is synergistic with the oxygen removal aspects of the enzyme system. The authors cited a Japanese patent in which catalase-free glucose oxidase was demonstrated to be effective in preserving other proteinaceous foods such as ground chicken and tofu (Fukazawa, 1980). Although the Rand et al. work did not specifically state the incorporation of enzymes into package materials, the implications were sufficiently clear in the examples of the enzyme-containing ice and the enzyme-containing algin blanket. Either of these could have been relatively easily substituted with a skin package material which had been surface tested with the enzyme system. The notion of hydrogen peroxide as an intentional active antimicrobial agent is somewhat of a contradiction since this chemical is quite reactive with many food constituents, especially lipids, and residual free hydrogen peroxide is not readily accepted by regulatory officials. If the hydrogen peroxide is fully reacted with microorganisms as in aseptic packaging, however, perhaps the proposed system may warrant further consideration. Unfortunately, work at the University of Rhode Island on this topic has been discontinued. A German patent assigned to Continental Group (Anon. 1977) describes incorporation of biologically active enzymes into polymers on the interiors of package structures to destroy microorganisms of contained products. The enzymes were intended to destroy microorganisms by breaking cell walls and also to consume oxygen, thus increasing shelf-life without heat. The applicable products were beer and fruit juices. Enzymes such as muramidase for cell wall destruction and glucose oxidase for oxygen interception were attached to the internal polymer by covalent bonds. 'Non-essential' functional groups such as NH2, COOH, OH phenol, imidazole and sulfhydryl were cited as examples. The polymer was described as a terpolymer of monomer alkyl acrylate and vinyl aromatic applied to the interior of a glass container from a solvent and dried by heat. The enzyme was subsequently applied as a coating from an aqueous dispersion.
Tests indicated highly significant reductions in oxygen concentrations within the glass jars due to the conversion of glucose to gluconate in an oxidative enzymatic reaction. This appears to be the first reference to actually incorporating an enzyme into an interior package wall to achieve an enzymatic antimicrobial effect. 7.6 Time-temperature integrator-indicators For many years, efforts have been underway to develop a practical, accurate, reliable and economic indicator of total temperature-time exposure of food products. Among the routes has been the application of the principles of temperature sensitivities of enzymes. Although the original objectives were aimed at frozen food defrosting devices, more recent interest has been focused on chilled foods. Among the issues are activation only when actually at the beginning of shelf-life, accuracy over the entire range, how reflective the integrator-indicator is of the actual temperature-time experience, and another basic question, how well the measurement represents the effect of the temperature-time integral on the food itself. Kramer and Farquhar (1976) listed a number of the problems in their evaluation of five commercial, time-temperature indicating and defrosting devices. No descriptions were given the mechanisms for sensing, integrating or measuring time-temperature. On the other hand, Blixt and Tiru (1976) described a commercial enzymatic time-temperature monitor, called I-point® TTM. The authors, of Kockums Chemicals of Malmo, Sweden, stated that their device met all the requirements of reliability, accuracy, size, cost, understandable message and ability to integrate ' . . . both length and degree of all temperature exposures.' The reaction was based on enzymatic degradation to colored end points. The device was a two-part system, one containing an enzyme and pH indicator since the system was based on pH change caused by enzymatic activity plus a substrate. Because of the enzymatic core of the pH change, the temperature response was exponential with increasing temperature, and so evidently indicative of actual biochemical changes arising due to the temperature-time experience. Although the indicators reportedly functioned very effectively, no reference was made to the type of enzyme used. One might speculate on the simple glucose oxidase-catalase system producing gluconic acid as the reaction proceeded. This product was another manifestation of the application of enzymes in package systems to an inactive mode. A 1989 US patent (Klibanov and Dordich, 1989) claimed a temperaturechange indicator composed of an enzyme and substrate, a colorimetric
indicator and a trigger mechanism of a solid organic solvent system that melted when a specific temperature range was reached to permit the enzyme system to respond to temperature stimulus over time. The enzyme and substrate cited in the reduction process was peroxidase and peroxide with a /7-anisdine colorimetric indicator. Another enzyme cited as being effective was polyphenol oxidase. The organic solvents claimed were basically paraffins. Applications were as monitors on the exterior of distribution packages of pharmaceutical and food products. No further reference to the use of this enzymatic temperature indicator has been found in the literature.
7.7 Lactose removal Lactose intolerance is a dietary problem affecting a minor but nevertheless substantial fraction of the population. Individuals affected by this problem suffer from a lack of the enzyme lactase in their intestinal wall. Lactase is necessary to break the disaccharide lactose, or milk sugar, into its component parts glucose and galactose. Since lactose cannot be absorbed from the gastrointestinal tract, its presence can cause discomfort in the form of cramps, bloating, flatulence and diarrhea. Persons with lactose intolerance either avoid milk or introduce lactase enzyme into their milk prior to consumption. A British patent assigned to Tetra Pak International AB (Anon., 1975) describes incorporation of lactase into pasteurized or sterilized milk prior to packaging to split the lactose after packaging. The lactose must be sterile and is added aseptically. The patent notes that the milk must remain for about a day at a temperature of at least 80C for the lactase to function. The Tetra Pak approach differs from the previously discussed examples of active packaging because the enzyme has no relationship to the packaging material. Rather, a solution of enzyme is added directly to the individual package just prior to sealing. In reality, the Tetra Pak approach is batch processing done on a miniature scale, within the individual container. However, this approach does point out that an active enzymatic process can be carried out in a sealed container. PharmaCal, Ltd. extended and improved the Tetra Pak approach and made the process a true enzymatic active packaging process. Budny, at PharmaCal, Ltd. (1990) incorporated the lactase, using proprietary technology of PharmaCal, Ltd. with the result that 30-70% of the lactose was removed in 24-36 hours at 3-4°C. PharmaCal, Ltd. has proprietary designs and approaches for commercializing this active package (Figure 7.2).
7.8 Cholesterol removal The widespread information on the effects of excess cholesterol in the diet does not require discussion here. To demonstrate the awareness in the USA of the cholesterol content of foods, all food packages in the United States must be labeled for cholesterol content. Co-author Budny (1990) suggests the removal of cholesterol which is present in whole milk by incorporating the enzyme, cholesterol reductase, in the package structure. Using much the same proprietary technology of PharmaCal, Ltd. as he employed for enzymatic oxygen removal or lactose splitting, the fluid milk contents are exposed to the enzyme to convert its cholesterol to coprosterol which is not absorbed by the intestine. This system, illustrated in Figure 7.3, reduces the extensive in-plant processing required by supercritical fluid extraction systems to produce cholesterol-reduced fluid milk products. Rather, active packaging and the technology of PharmaCal, Ltd. allows untreated fluid milk to be packaged,
Milk
Lactase enzyme
Glucose
Galactose
Lactose
Outside of container
Container wall
Inside of container
Figure 7.2 Lactose removal from liquid products.
Milk
Cholesterol reductase enzyme
Coprosterol
Cholesterol Outside of Container
container wail
Inside of container
Figure 7.3 Cholesterol removal from liquid products.
and in the time taken to transport the package to the consumer, it conceivably could become free of cholesterol. While the commercial implementation has not yet been completed, the component elements of the application have been successfully demonstrated. References Anon. (1977) Packagedfoods and drinks in containers coated internally with polymer carrying enzyme with sterilising action. German Patent DE2817854A. Anon. (1990) Packaged milk containing lactose enzyme-giving milk with reduced lactose content. UK Patent Application. Baker, D.L. (1949) Deoxygenation Process. 20 September. US Patent 2482724. Best, D. (1990) Fermentation opportunities ripen. Prepared Foods, 159, 5. Blixt, K. and Tiru, M. (1977) An Enzymatic Time/Temperature Device for Monitoring the Handling of Perishable Commodities. International Symposium on Freeze-Drying Biological Products, 36, 237. Budny, J. (1989) A transporting storage or dispensing container with enzymatic reactor. International Patent Application WO89/06273. Budny, J. (1990) Presentation at Pack Alimentaire, San Francisco, California, May.
Copeland, J . C , Adler, H.I. and Crow, W.D. (1991) Method and composition for removing oxygen from solutions containing alcohols and/or acids. XJS Patent 4996073. Copeland, R.A. (1994) Enzymes, the catalysts of life. Today's Chemist at Work, March. Courtland, S.B., McGrew, G.N. and Richey, L. (1992) Food packaging improvements, 30 June. US Patent 5126174. Ernst, R. (1991) Oxygen absorbent and use thereof 2 July. US Patent 5028578. Field, C , Pivarnik, L.F., Barnett, S.M. and Rand, A.G. (1986) Utilization of glucose oxidase for extending the shelf-life of fish. J. Food Science, 51. Fukazawa, R. (1980) Methods of preventing spoilage of foods. Japanese Patent 23071180. Hopkins, T.R., Smith, VJ. and Banasiak, D.S. (1991) Process utilizing alcohol oxidase, 10 December. US Patent 5071660. Klibanov, A.M. and Dordich, J.S. (1989) Enzymatic temperature change indicator, 2 May. US Patent 4826762. Kramer, A. and Farquhar, J.W. (1976) Testing of time-temperature indicating and defrost devices. Food Technology, 30, 56. Labuza, T. and Breen, W. (1989) Active Packaging. J. Food Processing and Preservation, 13, 1. Lehtonen, P., Karilainen, U., Jaakkola R. and Kymolainen, S. (1991) A packaging material which removes oxygen from a package and a method of producing the material. International Patent Application WO 91/13556. Sarett, B.L. and Scott, D. (1956) Enzyme treated sheet product and article wrapped therewith. US Patent 2765233. Scott, D. (1958) Enzymatic oxygen removal from packaged foods. Food Technology, 12(7), 7. Scott, Don and Hammer, F. (1961) Oxygen scavenging packet for in-packet deoxygenation. Food Technology, 15(12), 99. Scott, D. (1965) Oxidoreductase. Enzymes in Food Processing, Academic Press, NY. Thomas, K. and Harrison, RJ. (1985) Method and apparatus for secondary fermentation of beverages. UK Patent Application 2143544A. Wiseman, A. (1975) Enzyme utilization in industrial processes, Handbook of Enzyme Biotechnology, Ellis Horwood, UK.
8
The history of oxygen scavenger bottle closures F.N. TEUMAC
8.1 Background The early history of the use of scavenger chemicals with beer has played an important part in the development of oxygen scavenger closures. Gray, Stone, and Atkin (1948) measured oxygen content of bottled beer and correlated oxygen presence with off-flavor development. The report made to the American Society of Brewing Chemists concluded that the addition of anti-oxidants to beer should be studied. The prime candidates were sulfites and ascorbic acid. Thomson (1952) reported extensions of the earlier work in the Brewers' Guild Journal. He found that the use of reductones made from sugar reduces oxygen, but increases the level of calcium to a level that forms hazes. The reactions with sulfur dioxide, sodium formate, and phosphites were too slow. He recommended adding ascorbic acid just prior to bottle filling. Reinke, Hoag, and Kincaid (1963) reported that the inclusion of oxygen scavengers in the lining of cans improves the storage stability of canned beer. Glucose oxidase-catalase was preferred to sulfur dioxide and isoascorbic acid. Klimovitz and Kindraka (1989) published in the Master Brewers Association of the Americas Technical Quarterly that a combination of sodium isoascorbate and potassium metasulfite when added to the silica hydrogel mixing tanks significantly improved product flavor stability. 8.2 Oxygen measurements 8.2.1 Techniques for measuring the oxygen content of bottles Before withdrawing gas samples from a bottle for measurement, the bottle should be equilibrated by shaking. The foam should be allowed to settle. For a gas sample, this might require several hours. The sample is withdrawn with a Zahm-Nagel device. Oxygen and nitrogen can be measured in a gas sample directly by gas chromatography or by removing carbon dioxide, separating the other gases and measuring with a mass spectroscopy detector. Using chromatography, assumptions and corrections must be made to determine oxygen. The advantage of the mass spectroscopy detector is that
argon is detected directly. Because nitrogen and argon do not react with the bottle contents, other data can be gained by comparing the ratios of the three gases. The total oxygen and nitrogen can be calculated from the measured headspace, the temperature, and the oxygen and nitrogen concentration in the head-space. Liquid samples can be withdrawn and measured with polarographic techniques. Again, the bottle should first be equilibrated. The total oxygen of the bottle can be calculated. Since the Zahm-Nagel device pierces the closure, each bottle can only be sampled once. In order to follow the changes in the bottles, it must be assumed that all the bottles were the same at bottling. This requires the most reproducible conditions possible. 8.2.2 Results of measurements Depending upon the equipment capability of the brewer, the oxygen content of bottled beer can be seen to correspond to three categories of brewer: •
•
•
Brewers incapable of performing a final blow down with purified carbon dioxide and without new high technology fillers. The initial oxygen in the package is about 1700 ppb. This decreases by 30% during pasteurization, 42% the first day, 54% the second day, and 95% in a week. The reaction of oxygen with the bottle contents is rapid. These brewers usually compensate by adding 10 ppm or more of sulfur dioxide to the beer. Brewers capable of good oxygen control up to the last step, but do not have new high technology fillers. These brewers provide an initial oxygen content of about 900 ppb. The oxygen depletion in the bottle proceeds at the same percentage rate as in the first category. Brewers that use the best equipment available. The initial values vary because maintaining the lower value requires the filler to be in top condition. An initial value of 400 ppb is common. Brewers using 10 ppm or more of sulfur dioxide obtain values of 200 ppb oxygen; brewers with 8 or less ppm sulfur dioxide experience values of 350-800 ppb of oxygen depending on the maintenance of the equipment. The oxygen depletion in the bottle proceeds at the same percentage rate as for the other two classes.
The effect on flavor deterioration of bottling under different conditions is difficult to gauge. Each beer is different, so comparisons must be made on different crowns under the exact same bottling conditions on the same batch of beer. The few valid comparisons made prior to the introduction of scavenging crowns indicated that beer bottled with less oxygen had better shelf-life.
8.2.3 Oxygen ingress Closure of the bottle does not mean that the battle with oxygen is over. For years a crown or closure was defined as a hermetic seal. Wisk and Siebert (1987) at Stroh and Heyningen et al. (1987) at Heineken separately challenged this assumption and came to the same conclusion: crowns allow oxygen ingress. ZapatA Industries studied oxygen ingress in crowns; Teumac, Ross and Rassouli (1990) confirmed the earlier conclusions, and recommended some improvements to eliminate oxygen ingress. This work was extended to include both plastic and aluminum closures (1991). The concept of oxygen ingress into the bottle gained slow acceptance because it is difficult to envision how oxygen will penetrate a bottle with 3 atmospheres of pressure within from an ambient pressure of 1 atmosphere. The phenomenon has, however, been proven using several techniques by several workers in the references cited above. It is based on a well established equation that describes permeability through a permeable polymer (the liner or gasket). (A X p) PERMEABILITY = P — L Permeability as used here means the flow of any gas per unit of time. For a container, it is the flow of a specific gas through the portion of the container in question. P is the permeability coefficient; this is determined empirically for a specific polymer or polymer compound and is specific to the gas and the conditions of the test. A is the area of the compound surface involved in the transfer. As metal has no permeability, A for a crown is the area of the liner compound between the metal and the glass. An aluminum closure provides a very small area. A plastic closure is totally made up of permeable material, so the area is quite large. L is the length of the route followed by the gas. p is the driving force of each gas. It is the difference in the partial pressure between the respective sides of the liner. It should be emphasized that it is not the total pressure; it is the partial pressure of the particular gas. A pressure of three atmospheres in a bottle does not mean that all gases will move outward from a bottle. If there were a physical leak, that would be the case. For a polymeric material like PVC, EVA, or polypropylene, gas will flow from the higher partial pressure to the lower. Oxygen ingress can be measured by placing a closure on a bottle containing a known amount of oxygen and periodically measuring the oxygen in the bottle. The bottle must contain nothing that can react with oxygen. A simpler method uses an instrument sold by Modern Controls, Inc. There are several models of an instrument commonly called the Mocon. The instrument is used primarily to measure transmission through a permeable membrane. Because of uncertainties of the dimensions of L and A in a
crowned bottle, the measurements on a closure are best made by modifying the Mocon to measure the transmission directly for a closed bottle. The bottle is closed with the test closure and then cut and sealed to a metal block containing a sealed inlet and outlet. Figure 8.1 is a schematic of the apparatus. Oxygen-free nitrogen is flushed into the bottle carrying any oxygen in the bottle out to the detector. By keeping the nitrogen flow rate constant, a steady state is reached where the oxygen in the stream is a measure of diffusion of oxygen through or around the closure. 8.2A
Combining the effect of initial and ingress oxygen
The bottler must consider both the oxygen trapped in the bottle at filling and oxygen ingress. For example, a brewer with good oxygen control techniques will fill bottles with beer containing 50 ppb oxygen and entrap another 440 ppb. The initial oxygen level would be 490 ppb. A crowned 12 ounce bottle will allow another 750 ppb to ingress in 3 months or 2000 ppb in 8 months. The amount of oxygen available to react with the product can be calculated from measurements and extrapolated. The amount of oxygen that has reacted with the product can then be calculated by subtracting the measured oxygen from the total oxygen
Hot melt lueor min. epoxy
§ Brass mtg. plate
Solder
Figure 8.1 Schematic of Mocon apparatus.
exposure (initial + ingress). The instruments cited are capable of making meaningful measurements that reveal the oxygen chemistry taking place in the bottle. All of the oxygen that gets into the bottle is either measurable or has reacted. In other words: OXYGEN (Initial+ Ingress) = OXYGEN (Measured) - OXYGEN (Reacted) 8.3 Oxygen scavenger liners 8.3.1 Theoretical Removal of oxygen from a bottle by a closure requires that the reaction occurs with gaseous oxygen in the headspace of the bottle. About two-thirds of the oxygen in a bottle is in the headspace. Scavengers can be incorporated into the closure by two different means. (i) A compartment is placed in the closure that separates the scavenger via a membrane that allows oxygen and water vapor to permeate the liner, but prevents the scavenger from leaching back into the bottle. This approach lessens the concern of product contamination by the scavenger; thus, it increases the choices of potential scavengers. There are many patents describing this approach. In order to be practical, the design and placement of the compartment must allow normal closure handling and bottling procedures. The fabrication of such a closure would add significant cost and require process changes by the brewer. This approach has not been commercially tested. (ii) The scavenger is included in the liner compound. The scavenger must be effective at levels that do not interfere with compound processing, closure lining, or the closure performance on the bottle. To be effective, the compound must be permeable to water vapor and oxygen. The rate of oxygen removal will be determined by the concentration and reactivity of the scavenger, the permeability of the compound, and the surface area of liner exposed. The scavenger should not become degraded during processing thereby losing activity and should be immune to activity loss during normal handling. For example, enzymes such as glucose oxidase-catalase are very reactive, but are destroyed by plastics processing conditions and are much too reactive for normal filling procedures. Because of close contact with the product the scavenger should not be noxious from a health or organoleptic standpoint. It is not surprising then that the most successful scavengers are the materials tested earlier as direct beer additives. 8.3.2 Commercial activity PVC compounds lend themselves particularly well to use as scavenger additives. Plasticized PVC can tolerate fillers without significant loss of
properties and is sufficiently permeable to oxygen and water vapor to allow good reaction rates. Polyolefins have sufficient oxygen permeability, but are less permeable to water vapor. (i) W.R. Grace, through Tapon France (a crown manufacturer), introduced a scavenger product in a polyolefin liner to Heineken in the Spring of 1989. Heineken dropped scavengers when they became more interested in other aspects of crown performance. Grace has published and been granted several patents on their scavenger system. Essentially, they describe using ascorbates and similar chemicals with or without sodium sulfite in a thermoplastic matrix. By reading the patents and analyzing liner materials, it is evident that the Grace scavengers contain up to 7% sodium sulfite and up to 4% sodium ascorbate. The ascorbates by themselves are very weak scavengers, so the sulfite is required for the rate of activity. For some beers the low activity is not a disadvantage and may even be an advantage. The exact extent of Grace's commercial success is not known outside of Grace. It appears that a PVC compound is being used on a low alcohol beer, Foster's Special Bitter. Courage Beer uses Daraform 6490 for the AnheuserBusch beer produced under license in the UK. Daraform 6490 is a polyolefin liner compound containing an oxygen scavenger. A PVC compound from Grace is being intensively tested at one major Canadian brewer and several smaller US brewers. Trials are being performed with Daraform 6490 at several European brewers. (ii) Aquanautics Corporation, now Advanced Oxygen Technologies, Inc., had developed some expertise in removing oxygen from sea water and recognized an opportunity in removing oxygen from bottles of beer. An elaborate business plan was developed and eventually sold to ZapatA Industries, Inc. A joint effort was launched in early 1989. A system was developed that was based on the beer chemistry described earlier. Several patents have been granted and applications are in the process of being approved. The new technology employs ascorbic acid as the reducing agent. Alternatively, alkali metal ascorbates, alkali metal erythorbates, or erythorbic acid can be used. The addition of very small amounts of metal catalysts to ascorbate or erythorbate-containing liners greatly enhanced the rate of reaction with oxygen. The preferred catalysts are copper and iron salts, but all transition metal salts increase the reaction rate. The amount of ascorbate or erythorbate determines the oxygen reduction capacity; the type and amount of catalyst determines the reaction rate. Placing the same materials in a plastic liner earlier placed in beer greatly minimizes the fear of product contamination. The small amount of catalyst enclosed in the plastic liner yields undetectable amounts of leach in beer. Separating the scavenger system from the beer, besides keeping it out of the beer, gives a surprising benefit. Different reactions occur. Earlier workers had observed a
reversal of the benefits of adding ascorbates directly to beer; this is not observed when the ascorbate is placed in the liner. Another surprising benefit is that the reaction rate is significantly increased by placing the scavenger in the liner. The reaction with oxygen is enhanced by the paucity of moisture found in the liner. The Aquanautics-ZapatA liner, Smartcap®, was introduced in a controlled manner in 1991. There was concern that the new conditions created in the bottle would cause off-flavors to be created. The brewers that started using the liners commercially in 1991, and those that have started subsequently, have not had a documented incident of off-flavor development attributable to the liner. Smartcap and the improved PureSeal® crowns are marketed by ZapatA Industries and affiliated companies in other countries. In 1993, over 1 billion PureSeal crowns were sold. Other crown manufacturers have been provided with lining compound for crown trials. ZapatA provides trial recommendations and an oxygen testing service for PureSeal trials. Trials at 60 brewers have proven that the liners always reduce the oxygen level in the bottle and usually reduce oxygen damage to the beer during storage. The difference is first noticeable between 1 and 3 months of storage and is maintained to between 9 and 12 months. 8.3.3 Health and environmental concerns This history implies ready acceptance by health authorities; each supplier will document the acceptability to potential customers. The commercially employed oxygen scavenging chemicals have not been listed as environmentally harmful. 8.4 The effect of scavenging closures on beer flavor There are several large brewers, and they can afford and use excellent oxygen control. With one exception, they have been evaluating PureSeal oxygen control crowns for about 2 years. Extensive research and large trials are resolving their concerns and demonstrating the value of oxygen scavenging crowns. Working with these brewers has resulted in a better understanding of the role of oxygen in beer flavor chemistry. The original goal of the project was to remove as much oxygen as quickly as possible. For bottles containing more than 600 ppb oxygen, rapid removal is beneficial. As the initial oxygen approaches 250-350 ppb, rapid oxygen reduction is not always beneficial. The reason for this is that all beers contain trace amounts of organic compounds, actually hundreds of them. Some of the sulfide-containing organic compounds included in this number have a low flavor threshold. Oxygen participates in the reactions that reduce these flavors. 'Sulfury' beers bottled with low initial oxygen require a lower
rate of oxygen depletion to allow some of the oxygen to react with the sulfury components. PureSeal liner compounds are readily adjusted to achieve both goals. A summary of the results was reported at Pack Alimentaire (1993). 8.5 The advantages of oxygen control bottles The advantages are most obvious to exporters. There is little similarity between beer purchased in the area of origin and that purchased elsewhere. Beer carefully brewed to have certain flavor characteristics can now be delivered to customers all over the world in shipments in the same condition. The effect on the bottle of day-to-day variations in oxygen level at the filling line or between filling lines can be minimized. Most large brewers have rigid standards on initial oxygen levels and dump beer that exceeds the limit. Oxygen control bottles would allow raising of the limit. Most beer is sold through distributors. Brewers lack the control they would like on the distribution of their beer; by extending the expected shelflife, they can now lessen the concern on how long beer is on the shelves or give the distributors more leeway. Production departments can use oxygen control bottles as a tool to solve manufacturing problems; for example, the limiting factor on how fast a filling machine can operate is often the initial oxygen level. Limitations of filling lines, the amount of oxygen, and the amount of headspace place limitations on package design; with oxygen control bottles, new packages can be designed for the same filling line. Properly designed oxygen control bottles will provide a fresher tasting beer compared to a bottle with a standard crown after approximately 30 days. There is some evidence that reduction of oxygen in a package can reduce spoilage caused by organisms. Acidic beverages, like beer, can be protected by less severe heat or additive treatment. In fact, total removal of some organisms has been achieved by rapid depletion of oxygen in the bottle. This provides more freedom of design in the product, processing, package, packaging materials, and distribution. 8.6 The future of oxygen scavenging closures The use of oxygen scavenging crowns for beer is increasing rapidly. Brewers will become more comfortable with this trend. At the same time, the cost premium over standard crowns will diminish with increased volume. Oxygen control liners should be used in the standard crown
employed by the beer industry. Oxygen control liners have been introduced for aluminum roll-on closures to complete the closure requirements for beer. The use of oxygen control for other beverage products is a new frontier. It is a relatively new industry involved in this field, differing in many respects from the beer industry. Until recently, refined constituents such as sugar, corn syrup, artificial flavors, and citric acid have been used. There were relatively few substances that had the potential of becoming oxidized to off-flavors. As beverage makers begin to use more natural materials such as fruit juice, the potential for organoleptic problems increases. These problems can be off-set with additives, but additives must be listed on the label. Wines and coolers also contain hundreds of organic compounds that can react with oxygen. Wine chemistry has dealt with oxygen for centuries. Wine makers understand the role of oxygen in maturation and/or spoilage in wine; it is a matter of how much oxygen at what stage. Oxygen scavenging closures can be part of the oxygen control procedure of a winery. Many food products are damaged by oxygen. Damage might be in the form of discoloration, change in texture, loss of flavor, or the generation of off-flavors. The effect is obvious and well understood by food processors. Sacrificial reduction of metal and use of preservatives are becoming less acceptable. Package oxygen control affords a different means of protecting food from oxygen damage. Measuring techniques and equipment are now available for evaluation of the control of oxygen in any package. Nonetheless, 'quick and dirty' methods are commonly found. This lack of precision will lead to faulty conclusions or indicate no significant difference. Control of the initial oxygen content and a valid means of measuring a change in properties are essential features. The food scientist should become familiar with the latest developments and only then very carefully plan and execute experiments.
References Gray, P., Stone, I. and Atkin, L. (1948) Systematic study of the influence of oxidation on beer flavor. ASBC Proc, 101-12. Heyningen, D. et al. (1987) Permeation of gases through crown cork inlays. EBC Congress, 679-86. Klimovitz, R. and Kindraka, J. (1989) The impact of various antioxidants on flavor stability. MBAA Technical Quarterly, (30), 70-4. Reinke, H., Hoag, L. and Kincaid, C. (1963) Effect of antioxidants and oxygen scavengers on the shelf-life of canned beer. ASBC Proc, 175-80. Teumac, F., Ross, B. and Rassouli, M. (1990) Air ingress through bottle crowns. MBAA Technical Quarterly, (27), 122-6. Teumac, F., Ross, B. and Rassouli, M. (1991) Oxygen Ingress Into Soft Drink Bottles. Proceedings of the 38th Annual Meeting, Society Of Soft Drink Technologists, pp. 201-10.
Teumac, F. (1993) Case Studies of Oxygen Control in Beer. Proceedings of Pack Alimentaire '93. Thomson, R. (1952) Practical control of air in beer. Brewers' Guild Journal, 38(451), 167-84. Wisk, T. and Siebert, K. (1987) Air ingress in packages sealed with crowns lined with poly vinyl chloride. /. Amer. Soc. Brew. Chem., 45, 14-18.
9
Commercial applications in North America S. SACHAROW
9.1 Packaging overview Packaging exists because it performs four basic functions which may vary in importance depending on the nature of the products and their modes of distribution. The classic functions are: 1. 2. 3. 4.
Protection Containment Information Utility of use
In recent years, these properties have been expanded to include both the environmental disposability of the package material as well as the ability of the package to perform far beyond the inherent property of the package media. This may include characteristics such as enhanced shelf-life, the ability to 'cook' the product or other changes in the product caused by the packaging material. Active packaging is the term used for a package that changes the characteristics of the product packaged. Examples of active packaging existing in the North American marketplace will be discussed in this chapter. 9.2 Marketplace susceptors In its classical definition, an active package (within the microwave field) is one that changes the electric (or magnetic) field configuration and ultimately the heating pattern of the product packaged (Packaging Gp., 1987). Susceptors (also sometimes called receptors) are materials which convert sufficient microwave energy into heat to result in temperature increases that exceed those produced by either the direct heating of foods or the boiling of water into moisture vapour. Temperatures high enough to produce drying, crisping and ultimately browning result, thereby yielding the desirable effects associated with conventional infrared oven cooking. Microwave cooking alone produces temperatures limited by the temperatures developed by the food components, especially water, sugar and fats, in response to excitation.
•
• •
Foods containing mostly water, such as vegetables, thus reach boiling temperature. If heating continues in the absence of sufficient relief of internally-generated pressures, bursting can result. Thus potatoes baked in the microwave are first deeply pierced so moisture has exits. Foods containing fat, such as bacon, reach temperatures of frying, which may exceed 2000C (392°F). Foods containing mixtures of water and sugars or fats achieve temperatures determined by the concentrations and distributions of ingredients, limited of course by the exposure time. This complicates the heating of meals made up of foods differing in response to microwaves, such as vegetables with gravy-covered meat, mashed potatoes and a dessert of cherry cobbler.
Foods that require surface drying include pastries, breads, pizza crusts, and other dough-based compositions. Crisping and sometimes browning is needed in some of these same foods and additionally in certain meat products as well as roasts. Microwaves are not yet suited for crisping and browning. They must rather be implemented with some method of raising local temperatures to 1500C or higher (3000F or higher) to make them function as do browning dishes and conventional ovens or frying pans. Methods include (1) use of a browning element, an infrared heating source such as a heating coil, in the oven to provide air at temperatures up to 2500C (4800F); and (2) a surface, a susceptor, which reacts to microwaves by becoming hot enough to create the desired temperature. The characteristic temperature-time curves for foods in a microwave oven vary over a considerable range. Among dry foods, watery and high moisture foods, foods containing fats and oils, and sugary foods, there can exist orders of magnitude differences in heating rate, exacerbated by initial temperature and specific composition. Susceptors help to overcome these differences. Two outcomes are desired of a susceptor: • •
rapid rise to the required temperature constant temperature thereafter
Only the first of these has been achieved in practice, but self-limiting susceptors that satisfy the second are receiving considerable research attention and should be on the market within a few years. 9.2.7
Susceptor types
In the parlance of deposited films, 'thick' and 'thin' are differentiated by the form which the deposited material takes during the deposition process. •
thin films are direct condensations of individual atoms, ions or molecules onto a substrate
•
thick films are deposits onto a substrate from dispersions of the material as, for example, from a paste
Thin films may be only a few angstroms up to a thousand or more angstroms (1 angstrom = 10~8 cm = c. 4 x 10~9 inch) in thickness; as deposition continues, thickness increases. The most interesting and potentially most useful effects pertaining to aluminium in microwave packaging are those which occur at thicknesses corresponding to Macbeth optical densities (OD) between 18 and 28. These coatings are largely transparent to visible light (% transmission c. 50%) and in fact overlap the lower end of the range of thicknesses used in window films. In this range, particles form a discontinuous film of non-uniform thickness - an array of electrical resistances - which responds to microwaves by becoming increasingly hot, through ohmic heating. At thickness yielding OD = 35, arcing occurs. Thickness is a misleading term to apply to these extremely thin coatings because their surfaces are quite irregular; perhaps the root mean square thickness might be more apropos. Early susceptors (c. 1986 - 1987) yielded promising heating results which produced the desired cripsing and browning, but they also produced problems in some cases, which since that time have been largely corrected or eliminated entirely. Two examples are: (i)
(ii)
A strong unpleasant odour, emitting from the oven or on opening the door after heating, emanated from the paperboard substrate, the adhesive, the film base or metal or combinations; Uneven crisping, or lack of crisping in some areas of the food when other areas were done, detracted from the favourable impression this new technology offered.
In addition, the rapid growth in use of compact ovens, typically less than 500 W and 201 capacity (0.7 cf) sharply increased demand for convenience foods most likely to require crisping and browning. The accompanying rush to formulate suitable foods and packages led to some sub-optimal results. Most of the recent offerings of susceptor-crisped foods seem to overcome the early problems, though some remain, especially in the frozen category where uniform temperature attainment is difficult at best. In tests of oval cross-section frozen dough-encased pasties (meat pies), centre line temperatures from middle to ends after the recommended heating time varied from 71-27°C (160-80 0 F) and were not improved with additional heating up to the maximum recommended using the sleeve susceptors provided. Moreover, the susceptor efficiency was noticeably better at the base of the pies than at the upper surfaces. Standing time of 5 min narrowed the difference between highest and lowest temperatures from 44 to 33°C (80 to 60 0 F).
Further advances in susceptors technology are anticipated. A key need is a susceptor the temperature of which rises quickly to the desired value and holds it nearly constant for the time required. Such a self-limiting device may be found in current early stage research and development activities. Susceptors are made of either aluminium or stainless steel deposited on substrates. Other metals may be used in the future. The practical application of microwave-susceptible materials to heating of foods is to produce crisping and browning (see Table 9.1). Two classes of materials are available for producing susceptors; (i)
Resistive coatings, i.e. materials whose electrical resistance in the form in which they are deposited is high enough to produce ohmic heating; (ii) Ferromagnetic/electric materials. Resistive coatings are currently used exclusively in susceptors, but the second class may become important when the expected technology is developed during the next five years. These latter offer the possibility of setting specific upper temperature limits on susceptors, thus overcoming some of the disadvantages of resistive coatings, such as local hot spots, and charring. In an experiment with a sleeve susceptor around a frozen waffle, the time of microwaving was extended by one-fourth with the result that the sleeve and the waffle began to char. Susceptor substrates thus far have been limited to polyester films and paperboard. Other materials may find use for specific reasons, not least of which could be greater resistance to heat as higher temperature performance is achieved. Engineering thermoplastics including the liquid crystal polymers, polysulfones, polyarylenes, nylons, and others listed among high temperature tray materials for dual oven ware, are potential candidates. 9.2.2 Field intensification devices Field intensification devices focus microwave energy to increase local intensity above that which would otherwise exist. FIDs therefore function in a manner similar to optical focusing lenses. The extent of intensification depends on the geometrical design of the focusing system - a metal antenna - and the distance of the target plane(s) from the antenna. Alcan, Ltd., calls their MicroMatch™ system for field intensification a field management system which focuses and directs the incoming energy. In development of the MicroMatch container, Alcan found that efficient designs consisted of patches of aluminium arranged on a polymeric, microwave-transparent, snap-fit dome used as a cover for the food tray. The dome positions the aluminium array with respect to the food and, by virtue of an overlap of the tray, precludes direct contact of the tray with oven walls, thereby reducing the chance for arcing to occur.
Table 9.1 Comparative performance of various susceptor technologies Technology name
Even heating
Commercial status
Depends on product packaged
Excellent
Limited success. Now being licensed under technical agreeemnt. Presently there are two paid licensees in North America. Only one commercial product in North America, 'Meals on Wheels'.
Cumbersome Expensive Over-engineered No tray use
Good
Good
1. Excellent 2. Excellent 3. Good
'Accu-Crisp is the mainstay' as a 'patterned susceptor'. Other two are not yet commercial.
Gaining one customer per month
Printpak, Inc. Deposition Technologies, Inc. (San Diego, CA)
Good
Excellent rapid heating
Good
Only use is in pizza boxes (Healthy Choice brand).
Rapid heating for French fries and pizza
Dupont, Inc. 1. Cello-based demetallized 2. PET stainless steel metallized film
Printpak, Inc. (licensees)
Good
Good
Not yet commercial; however, one product on market has been withdrawn.
Still somewhat on drawing board
'Micromet' (pattern susceptor)
Lawson Mardon Midsomer North (UK)
Excellent
Only limited commercial trials. No market success.
About to be taken off R & D program
Firm
Browning
Crisping
'Micro-Match'
Alcan, Ltd (Montreal PQ)
Good
1. 'Acan-Crisp' 2. Accu-Wave' 3. 'BarrierWave'
Printpak, Inc. Advanced Dielectric, Inc. (Taunton, MA)
'Susceptor Film' 'InconaT (Alloy metal)
Excellent
Good Good
Comments
No commercial application for MicroMatch is yet in place, but licensing is reportedly underway in the USA and Germany. Alcan has concluded that acceptance would be enhanced if packaging companies better known in the food industry were to handle commercialization. Aspects of the system which appear to make it attractive include: • • • •
the ability to design the antennae to provide optimal focusing on different areas of food, thereby facilitating heating of each food in a multi-component meal to its proper temperature faster heating relatively direct application in manufacturing and ease of changing the required patterns to fit individual food suppliers' needs functioning from above and without direct contact with the food makes possible the browning and crisping of foods having soft and sticky surfaces, which is not feasible with contact susceptors
Use of the MicroMatch container does not always obviate the need to rotate the food to achieve even heating - many cheaper MW ovens have no mode stirrer. The aluminium tray is coated to lend greater assurance against arcing. Coating increases the electrical potential required for arcing from 30 000 V to c. 50 000 V, but arcing can occur in either case. Another reason for coating that is not usually mentioned is that coating improves appearance and corresponding consumer appeal. Thus, a FID dome on an aluminium tray directs and focuses energy to provide both control for uniformity and a means of regaining the speed lost by virtue of the tray's inability to transmit MW energy. A FID dome on an aluminium composite tray in which the tray base is plastic or paperboard would overcome the speed loss and would supposedly heat food faster than the same tray without the field intensification. Cost of the FID dome will no doubt be a major factor in determining its market niche. The extra space required to accommodate the dome shape and its manufacturing cost will likely limit its use to the more expensive meal offerings. Consumers will require convincing evidence of resulting better quality. 9.2.3 Susceptor applications There are numerous packages in the supermarket that utilize susceptors from microwaveable popcorn (reducing the amount of unpopped kernels) to microwaveable pizza (offering a crisp crust). In addition, entrees, fruit pies, meat pies and various 'crust' items lend themselves quite well to susceptor utilization. Susceptors have been used in Israel for bourekas, New Zealand for French Bread, the UK for pappadums, and in Sweden for frozen meat entrees.
9.3 Application of temperature indicator to microwaveable packaging An interesting American microwave innovation not utilizing a susceptor, but still an * active package' form is a microwaveable polypropylene bottle for pancake syrup. Squat PP jugs of Hungry Jack pancake syrup to be heated in household microwave ovens feature thermographic 'temperature indicator' labels that tell consumers when the syrup is hot. Developed by Pillsbury Co., Minneapolis, MN, the microwave-ready bottles are currently in supermarkets across the USA. The 24 oz bottles stand 16.8 cm (6 § in) high 13.0 cm (5 g in) wide, and 7.0 cm (21 in) deep. Like their counterparts, the MW bottles incorporate front and back paper spot labels. But, on the MW bottles, the face of the front labels features an illustration of a microwave oven. On the shelf, the door of the oven appears black. But, when the bottle is put in an oven and heated to bubbling according to directions on the black label, the black oven door on the front label fades to yellow and the word 'HOT appears in the centre. Extrusion blowmoulded by Continental Can Co., Syosset, NY, the squat PP bottle incorporates a fat, hollow handle that is pinched closed where it joins the container's body and shoulder. The handle design is meant to warn consumers and prevent them from burning themselves when handling the heated bottle. 9.4 Active packaging - produce 9.4.1 Oya produce bags Evert-Fresh, a company in Houston, Texas makes a new kind of bag for storing produce (Evert-Fresh, 1994). The greenish polyethylene bags are impregnated with a finely ground stone of the zeolite family that has high absorption properties. (Similar minerals are used to make products like Odor Eaters for shoes.) In the bags, the mineral absorbs ethylene gas, which is given off by many fruits and vegetables and hastens ripening. By absorbing the gas, the bags slow down the ripening process and keep foods fresh longer. The bags also have minute pores that allow the gas to escape and prevent the accumulation of moisture that could result in the development of bacteria. The bags are re-usable if rinsed and turned inside out to dry. 'Evert-Fresh Bags' are reported to be impregnated with processed Oya Stone which has its origins in a cave in Japan. The cave has been used for three centuries to store fresh produce. The success of this cave as an ideal storage space can be attributed to constant levels of high humidity, static temperature, darkness and, most importantly, the ability to absorb the gases discharged by the stored produce. The study of the caves gave scientists the
key to developing the Evert-Fresh film that absorbs ethylene, maintains humidity, is permeable to other gases, and, when refrigerated, maintains temperature control. AU of these are major factors in successful long-term storage of produce. 9.4.2 Oya test results A substantial portion of the vitamins and minerals in the American diet come from fruits and vegetables. Approximately 50% of the vitamin A and over 90% of vitamin C come from this food group. Stability of vitamins in produce is affected by a number of factors, including heat, light, oxygen and pH. For example, testing with vegetables, such as whole cabbage and green beans, has demonstrated that the susceptibility to heat destruction of 3-carotene, which is a pre-cursor for vitamin A, can depend upon the nature of the vegetable. Control of temperature and humidity are necessary since if low humidity conditions prevail, rapid transpiration occurs and vegetables wilt. Under these conditions, vitamin C and (3-carotene losses in leafy vegetables are well over 50%. Also proven is the use of modified atmosphere storage to control the carbon dioxide and oxygen levels which affect vitamin C retention. It is reported that this bag does effectively reduce vitamin loss. Specifically, the bag is green in colour to reduce light transmission and made breathable to enhance the transpiration of gases (O2 and CO2). Combine these two elements with modern refrigeration (temperature) and three of the four factors that affect vitamin retention are excluded. Storage evaluations of leaf spinach, lettuce, broccoli, cabbage and green beans indicated that vitamin C loss was reduced in excess of 50% using the Evert-Fresh bag for long term storage. Tests regarding vitamin C retention using the Ever-Fresh bag and ordinary polythylene bags were conducted in Japan by the Consumer's Products Company. The Vitamin C contents were determined using the Indophenol Method. Broccoli After 3 days After 6 days After 12 days
Evert-Fresh 95% 90% 77%
Polyethylene 90% 80% 50%
Final results indicated that Evert-Fresh reduced vitamin C loss by 54% over a 12 day storage period during the broccoli test. Crown Daisy Evert-Fresh Polyethylene After 12 days 60% 40% Final results indicated that Evert-Fresh reduced vitamin C loss by 50% over a 12 day storage period during the Crown Daisy test. •CROWN DAISY is an edible flower popular in Japan
9.4.3
Modified atmosphere produce
With the exception of the Oya type consumer produce bags, the trend toward fresh produce has resulted in products such as fresh cut packaged vegetables. These are prepared using ultra-clean processing and packaging. The rate of package material gas permeation is controlled to allow for natural respiration to occur with the product distributed under refrigerated conditions. While not strictly an 'active' package form, the concept does utilize controlled permeation. 9.5
Oxygen absorber food applications
In 1977, Mitsubishi Gas Chemical introduced 'Ageless' oxygen absorbers in Japan, and now reportedly command over 70% of the 10 billion unit per year Japanese market; the remaining 30% is shared by approximately 15 other Japanese producers. In 1988 Multiform Desiccants introduced Fresh Pax™ oxygen absorbers (Figure 9.1), as the first US producer of oxygen absorbing packets. Between 1988 and 1994 Multiform developed a family of products to meet the specific needs of the North American marketplace (Multiform, 1994). Products are designed to be moisture-activated preventing primary oxidation until time of use, while others are for dry applications, or for
Figure 9.1 Fresh Pax oxygen absorbers introduced by Multiform Desiccants, Inc. for use in a wide variety of food packs.
situations where carbon dioxide is present, or to control oxygen removal rate at a wide range of temperature conditions. In 1992 Multiform introduced FreshMax® oxygen absorbing labels (Figure 9.2) designed to meet a market desire to make the absorber an integral part of the package system. Formulations are being adapted from FreshPax development, and a wide variety of substrates, adhesives and custom print are available. Fresh Max can be automatically applied within packages using conventional labelling equipment. Outside the Orient, the most common uses for oxygen absorbers are in protecting processed and cured meats, peanuts, and other nut varieties, high value baked goods, refrigerated pasta, snack foods, and dehydrated foods. In addition to human foods, oxygen absorbers can be found in medical devices, artemia, pet foods, treats, vitamins and in protecting valuable collectibles. Some common items containing the absorbers manufactured by Multiform Desiccants Inc., are as follows: • • • • •
Hormel Foods corp. - FreshPax 3 oz bottled bacon bits, 8 oz refrigerated sliced peperoni Marks & Spencer - St Michael's sliced meat products (UK) Kraft - DiGiorno refrigerated pasta Goodmark - beef jerky Melody Foods - Pioneer beef jerky
Figure 9.2 Multiform Desiccants, Inc. introduces FreshMax oxygen absorbers for processed, smoked and cured meats.
• • • •
US Military - shelf stable bread, cake, hamburger buns, chow mein noodles, potato sticks John B. Sanfillipo - bulk peanuts and almonds NASA - shelf stable tortillas Dietary specialties - shelf stable bread
In addition to this list, the institutional and food service markets have used oxygen absorbers to protect such products as processed meats, nuts, potato chips, and whole fat powdered milk. New applications are evolving almost weekly. Mitsubishi Gas Chemical Americas' 'Ageless' absorbers (Mitsubishi, 1994) are used as follows: • • • • • • • •
Hormel Foods Co. - sliced peperoni, bacon bits Kraft General Foods - fresh pasta (DiGiorgio brand) Penge Foods - beef jerky Goodmark Foods - beef jerky Victor coffee - coffee beans Advanced Development Corp - powdered drink Tyson Foods - poultry Dokosil Foods - sliced ham and poultry
9.5.1 Bottle closures - oxygen scavengers This subject has been extensively discussed in Chapter 8. Aquanautics Corp is the developer of 'Smartcap'®, marketed by ZapatA Industries, and used in various beer bottle crown liners. At present, an estimated 20 microbreweries use the Pureseal® liner in various applications. No large volume beer application yet exists; however, ZapatA is actively pursuing this market (ZapatA, 1994). Microbreweries using ZapatA's PureSeal® include Sierra Nevada Brewing Co., Cellis Brewing Co., Abita Brewing Co., and Full Sail Brewing Co. 9.6 Other applications International Paper, Purchase, NY, has developed an odour-trapping paper in conjunction with UOP, Des Plaines, IL supplier of Abscents deodorizing powder. The powder, which absorbs odours instead of masking them, makes up 30-35% of the paper's weight, replacing clay and other fillers typically used in paper. Paper made with Absents powder has been tested in surgical face masks, feminine hygiene products, and filters. International Paper is still investigating uses to see if paper with Absents is a viable, cost-effective product.
International Paper foresees applications in the medical industry, food packaging and household deodorization. The company is also testing uses for automotive products. References Evert-Fresh (1994) pers. commun. with Evert-Fresh (Houston, TX), August, 1994. Mitsubishi (1994) pers. commun. with Mitsubishi Chemical (New York City., NY) September, 1994. Multiform (1994) pers. commun. with Multiform Dessicants (Buffalo, NY), September, 1994. Packaging Gp. (1987) Microwave Packaging. A multi-client study published by the Packaging Group, Inc. (Milltown, NJ). ZapatA (1994) pers. commun. with ZapatA Industries, October, 1994.
10 Time-temperature indicators J.D. SELMAN
Time-temperature indicators are part of the developing interest in intelligent packaging, and there has been considerable interest in small temperature indicators (TIs) and time-temperature indicators (TTIs) for monitoring the useful life of packaged perishable products. There are over 100 patents extant for such indicators based on a variety of physico-chemical principles; however, widespread commercial use has been very limited for a number of reasons. For example, TTIs must be easily activated and then exhibit a reproducible time-temperature dependent change which is easily measured. This change must be irreversible and ideally mimic or be easily correlated to the food's extent of deterioration and residual shelf-life. TTIs may be classified as either partial history or full history indicators, depending on their response mechanism. Partial history indicators will not respond unless some temperature threshold has been exceeded, while full history indicators respond independent of a temperature threshold. This chapter reviews some of the physico-chemical principles utilised by different types of indicator, and discusses the various issues concerning their application, including consumer interests. Similar principles are being used in indicator systems for validating heat processes, and some of the latest research directions are highlighted. 10.1 Introduction Time-temperature indicators are one example of intelligent packaging, and interest in this is growing because of the need to provide food manufacturers, retailers and consumers alike with assurances of integrity, quality and authenticity. Other intelligent product quality indicators might include microwave doneness indicators, microbial growth indicators, and physical shock indicators. No microbial growth indicators are commercially available yet, but they are likely to be based on the detection of volatile microbial metabolites such as CO2, alcohols, acetaldehyde, ammonia and fatty acids. Tamper evidence and pack integrity indicators are perhaps the most well developed category. The most familiar types include the physical barriers such as plastic heat shrink sleeves and neck bands; tape and label seals; and paper/plastic/foil inner seals across the mouth of a container. More sophisticated systems include Vapor-Loc introduced by Protective Packaging Ltd. (Sale, UK) which provides a tamper evident recloseable pouch that
combines the security of a barrier pouch with the ease of a recloseable zipper seal. Secondary tamper evident features rely on subtle devices based on chemical reactions, biological markers, and concealing techniques. Some that are now commercially available utilise pattern adhesive labels and tapes, solvent soluble dyes and encapsulated dyes, optically variable films and holographic tear tapes. A number of other developments are on the horizon, including the application of smart cards within caps, magnetically coded closures and electrochemical devices. However, gas sensing dyes are the most advanced, especially for modified atmosphere packs. For example, a CO2 sensing dye could be incorporated into the laminated top web film of a modified atmosphere pack, and this could be designed to change colour when the CO2 level falls below a set concentration. In the area of product authenticity and counterfeiting, there is a large range of intelligent package devices which are being developed for use in various industrial sectors. Some of these will be applicable to the food industry and include the use of holograms, thermochromic and photochromic inks, IR and UV bar codes, biotags, optically variable films, computer scrambled imaging, electromagnetic ink scattering, and so on. There is continuing interest in the monitoring of temperature in the food distribution chain from factory to the consumer, and temperature monitoring and measurement, particularly of chilled foods, have been discussed by others (Woolfe, 1992). As part of the approach to assuring product quality through temperature monitoring and control, attention has focused on the potential use of indicators. Temperature indicators may either display the current temperature or respond to some predefined threshold temperature such as a freezing point or a chill temperature such as 80C. TTIs usually utilise a physico-chemical mechanism that responds to the integration of the temperature history to which the device has been exposed. Many different types of indicator have been devised over the years and general reviews have been presented by several authors, including Schoen and Byrne (1972) covering patent literature from 1933 to 1971, Cook and Goodenough (1975), Kramer and Farquhar (1976), Olley (1976, 1978), Farquhar (1977), Schoen (1983), Ulrich (1984), Selman and Ballantyne (1988), Bhattacharjee (1988), and Selman (1990). In general terms, indicators must be able to function in order to monitor one or more of the following. • • • • •
Chill temperatures (go/no go basis). Frozen temperatures (go/no go basis). Temperature abuses. Partial history (response over threshold). Full history (continuous response).
In order to achieve the monitoring objectives, there are several important requirements for indicators, including: •
Ease of activation and use. - Indicator may need to be stored and stabilised below threshold temperature for several hours before use • Response to temperature or to cumulative effect of time and temperature. • Response accuracy, time and irreversibility. • Correlation with food deterioration. • Correlation with distribution chain temperature/time. The sensory quality of food deteriorates more rapidly at higher temperatures due to increasing biochemical reaction rates. Such increasing reaction rates are often measured in terms of Q10 (the ratio of the rate at one temperature to that at a temperature 100C lower). For many chemical reactions Q10 has a value around 2, i.e., the reaction rate approximately doubles for each 100C temperature rise. As different foods lose quality at different rates, it may therefore be important that the indicator reaction has an activation energy that is similar to that of the food deterioration (Taoukis and Labuza, 1989a; 1989b). This is important for two reasons: firstly, the deterioration rates of stored foods follow similar patterns, although Q10 values may be higher, say from 3 to 20; and secondly, chemical reactions can be used in indicator systems so that by design the reaction rate can be made similar to that of the rate of deterioration of the food. Tables of product activation energies or Q10 values have been given by Hu (1972) for ambient shelf-stable foods, by Schubert (1977) and Olley (1978) for frozen products, and by Labuza (1982), and Hayakawa and Wong (1974) for the scientific evaluation of shelf-life. 10.2 Indicator systems There are a variety of physico-chemical principles that may be used for indicators, including melting point temperature, enzyme reaction, polymerisation, corrosion, and liquid crystals. Using these systems, many indicators give one of three responses: colour change, movement, or both colour change and movement. A variety of patents have been recorded and some of these are summarised in Table 10.1; a number of types of labels are discussed below. Liquid crystal graduated thermometers may be familiar to some (e.g. those manufactured by Liquid Crystal Devices Ltd., Ruislip, UK), and they can be engineered in different ways, e.g. as a sticky-backed paper label (Avery Label Systems Ltd., Maidenhead, UK) or designed to show selected temperatures as with the Hemotemp II (Camlab, Cambridge, UK). The
Table 10.1 Some recent patents - Cold chain monitoring systems Thaw Indicators - Based on Ice Melting Bigand, F.M.
French Patent 2626-668A 29.01.88
This device reveals an indicator when the frozen liquid thaws
Fauvart, J.
French Patent 2616-596A 06.01.89
This is a defrost indicator which consists of blotting paper that becomes coloured by afrozenaqueous dye when it thaws
Gradient, F.
French Patent 2641-61IA 09.01.89
A defrost indicator for frozen foods; it uses a windowed packaging system to observe change of shape due to thawing
Holzer, W.
W. German Patent 3716-972A 20.05.87
This device makes use of an ice tablet and an empty chamber which willfillup with water if the temperature rises
Holzer, W.
W. German Patent 3731-268A 17.09.87
This device consists in developingfrozenhemispheres of ice on the surface. When these thaw they lose their shape
Japanese Patent 0031-809 21.07.82
This device consists of an evaluation indicator which is stable when frozen but separates on thawing
British Patent 2209-396A 04.09.87
This indicator uses an irreversible change of state system: once a temperature change occurs it is recorded
Minnesota Mining MFG
European Patent 310-428A 02.10.87
This consists of a microporous sheet which becomes wetted when the liquid thaws. The process is irreversible and operates quickly
Mitsubishi Heavy Ind. KK
Japanese Patent 2021-229A 08.07.88
Use of vegetable leaves to indicate thawing - green colour turns to black; irreversible on thawing
Perez Martinez, F.
European Patent 2002-585A 10.03.87
This device is a sealed unit containing ice which changes shape on thawing
Perinetti, B.
French Patent 2625-599A 28.01.88
Sphere of ice suspended in the centre of a capsule
Toporenko, Y.
French Patent 2626-072A 20.01.88
This device has a geometrically shaped column of ice coloured with phosphorescent material at the centre. Loss of geometry indicates thawing
Uberai, B.S.
French Patent 2441-076A 23.12.88
Solvent/membrane indicator; when solvent melts colour is developed
Wanfield-Druck KaId
W. German Patent 2824-903C 13.10.88
Bi-metal stripflexesto display colour to indicate critical temperature reached
KAO Corp. Levin, D.
Table 10.1 Continued Electrochemical Time-Temperature Devices Grahm, I.
World Patent 9004-765A 24.10.88 Also US Patent 4929-020A
Temperature history indicating label; the electrodes of a galvanic circuit form a temperature-responsive device
Johnson Matthey
US Patent 4804275 14.02.89
Tungsten trioxide electrode/weak acid
Toppan Printing KK
Japanese Patent 1141-973A 28.11.87
This is a time indicator to show the expiry of foods started at ambient temperature. The device consists of a dye diffusing into a gel; the rate is determined by time and temperature
Toppan Printing KK
Japanese Patent 1250-090A 03.12.87
Twin lapse display. Dye diffusion in agar. With retarder, e.g. albumin
Badische Tabakmanuf
W. German Patent 3907-683A 09.03.89
Time-temperature indicator based on colour development with time when two chemicals are brought into contact, e.g. amino compounds, hydroquinones, quinones and nitro compounds
Bramhall, J.S.
US Patent 4825-447A 21.09.87
This sytem comprises liposomes containing a quenched fluorescent dye. Thefluorescenceis released by lysis when the product temperature fluctuates. It measures positive and negative temperature deviations
Lifelines Tech. Inc.
US Patent 4892-677 19.12.84
Diacetyiene monomer which polymerises to a dark compound, the intensity of which depends on time-temperature exposure
Rame, P.
French Patent 2613-069A 25.03.88
A thermal inertia temperature indicator which reacts at a certain preset threshold temperature. It is enclosed in a transparent case. It does not react to short temperature changes
Three S Tech BV
Japanese Patent 1012-237A 22.06.87
This device consists of a microcapsule layer containing an achromatic lactone compound pigment precursor and solvent. The sheet indicates the time elapsed at 50C temperature intervals
Dry Diffusion in Gels
Chemical Reactions
Freezewatch indicator (PyMaH Corp., Flemington, NJ, USA) is, by contrast, a simple irreversible indicator based on some threshold temperature, compared to the reversible technology exhibited by liquid crystals. When frozen, the liquid inside the ampoule freezes, causing it to break. If the temperature rises to -4°C, the liquid thaws and flows out, staining the backing paper. Chillchecker operates by means of a meltable, dyed compound contained in a porous reservoir (Thermographic Measurements Ltd., Burton, UK). In the inactivated form, a domed indicator paper is separated from a reservoir by a small distance. When the dome is pressed, the two materials come into contact, allowing wicking to occur when the melt temperature is reached. The Chillchecker can be designed for different threshold temperatures, e.g. + 9 or + 200C. Thermographics (see above) have now launched the Thawalert, a self-adhesive label (18 mm in diameter) which utilises temperature sensitive paints chosen to respond at a variety of threshold freezing and chilling temperatures. The above types are based on simple colour development; others quantify the change. Ambitemp (Andover Monitoring Systems Corp., Andover, USA) was a time-temperature integrator which functioned with a fluid that has a specific melting point related to the product to be monitored. Under abuse conditions the melted liquid moves along the capillary tube. Tempchron (Andover Laboratories Inc., South Weymouth, USA) was a more recent version of Ambitemp which gave a read-out in degree minutes that could be interpreted from a chart. Although these two did semi-quantify the changes, their size and cost did not meet the further important requirements for the indicators to be simple, small and inexpensive. 3M Monitormark indicators consist of a paper blotter pack and track separated by a polyester film layer (3M Packaging Systems, Bracknell, UK). Incorporated into the paper blotter pad are chemicals of very specific melting points and a blue dye. The indicator is designed as an abuse indicator which yields no response unless a predetermined temperature is exceeded. The response temperature of the indicator is therefore the melt point of the chemical used. To activate this partial history indicator, the polyester film layer is removed, allowing the melted chemical and dye to diffuse irreversibly along the track. The higher the temperature above the response level, the faster the diffusion occurs along the track. If the temperature falls below the response level of the tag, then the reaction stops. Each indicator has five distinct windows which allow an estimate of exposure time above present values to be made. Before use the indicator has to be preconditioned by storing at a temperature several degrees below the response temperature of the indicator, so that at the start of the reaction the chemical/dye mix is solid. Response of the indicator is measured by the progression of the blue dye along the track, and this is complete when all five windows are blue. An indicator tag labelled 51, for example, would indicate a response temperature
(melt temperature) of 5°C with a response time of 2 days. This response refers to the time taken to complete blue colour for all five windows at a constant 2°C above the response temperature of the tag. Similarly, response times of 7 days and 14 days are available on tags, with response temperatures varying from -170C to + 48°C (Byrne, 1976; Manske, 1983, 1985; Taoukis and Labuza, 1989a, 1989b; Morris, 1988; Ballantyne, 1988). I Point labels are 'full history' indicators showing a response independently of temperature threshold (I Point A/B, Malmo, Sweden). The device consists of a two-part material, one part containing an enzyme solution, the other a lipid substrate and pH indicator. To activate, the seal between the two parts of the indicator is broken and the contents become mixed. As the reaction proceeds, the lipid substrate is hydrolysed and a pH change results in colour change through four colour increments (0-3, green to red). This reaction is irreversible and will proceed faster as temperature is increased and slower as temperature is reduced. Each label has a colour scale to be used as a matching reference, which can also be expressed as a percentage of set time-temperature tolerance (TTT) elapsed (colour 1: 80% TTT; colour 2: 100% TTT; colour 3: 130% TTT). These labels have been the subject of several studies (Byrne, 1976; Blixt and Tiru, 1977; Blixt, 1984; Singh and Wells, 1987; Grisius et ai, 1987; Ballantyne, 1988; Taoukis and Labuza, 1989). An alternative I Point indicator (type B) is also available. Each indicator model is provided with the same time-temperature characteristics as type A, but the difference occurs in the colour change interval. In model B only two visible colours are seen: green and yellow. Only in the final 5% of preset TTT (95-100%, time to colour in type A) does the indicator change from green to yellow. So, whilst responding to the temperature history, the indicators actually remain green for most of the storage life. The development of a yellow colour then indicates product approaching the end of its shelf-life. This single colour change was designed to reduce variability in colour determination by different personnel, which was a common complaint with type A models. A range of indicators (A and B with varying TTT) are available, lasting from 2 years at -18°C to 2 days at + 300C. Activation energies of the models 2140, 2180 and 2220 range from 14.0 to 14.3 kcal/g mole (Wells and Singh, 1988c). The biochemical solutions must be accurate; results may tend to become less reproducible at longer intervals. Using the same technology, I Point have made a freezer indicator. Another enzyme based time-temperature indicator has been experimentally developed by Boeriu et ah (1986). This is based on enzymic reactions taking place many orders of magnitude faster in liquid paraffins than in solid ones. The device works as a thaw indicator by triggering off an enzymic colour reaction when the solid paraffin melts.
Lifelines' Fresh-Scan labels provide a full-history TTI, again showing a response independently of a temperature threshold. The Lifelines system consists of three distinct parts: a printed indicator label incorporating polymer compounds that change colour as a result of accumulated temperature exposure; a microcomputer with an optical wand for reading the indicator; and software for data analysis (Lifelines Technology Inc., Morris Plains, USA). The indicator label consists of two distinct types of bar code. The first is the standard bar code, providing information on product and indicator type, and the second is the indicator code containing polymer compound that irreversibly changes colour with accumulated temperature exposure. The colour change is based on polymerisation of diacetylenic monomers, which proceeds faster at higher temperatures, leading to more rapid darkening of the indicator bar (Fields and Prusik, 1983,1986; Byrne, 1990). Initially, reflectance of the indicator code is high (approximately 100%), subsequently falling during storage as the reaction proceeds and the colour darkens. Once manufactured, Lifelines' labels immediately start reacting to environmental temperature. Therefore, to maintain high initial reflectance values, indicators must be stored at temperatures of - 200C and below. Studies have found that the colour changes correlate well with quality loss in tomatoes and UHT milk, with activation energies for the indicators ranging from 17.8 to 21.3 kcal/g mole (Wells and Singh, 1988a, 1988b). The portable hand-held computer reads both the bar codes and the indicator codes. The software package has been designed to correlate reflectance measurements to predetermined time-temperature characteristics. Data from the hand-held computer are transferred to a host computer, product freshness measurements are entered into the system, and a comparison is made between the product freshness curve and the response kinetics of the Lifelines labels (ZaIl et al., 1986; Krai et ai, 1988). A mathematical model can then be prepared to compensate for the differences in reaction rates of indicators and product degradation and allow prediction of product quality from one indicator reading. Trials at Campden and Chorleywood Food Research Association found these labels to be more reliable than I Point indicator labels (Ballantyne, 1988). The Lifelines Fresh-Check indicator has been developed for the consumer in a simple visual form (Anon., 1989). A small circle of polymer is surrounded by a printed reference ring. The polymer, which starts out lightly coloured, gradually deepens in colour to reflect cumulative temperature exposure. Again, the higher the temperature, the more rapidly the polymer changes. Consumers may then be advised on the pack not to consume the product if the polymer centre is darker than the reference ring, regardless of the use-by date (Fields, 1989). Once again the required polymer response can be engineered. During the last two years several American companies have been using these labels on a trial basis, and the system has been found
useful for determining shelf-life expiry when products are held under proper refrigerated conditions. However, use is still limited by the lack of response to short periods of temperature abuse, and the polymerisation reaction is influenced to some extent by light. The latest types are light-protected by a red filter. There is at present considerable interest in these indicators, for example for fresh eggs where short time-temperature rises may not directly affect quality. Lifelines Inc. also claim good correlation with the quality life of cooked ready meals, fresh chicken and yoghurt. During 1991, Lifelines continued to evaluate their polymer-based indicators used in both the food and pharmaceutical industries, and their Fresh-Check label has been trialled in some of the department stores of the French company Monoprix, where they have been applied to over a dozen types of chilled retail products (Monoprix, 1990). The most prominent of the indicators to date have been the three referred to above, i.e., 3M Monitormark, the I Point type, and the Lifelines Fresh-Scan and Fresh-Check. These have been the subject of a number of independent validation tests, and the test systems and references are given in Table 10.2. Marupfroid (Paris, France) has developed a partial history freezer label based on the melting point of ice. The part of the tag containing the redcoloured ice is located inside the pack next to the frozen food, with a hazard warning area visible externally. If thawing has occurred, the red dye moves along the label and exposes a warning printed in hydrophobic white ink. One very important point must be highlighted here, and that is that all other indicators are placed on the outside of a pack and therefore respond to the environmental temperature. The packaging itself may provide the food with some insulation from the environment and the food temperature will therefore lag behind any changes in outside temperature. In the case of this label, the indicator system is placed inside the pack but with its response change visible externally. Johnson Matthey has patented a system based on the corrosion of an indicator strip (US Patent, 1989). It consists of a film of electrochromic material (in this case tungsten trioxide), with a metal overprint at one end, printed onto a card. The dissolution of the metal anode in acid is temperature sensitive and results in a colour boundary which moves down the strip at a rate governed by the temperature. The indicator can be engineered to respond to short total times and shows some promise in this respect, and the potential exists for miniaturisation of such indicators. Oscar Mayer Foods Corp. (Madison, USA) have developed a quality freshness indicator. This is based on pH-sensitive dyes in contact with a dual reaction system which simultaneously produces acid and alkali to maintain a constant pH. When one of the substrates becomes depleted, a rapid pH change occurs, resulting in a sharp visual colour change (green to pink). A rise in temperature causes a shift in the equilibrium and the colour changes.
Table 10.2 Validation tests on time-temperature indicators Model Lifelines Fresh-Scan Fresh-Check
I Point
System test
Reference 0
Tomato firmness (10-20 C) Microbial growth in pasteurised milk (0-50C) Green tomato maturity (10-200C) UHT sterilised milk (5-37°C) Fruit cake Lettuce Pasteurised milk (pallet) Milk, cream and cottage cheese Orange juice UHT milk freshness Orange juice concentrate (frozen) Fresh produce (chilled) Hamburger patties UHT milk freshness (21-45°C) Orange juice (7.2°C) Response to isothermal conditions (4-300C) Response to non-isothermal conditions (4-300C) Response to temperature (0-370C) Response to temperatures (5°C and 100C)
Wells and Singh (1988a) Grisius et al. (1987) Wells and Singh (1988b) Wells and Singh (1988b) Wells and Singh (1988b) Wells and Singh (1988b) Malcata (1990) Chen and ZaIl (1987a) Chen and ZaIl (1987b)
Green tomato maturity (10-200C) UHT sterilised milk (5-37°C) Fruit cake Lettuce
Wells Wells Wells WeUs
ZaHetal. (1986) Krall et al. (1988) Krall et al (1988) Singh and Wells (1986) Taoukis and Labuza (1989a) Taoukis and Labuza (1989b) WeUs and Singh (1988c) Fields (1985) Fields (1985) Ballantyne (1988) and and and and
Singh Singh Singh Singh
(1988b) (1988b) (1988b) (1988b)
Table 10.2 Continued Model
System test
Reference 0
3M Monitormark
Unspecified (two models)
0
Pasteurised whole milk (0 C, 5°C and 10 C) Hamburger rancidity (frozen) Hamburger rancidity Strawberries (- 12 to + 350C) Seafood salad (pallets) (- 20 to - 100C) Codfish(frozen) (pallets) Steak, beef patties, macaroni cheese (pallets) (- 20 to + 300C) Pizza (- 20 to + 300C) Milk (4.4-100C) Response to isothermal conditions (4-300C) Response to non-isothermal conditions (4-300C) Response to isothermal conditions Response to isothermal conditions (- 18 to + 5°C) Response to isothermal conditions (+ 2C, + 100C, - 12°C, -100C)
Grisius et al. (1987) Wells et al. (1987) Singh and Wells (1985a) Singh and Wells (1987) Singh and Wells (1985b) Olsson (1984) Olsson (1984) Kramer and Farquhar (1977) Mistry and Kosikowski (1983) Taoukis and Labuza (1989a) Taoukis and Labuza (1989b) Wells and Singh (1988c) Wells and Singh (1985) Ballantyne (1988)
Hamburger rancidity (>- 17°C)
Steak, beef patties and macaroni cheese (pallet loads) (- 23.4 to - 15°C) Milk (4.4-100C) Response to isothermal conditions (4-300C) Response to non-isothermal conditions (4-300C) Response to isothermal conditions (4 - 1O0C)
Wells et al. (1987) Singh and Wells (1986) Wells and Singh (1985) Kramer and Farquhar (1977) Mistry and Kosikowski (1983) Taoukis and Labuza (1989a) Taoukis and Labuza (1989b) Ballantyne (1988)
Response to isothermal conditions
Arnold and Cook (1977)
Imago Industries (La Ciotat, France) have launched their re-usable thermomarker. This is solid and relatively large (88 x 53 mm), and the principal element in its makeup is a shape memory alloy. The alloy effectively 'memorises' two distinct shapes associated with predefined temperatures. In the device itself, a spring made of shape memory alloy changes size according to predetermined temperatures within a programmed range. This in turn activates a system which ejects different coloured balls that signal the reaching of the various temperature thresholds. A patent from Microtechnic (Germany) apparently uses the alignment of two magnets as an indication of the thawing of a frozen food. At the point of freezing, two magnets are held unaligned in a small liquid container. However, if the liquid thaws, then the attraction by the opposite poles of the magnets will promote movement and the two magnets come together, indicating that thawing has occurred. Albert Browne (Leicester, UK) make cold chain indicators which can produce either an abrupt change of colour (yellow to blue) at its end point, or a more gradual change depending on its application. They have specialised in thermal indicators for many years and are now promoting their time-temperature cold chain indicators in both the food and pharmaceutical industries. Food Guardian (Blandford, UK) have begun to promote their label which has a thermometer profile. The label indicates the time on the scale for which the temperature has been above the designated temperature. Senders (London) have developed a threshold label for application to large boxes and pallets, and this consists of both a warning indicator that the temperature is getting too high, and a second indicator showing the need for rejection. Courtaulds Research (Coventry, UK) have considered developing a temperature-sensitive colour in acetate film. This could be used to detect when a product is fully defrosted and ready for cooking, assuming no storage abuse. Bowater Labels (Altrincham, UK) have recently launched their Reactt TTI self-adhesive label for monitoring freezing and chilling distribution temperatures (Pidgeon, 1994). The labels remain inert until activated, then change from blue to red to reveal underlying graphics when preset time/temperature limits are exceeded. Trigon Industries Ltd. (Telford, UK) has also just launched its Smartpak label, which is self-activating before use and shows an irreversible colour change to reveal an underlying symbol warning. For example, the Smartpak 1812 label self-activates when it is frozen below -18°C, and subsequently indicates the temperature rising above -12°C. In the case of microwaveable products, research has shown that for microbiological and other quality criteria, all points within the food should be reheated to an equivalent of 700C for 2 min. To date only two doneness indicators are available. That from 3M (Bracknell, UK) uses a thermochromic ink which undergoes an irreversible colour change (Summers, 1992). The Reactt doneness indicator from Bowater Labels is a modification
of the TTI self-adhesive label and works on the same colour-change principle described earlier. Other devices are being developed at this time, although the challenge of measuring and correlating cold point temperatures with overall pack temperatures remains considerable. Risman (1993) refers to the gel indicator technique developed at the Swedish Food Research Institute for assessing the reheating performance of domestic microwave ovens for ready meals. 10.3 Indicator application issues and consumer interests It is generally agreed that there are a number of potential applications for which the above-mentioned indicators could be used regarding the monitoring of various aspects and parts of the chilled and frozen distribution chains (Singh and Wells, 1990). However, the industry has been expressing concern regarding several issues about all types of indicator. TIs and TTIs represent new applications of technology, with little or no history of successful and reliable application, and until recently there has been no standard against which their performance could be assessed. Also, the proliferation of TIs and TTIs now being offered, involving many different forms of indication, is of concern as this is likely to confuse the consumer. Provided these concerns are addressed by a given indicator for a specified product (or range), the potential exists for indicators to be used in several ways, including on pallets or consumer packs, for stock rotation, parts or all of the distribution chain, retail shelf-life, and as a simple consumer guide. Ideally, chilled and frozen foods should be stored at the appropriate temperature, which should remain constant. However, there may be several points in the distribution chain where the environmental temperature is raised. Such periods may be short, from a few minutes to several hours. To date, most indicators will not react rapidly enough to respond to such regimes. For example, a Lifelines indicator subject to 24 hours at 5°C, six hours at 100C, and two hours at 200C did not show a response that was significantly different to the control at 5°C (Ballantyne, 1988). Lifelines have done work over the last two years and now claim that a dual chemistry system can be engineered to specifications required. Therefore, there may be some important limitations of some indicators that must be recognised, in particular relating to reliability and reproducibility, sensitivity to short-timetemperature abuse, response to environment temperature but not necessarily food temperature, and cost benefits. For example, in 1988 Lifelines bar code labels cost 30-70p each (scanning system US$20 000), I Point labels 15-2Op each, and the 3M Monitormark about £1.50, for small trial quantities. In 1991, Lifelines' prices in the USA ranged from 7.5 to 3.50 for bar code labels and 3.5 to 1.250 for Fresh-Checks. The latter lower cost related to production runs in excess of 10 million units.
To be effective and of value to manufacturer and consumer, TIs and TTIs must provide an indication of the end-life of the product. This should be no less clear and unambiguous to the great majority of the population than the current minimum durability instruction. In particular, some consumers may have difficulty in detecting the difference between two colours, or shades of one colour, where this forms the end point. Related to this, the point at which product life starts can be clearly defined for the purposes of declaring a 'best before' or 'use by' date. It is essential that the start point of the life of the TTI, i.e. when it is activated, can also be known for certain, with selfindication that this has occurred, and no reasonable possibility of preactivation, partial activation, or especially post-activation. The legal requirement for a best before and use by date on the pack will continue for the foreseeable future. Therefore, consumer instructions on the pack will need to clearly indicate the action to be taken when there is conflict between end of product life indication as given by the best before and use by date and the TTI. There is also concern that where TIs and TTIs may have a role to play with regard to product quality over life, unsubstantiated claims should not be made regarding any role in relation to safety. TTIs in general do not measure product temperature. Only one commercially available type is known, which is claimed to measure food surface temperature. None is known to measure food centre temperature. Almost all respond to temperatures on the outside of the pack, where there may be some thermal insulation between product and indicator (Malcata, 1990). Measurement at this point may be of value, but the limitations in terms of usefulness and relevance of such measurement need to be made clear to the user and the consumer. A TI or TTI which reflects product temperature would be of far greater value and relevance than one which responds to the temperature on the outer surface of the pack. A TI or TTI also needs to be able to cope with fluctuating temperatures (including elevated temperatures for a short time) and to respond accurately and reproducibly at the extremes of temperature likely to be experienced by the product. A TTI may need to mimic the growth of food spoilage microorganisms, or whatever other timetemperature related factor is liable to affect the quality of the foodstuff, over the full range of temperatures likely to be experienced and when the temperature fluctuates. The quality management of the manufacture, distribution and storage of the TTI and the reproducibility of its performance must be of at least as high an order as the food product it seeks to monitor. In addition, there is concern that the wrong TI or TTI may be applied to a given product. An incorrectly applied date mark is self-evident, at least to the manufacturer at the point of application. As manufacturers may be producing simultaneously a range of products with different predicted lives, they will require a range of TIs or TTIs designed with related performance characteristics. Hence, every indicator should be supplied with a clear indication to the manufacturer,
distributor, retailer, and the enforcement authorities of the precise temperature threshold or time-temperature integration to which the indicator will respond. The TI or TTI needs to be no less resistant to malpractice and tampering than is the printed date on the pack. The indicator or the package should self-indicate if removed from the product; at the same time, if removed it should damage the packaging in such a way that a fresh indicator cannot be applied without detection. Finally, TIs and TTIs in themselves must not represent a hazard to the consumer, e.g. if swallowed. In particular, care needs to be taken to make the indicator 'child-proof. In order to address these issues of concern, the industry concluded recently that a specification was required which could be common to all types of TIs and TTIs, and which could be used by manufacturers of such indicators in order to meet the requirements of the industry and of the consumer. Such a specification would address the basic technical requirements for the performance of such indicators, although it is accepted that commercial reasons may influence the decision to use indicators for a particular application. A joint Ministry of Agriculture, Fisheries and Food (MAFF)/industry working party met during 1991 at the Campden and Chorleywood Food Research Association, and has completed a food industry specification (George and Shaw, 1992). It is hoped that this will provide a basis for indicator manufacturers to design the performance of their indicators to meet the needs of the food industry, and at the same time provide a basis for the users of such indicators to check the indicator performance against their requirements. This specification defines the testing scope for indicator type and application. It refers to the quality management of the indicator manufacture, the indicator compatibility with food, the need for evidence of tamper abuse, and indicator labelling. It then outlines test protocols for indicator response to temperature, including temperature cycling and abuse, and the evaluation of the kinetic constants of the indicator. It covers evaluation of the accuracy of indicator activation point, and the clarity and accuracy of end point determination, and finally simulated field testing. A survey of 511 UK consumers, carried out by the National Consumer Council (MAFF, 1991), indicated that almost all respondents (95%) thought that TTIs were a good idea, but only grasped their concept after some explanation, indicating that substantial publicity or an education campaign would be required. Use of TTIs would have to be in conjunction with the durability date, with clear instructions about what to do when the indicator changed colour. The relationship and possible conflict between the indication of the TTI and the durability date on the food was considered a problem. In the retail situation, nearly half those questioned would trust the TTI response if it had not changed but the product was beyond its durability date. If the TTI changed before the end of the durability date when stored at home, the majority of respondents (57%) would use their own judgement in
deciding whether a food was safe to eat, with at least 25% putting some of the blame on the food suppliers. However, the value of TTIs was recognised for raising confidence in retail handling, and improving hygiene practices when food is taken home and stored in refrigerators. It is clear that there is a future for TTIs in monitoring the chill chain. Development of different indicators is still in progress and technical difficulties have to be overcome by carrying out the appropriate tests (George and Shaw, 1992). However, the consumer can appreciate the concept, and the advantages and benefits of increased food safety for the higher-risk foods that would result. 10.4 Chemical indicators for thermal process validation Similar approaches to temperature indication have been taken for assessing pasteurisation and sterilisation processes, and some examples of commercially available indicator systems are summarised in Table 10.3. Most of these tend to give qualitative indications. Current research is directed towards evaluating new systems which may give precise quantitative indication. Hendrickx et al (1993) have conducted an extensive review and have classified time-temperature indicators, as shown in Figure 10.1, in terms of working principle, type of response, origin, application in the food material, and location in the food. For biological TTIs, the change in biological activity such as of microorganisms, their spores (viability) or enzymes (activity) upon heating is the basic working principle. The use of inoculated alginate particles is an example of the use of spores (Gaze et al, 1990). Recent studies on enzyme activity have shown potential for the use of a-amylase, using differential scanning calorimetry to measure changes in protein conformation (De Cordt et al, 1994). Brown (1991) studied the denaturation of several enzymes and suggested that an approach which measures the status of a number of enzymes in terms of pattern recognition would be better than using a single enzyme to indicate retrospectively the heat process that had been applied. Brown (1991) also determined the feasibility and potential for ELISA techniques for retrospective assessment of the heat treatment given to beef and chicken. Marin et al (1992) studied the effects of graded heat treatments of 30 min from 40 to 1000C on meat protein denaturation. They measured the remaining antigenic activity of the meat proteins and found this was significantly correlated with the heating temperature. Varshney and Paraf (1990) used specific polyclonal antibodies to detect heat treatment of ovalbumin in mushrooms, and could identify whether the ovalbumin had been heated to lower than 65°C or higher than 85°C. In terms of chemical systems, potential has been shown for correlating the loss of food pigments such as chlorophyll, and changes in anthocyanins, with heat treatment (El Gindy et al, 1972). Other food compounds may
Table 10.3 Commercially available time-temperature thermal process indicator/integrators Manufacturer
Trade name
Colour
Change characteristics
3M Industrial Tapes and Adhesives (Manchester, UK)
Autoclave Tape
White to black (stripes)
121°C for 10-15 min and 134°C for 3-4 min for fully developed colour change
3M Industrial Tapes and Adhesives (Manchester, UK)
Thermometer Strips
Silver to black
Immediately temperature reached
Albert Browne Ltd. (Leicester, UK)
TST
Yellow to mauve
Set to 121°C for 15 min or 134°C for 5.3 min
Albert Browne Ltd. (Leicester, UK)
Steriliser Control Tube
Red to green
Steam autoclaves - colour change over 100-1800C for a range of exposure times Dry heat = 1600C for 120 min to 1800C for 12 min
Ashby Technical Products Ltd. (Ashby de Ia Zouch, UK)
ATP Irreversible Temperature Indicators
Silver to black
Self-adhesive segmented labels giving colour change when temperature exceeds set point by 1°C
Cardinal Group (Tiburon, CA, USA)
Easterday
Black to red
Set at 2400C for 20 min, ketone based
Colour Therm (Surrey, UK)
Colour-Therm
White to black or red
Immediately temperature is reached
PyMaH Corp. (Flemington, NJ, USA) (Temperature Indicators Ltd., Wigan, UK Agent)
Cook-Chex
Purple to green
Irreversible indicator, eight ranges selectable, semiintegrators using chromium chloride complex for different temperatures (110-126.70C) and times (0-150 min) calibrated against spore destruction
PyMaH Corp. (Flemington, NJ, USA) (Temperature Indicators Ltd., Wigan, UK Agent)
SteriGage Thermalog S
A blue colour front diffuses along a transparent window of an accept/reject band
The presence of saturated steam lowers the melting point of a chemical tablet Diffusion of the blue colour front has been calibrated against spore destruction (B. stearothermophilus) over a range of timetemperature combinations
Reatec AG (Switzerland) (Barbie Engineering, Twickenham, UK Agent)
Reatec
White to black
Immediately temperature is reached: 54.4-1040C
Table 103 Continued Manufacturer
Trade name
Colour
Change characteristics
Redpoint (Swindon, UK)
Spectratherm
From light blue to a colour in the spectrum donating maximum temperature
Liquid crystal colour change immediately temperature is reached
S.D. Special Coatings (Barking, UK)
Temperature Tabs
Spirig Earnest (Germany) (Cobonic Ltd., Surrey, UK Agent)
Celsistrip Celsidot Celsipoint Celsiclock
Irreversible colour labels, 40-2600C; lacquers, 40-lOloC; reversible strips, 40-700C
SteriTec (Colorado, USA) (Temperature Indicators Ltd., Wigan, UK Agent) SteriTec (Colorado, USA) (Temperature Indicators Ltd., Wigan, UK Agent) Thermindex Chemicals & Coatings Ltd. (Deeside, Clwyd, UK)
White to black
Immediately temperature is reached: 40-2600C
Mauve to green
Three-stage semi-integrator using chromium chloride
Brown to black
Selected precise time and temperature, 121 to 134°C
White to black
Adhesive strips 40-2600C
For crayons and paints, a range of colours dependent on temperature reached
Reversible and irreversible inorganic pigment colour change either immediately temperature is reached or after a few min exposure, 50-10100C
Integraph
Cross-checks
Thermindex
Thermographic Measurements Ltd., Burton, S. Wirral, UK (Temperature Indicators Ltd., Wigan, UK, European Agent)
Pasteurisation Check
White to black or white to red
Immediately temperature reached: 71, 77, 82°C and 88°C ratings ± 1°C. Other temperature ratings on request
Thermographic Measurements Ltd. (Burton, S. Wirral, UK)
Thermax
Silver grey to black
Adhesive strips, irreversible colour change paints, 37-2600C
Thermographic Measurements Ltd. (Burton, S. Wirral, UK)
Autoclave Indicator
Red to green
Autoclave ink. Change set for 30 min at 116°C or 15 min at 127°C
TLC Ltd. (Deeside, UK)
TLC 8
Red to black
Organic thermo-chromic ink; colour changes immediately temperature is reached
exhibit heat-induced changes. For example, Kim and Taub (1993) have been studying the thermally produced marker compounds 2,3-dihydro-3,5-dihydroxy-6-methyl-(4H)-pyran-4-one and 5-hydroxymethylfurfural. Both these compounds are produced when D-fructose is heated, and glucose yields only the latter compound. Hence, where a food contains either of these sugars, there is some basis for assessing heat treatment received as the kinetic characteristics make them suitable as markers for bacterial destruction. As before, the kinetic response requirement which a TTI should fulfil can be derived theoretically and should match the response of the target index, such as a spore or a nutrient, when subjected to the same thermal process. Potential exists for multicomponent TTIs in the evaluation of thermal processes (Maesmans et al, 1994). Regarding the origin of the TTI, an extrinsic TTI is a system added to the food, while intrinsic TTIs are intrinsically present in the food. In terms of the
Working principle
Response
Origin
Application
Location
Chemical
Biological
Dispersed
Volume average
Physical
Single
Multi
Intrinsic
Extrinsic
Permeable
Isolated
Single point
Figure 10.1 General classification of time-temperature indicators (after Hendrickx et al, 1993).
application of the TTI in the food product, dispersed systems allow the evaluation of the volume average impact, whilst all three approaches (see Figure 10.1) can be used as the basis for single point evaluations. When using intrinsic components as the TTI, the TTI will be more or less evenly distributed throughout the food, and this also eliminates heat transfer limitations. This whole field is currently the subject of a major European collaborative research study co-ordinated by the Centre for Food Science and Technology at the University of Leuven in Belgium. 10.5 Conclusions The interest in this subject has generated numerous research studies and practical evaluations of indicator systems. It is clear that the food industry, and indeed other sectors such as the medical and pharmaceutical industries, as well as the consumer, recognise a variety of benefits that can stem from the application of indicators in aiding the monitoring and assurance of distribution chains. This, in turn, is leading to the development of new indicators that are much more precisely designed to meet the needs of the food industry. In the broader context of time-temperature integration, applications for thermal process assessment are receiving further attention and novel approaches are actively being researched. Such developments will assist in the assurance in and broader introduction of new heat processes such as microwave sterilisation. Overall, it is likely that there will continue to be exciting developments during the next five years. References Anon. (1989) Is it time for time-temperature indicators? Prepared Foods, 158(12), 219-30. Arnold, G. and Cook, DJ. (1977) An evaluation of the performance claimed for a chemical time/temperature integrating device. Journal of Food Technology, 12, 333-7. Ballantyne, A. (1988) An Evaluation of Time-Temperature Indicators, Technical Memorandum No. 473, Campden Food and Drink Research Association, Chipping Campden, Glos., UK. Bhattacharjee, H.R. (1988) Photoactivatable time-temperature indicators for low-temperature applications. Journal of Agricultural and Food Chemistry, 36(3), 525-29. Blixt, K. (1984) The I-Point TTM - a versatile biochemical time-temperature integrator. In: Thermal Processing and Quality of Foods, P. Zeuthen, J.C. Cheftel and C. Eriksson (eds), Elsevier Applied Science Publishers, London. Blixt, K. and Tim, M. (1977) An enzymatic time/temperature device for monitoring the handling of perishable commodities. Developments in Biological Standards, 36, 237. Boeriu, C.G., Dordick, J.S. and Klibanov, A.M. (1986) Enzymatic reactions in liquid and solid paraffins: application for enzyme-based temperature abuse sensors. Bio/Technology, 4, 997-9. Brown, H.M. (1991) The Use of Chemical and Biochemical Markers in the Retrospective Examination of Thermally Processed Formulated Meals, Technical Memorandum No. 625, Campden Food and Drink Research Association, Chipping Campden, Glos., UK. Byrne, CH. (1976) Temperature indicators - the state of the art. Food Technology, 30(6), 66-8.
Byrne, M. (1990) Chill check. Food Processing, 59(5), 29. Chen, J.H. and ZaIl, R.R. (1987a) Packaged milk, cream and cottage cheese can be monitored for freshness using polymer indicator labels. Dairy and Food Sanitation, 7(8), 402-4. Chen, J.H. and ZaIl, R.R. (1987b) Refrigerated orange juice can be monitored for freshness using indicator label. Dairy and Food Sanitation, 7(6), 280-2. Cook, DJ. and Goodenough, P.W. (1975) Cold chain and chilled chain - temperature integrity monitoring devices. Proc. Institute Refrigeration, 72, 47-58. De Cordt, S., Hendrickx, M., Maesmans, G. and Tobback, P. (1994) Convenience of immobilised Bacillus licheniformis a-amylase as time-temperature indicator. Journal of Chemical Technology and Biotechnology, 59, 193-9. El Gindy, M.M., Raouf, M.S. and El Manawaty (1972) A study on the effect of processing temperature and time on the colour loss of anthocyanin pigments in grapes. Agricultural Research Review, 50(4), 281-90. Farquhar, J.W. (1977) Time-temperature indicators in monitoring the distribution of frozen foods. Journal of Food Quality, 1, 119-23. Fields, S.C. (1985) Computerised Freshness Monitoring System - Case Studies on Perishable Foods and Beverages. In: Proceedings of HR Symposium, Technology Advances in Refrigerated Storage and Transport, 85/11, pp. 117-23. Fields, S.C. (1989) Monitoring of Product Quality Using Time-Temperature Indicators. In: Proceedings of Eastern Food Science Conference VI, Food Technology: A View of the Future, Hershey, Pennsylvania, 1-4 October. Fields, S.C. and Prusik, T. (1983) Time-Temperature Monitoring Using Solid-State Chemical Indicators. In: Proceedings of HR XVIth International Congress of Refrigeration, Paris, France, pp. 839-46. Fields, S.C. and Prusik, T. (1986) Shelf-life estimation of beverages and food products using bar coded time-temperature indicator labels. In: The Shelf-Life of Food and Beverages, J.G. Charalambous (ed.), Elsevier Science Publishers, Amsterdam. Gaze, J.E., Spence, L.E., Brown, G.D. and Holdsworth, S.D. (1990) Microbiological Assessment of Process Lethality Using Food/Alginate Particles, Technical Memorandum No. 580, Campden Food and Drink Research Association, Chipping Campden, Glos., UK. George, R.M. and Shaw, R. (1992) A Food Industry Specification for Defining the Technical Standards and Procedures for the Evaluation of Temperature and Time-Temperature Indicators, Technical Manual No. 35, Campden Food and Drink Research Association, Chipping Campden, Glos., UK. Grisius, R., Wells, J.H., Barrett, E.L. and Singh, R.P. (1987) Correlation of time-temperature indicator response with microbial growth in pasteurised milk. Journal of Food Processing and Preservation, 11, 309-24. Hayakawa, K-I. and Wong, Y.F. (1974) Performance of frozen food indicators subjected to time variable temperatures. ASHRAE Journal, 16(4), 44—8. Hendrickx, M., Maesmans, G., De Cordt, S., Noronha, J., Van Loey, A., Willocx, F. and Tobback, P. (1993) Advances in Process Modelling and Assessment: The Physical Mathematical Approach and Product History Indicators. In: Proceedings of Workshop, Process Optimisation and Minimal Processing of Foods, Porto, Portugal, 20-3 September, 1993. Hu, K.H. (1972) Time-temperature indicating system 'writes' status of product shelf-life. Food Technology, 26(8), 56-62. Kim, H.-J. and Taub, LA. (1993) Intrinsic chemical markers for aseptic processing of particulate foods. Food Technology, 47(1), 91-9. Krai, A.H., ZaIl, R.R. and Prusik, T. (1988) Use of remote communications to transmit product quality information from polymer-based time-temperature indicators. Dairy and Food Sanitation, 8(4), 174-6. Kramer, A. and Farquhar, J.W. (1976) Testing of time-temperature indicating and defrost devices. Food Technology, 30(2), 50-6. Kramer, A. and Farquhar, J.W. (1977) Experience with T-T Indicators. In: Proceedings of IIR Symposium, Freezing, Frozen Storage & Freeze Drying, 77/1, 401-6. Labuza, T.P. (1982) Scientific evaluation of shelf-life. In: Shelf-Life Dating of Foods T.P. Labuza (ed.), Food and Nutrition Press Inc., Trumbull, pp. 41-87.
Maesmans, G., Hendrickx, M., De Cordt, S. and Tobback, P. (1994) Theoretical considerations on design of multi-component time-temperature integrators in evaluation of thermal processes. Journal of Food Processing and Preservation, 17, 369-89. Malcata, F.X. (1990) The effect of internal thermal gradients on the reliability of surface mounted full-history time-temperature indicators. Journal of Food Processing and Preservation, 14, 481-97. Manske, WJ. (1983) The Application of Controlled Fluid Migration to Temperature Limit and Time-Temperature Integrators. In: Proceedings of HR XVIth International Congress of Refrigeration, Paris, France, pp. 797-804. Manske, WJ. (1985) Experience with Monitormark product temperature exposure indicators. In: UR Annex Bulletin 85-5, pp. 311-17. Marin, M.L., Casas, C , Cambero, M.I. and Sanz, B. (1992) Study of the effect of heat (treatments) on meat protein denaturation as determined by ELISA. Food Chemistry, 43, 147-50. Ministry of Agriculture, Fisheries and Food (1991) Time-temperature Indicators: Research into Consumer Attitudes and Behaviour, National Consumer Council, London. Mistry, V.V. and Kosikowski, F.V. (1983) Use of time-temperature indicators as quality control devices for market milk. Journal of Food Protection, 46(1), 52-7. Monoprix (1990) La Plaisir du Frais: Monoprix Lance Ia Puce Fraicheur, Monoprix, Paris, France. Morris, CE. (1988) Monitoring fresh food shelf-life with IfT labels. Food Engineering, 60(4), 52-7. Olley, J. (1976) Temperature indicators, temperature integrators, temperature function integrators and the food spoilage chain. In: HR Annex Bulletin 76-1, pp. 15-131. Olley, J. (1978) Current status of the theory of the application of temperature indicators, temperature integrators, and temperature function generators to the food spoilage chain. International Journal of Refrigeration, 1(2), 81-6. Olsson, P. (1984) TT integrators - some experiments in the freezer chain. In: Thermal Processing and Quality of Foods, P. Zeuthen, J.C. Cheftel and C. Eriksson (eds), Elsevier Applied Science, London, pp. 782-8. Pidgeon, R. (1994) High-tech labels help colour your judgement. Packaging Week, 9(38), 18. Risman, P.O. (1993) Microwave oven loads for power measurements. Microwave World, 14(1), 14-19. Schoen, H.M. (1983) Thermal Indicators for Frozen Foods. In: Proceedings of HR XVIth International Congress of Refrigeration, Paris, France, 588-592. Schoen, H.M. and Byrne, CH. (1972) Defrost indicators. Food Technology, 26(10), 46-50. Schubert, H. (1977) Criteria for the application of T-T indicators to quality control of deep frozen food products. In: HR Commissions C1/C2 77-1, Ettlingen, Germany, pp. 407-23. Selman, J.D. (1990) Time-temperature indicators - how they work. Food Manufacture, 65(8), 30-4. Selman, J.D. and Ballantyne, A. (1988) Time-temperature indicators: do they work? Food Manufacture, 63(12), 36-8, 49. Singh, R.P. and Wells, J.H. (1985a) Use of time-temperature indicators to monitor quality of frozen hamburger. Food Technology, 39(12), 42-50. Singh, R.P. and Wells, J.H. (1985b) Application of time-temperature indicators in food storage and distribution. In: Proceedings of HR Symposium, Technology Advances in Refrigerated Storage and Transport, Orlando, Florida, USA, 85/11, pp. 124-30. Singh, R.P. and Wells, J.H. (1986) Keeping track of time and temperature. Meat Processing, 25(5), 41-2, 46-7. Singh, R.P. and Wells, J.H. (1987) Monitoring quality changes in stored frozen strawberries with time-temperature indicators. International Journal of Refrigeration, 10(5), 296-300. Singh, R.P. and Wells, J.H. (1990) Time-temperature indicators in food inventory management. Food Technology International Europe, Sterling Publications Ltd., London, 283-6. Summers, L. (1992) Intelligent Packaging, Centre for Exploitation of Science and Technology, London, UK.
Taoukis, P.S. and Labuza, T.P. (1989a) Applicability of time-temperature indicators as shelflife monitors of food products. Journal of Food Science, 54(4), 783-8. Taoukis, P.S. and Labuza, T.P. (1989b) Reliability of time-temperature indicators as food quality monitors under non-isothermal conditions. Journal of Food Science, 54(4), 789-92. Ulrich, R. (1984) Indicators and integrators of time-temperature and frozen products. Revue Generale du Froid, 74(6), 337-41 US Patent (1989) Indicator device for indicating the time integral of a monitored parameter, US Patent 4804275. Varshney, G.C. and Paraf, A. (1990) Use of specific polyclonal antibodies to detect heat treatment of ovalbumin in mushrooms. Journal Science of Food and Agriculture, 52(2), 261-74. Wells, J.H. and Singh, R.P. (1985) Performance evaluation of time-temperature indicators for frozen food transport. Journal of Food Science, 50, 369-71, 378. Wells, J.H. and Singh, R.P. (1988a) Application of time-temperature indicators in monitoring changes in quality attributes of perishable and semi-perishable foods. Journal of Food Science, 53(1), 148-52, 156. Wells, J.H. and Singh, R.P. (1988b) A kinetic approach to food quality prediction using fullhistory time-temperature indicators. Journal of Food Science, 53(6), 1866-71, 1893. Wells, J.H. and Singh, R.P. (1988c) Response characteristics of full-history time-temperature indicators suitable for perishable food handling. Journal of Food Processing and Preservation, 12(3), 207-18. Wells, J.H., Singh, R.P. and Nobel, A.C. (1987) A graphical interpretation of timetemperature related quality changes in frozen food. Journal of Food Science, 52(2), 436-41. Woolfe, M.L. (1992) Temperature monitoring and measurement. In Chilled Foods: A Comprehensive Guide, C. Dennis and M. Stringer (eds), Ellis Horwood, London, pp. 77-109. ZaIl, R., Chen, J. and Fields, S.C. (1986) Evaluation of automated time-temperature monitoring system in measuring freshness of UHT milk. Dairy and Food Sanitation, 6(7), 285-90.
11
Safety considerations in active packaging J.H. HOTCHKISS
11.1 Introduction Ever since Appert discovered that heating food in sealed glass jars produced a stable product, a major goal of packaging has been to safely preserve foods for extended periods. A scientific understanding of the relationship between shelf-life, safety, processing/storage conditions, and packaging began to evolve in the late 180Os as the theoretical basis for the thermal inactivation of pathogenic spores was developed (Goldblith, 1989). This understanding is still evolving. The use of packaging to safely protect and preserve foods has remained a central focus of packaging development. The primary roles of packaging in food safety have traditionally been to withstand thermal processing conditions and to act as a barrier to contamination. It would be of little benefit to process food if there was no way to prevent recontamination. The success of the metal can over the last 150 years is due to its ability to withstand thermal processing and provide a barrier against chemical and biological contamination. Modern food packaging can also influence the nutritional and quality attributes of foods and ensure the year-round availability of many foods. These factors are important in the health and nutritional aspects of foods. The major advances in food packaging over the last two decades have been the development of new materials, combinations of materials, and containers with specific technical and economic benefits (Downes, 1989). Most of these new materials and containers are inactive technologies in that they act primarily as passive barriers which separate the product from its environment. However, current research is shifting to the development of packaging which actively contributes to the preservation and safety of foods (Labuza and Breene, 1989). Such packaging interacts directly with the food and the environment to extend shelf-life and/or improve quality.
11.2 Packaging and food safety Packaging has often been thought of as a source of risk for foods and seldom as a technology which could be used to enhance food safety (Wolf, 1992).
Certainly, when packaging fails to preform its protective functions the result is an unsafe product (Downes, 1993). For example, safety may be compromised when package components migrate to a food or when there is a loss of integrity resulting in contamination by pathogenic microorganisms. Table 11.1 lists several general ways in which packaging can detract from safety. However, active packaging can directly enhance food safety. Active packaging can not only prevent contamination but it can also improve food safety in several other ways. Examples of 'active' packaging which improves food safety include antimicrobial polymers and films which inhibit the growth of pathogenic and spoilage microorganisms, packages which react with toxins and indicate their presence, packaging materials which prevent the migration of contaminant, and packages which indicate if packages are leaking. These and other types of active packaging which improve safety and quality are areas of current research and commercial interest (Ishitani, 1994). Table 11.1 Types of food safety problems associated with packaging Examples Microbial contamination Loss of integrity Anaerobiosis
Chemical contamination Migration Environmental contamination Recycled packaging Insect contamination Post packaging
Consequences Seal rupture, leaking cans, incomplete glass finishes allow contamination by pathogenic m.o. Low oxygen environment resulting from product or microbial respiration. Can lead to toxin formation by anaerobic pathogenic microorganisms Transfer of package components to foods Environmental toxicants can permeate films Examples include preservatives used in wooden pallets, diesel exhaust Contamination of post-consumer packaging is transferred to foods after recycling Some insects can bore through many common packaging materials
Foreign objects
Glass shards, metal pieces
Injury Exploding pressurized containers Broken containers
Soft drinks, beer in glass, etc. Cuts, lacerations
Environmental impact
Disposal, recycling, CFCs
Loss of nutritional and sensory quality
Aroma and nutrient sorption by polymers
Tamper evidency
Malicious and innocuous
Inadequate processing Conventional Aseptic
Underprocessing can lead to food poisoning Loss of integrity or insufficient sterilization of packaging can lead to food poisoning
11.3 Passive safety interactions 11.3.1 Barriers to contamination The major safety and quality function of packaging is to act as a barrier between food and the environment. The purpose is to prevent contamination (or re-contamination after processing) of the food from both environmental chemicals and pathogenic microorganisms. With glass and metal food packaging, which are, for practical purposes, absolute barriers, preventing contamination is usually a function of closure integrity. Considerable experience with such closures has resulted in a remarkably low risk packaging system. The economic and functional disadvantages of metal and glass have led to the development of polymeric packaging materials. The barrier properties of these polymeric materials has been the central focus of packaging development in recent years. Polymers which are high barriers to both oxygen and water vapor are now available. Very recent efforts have focused on improved aroma/flavor barriers. Post-packaging microbial contamination of foods is now not only a function of closure integrity and material integrity (Downes et a/., 1985). There are two aspects of package integrity: strength and completeness. Strength implies that the closure or seal is sufficiently strong to withstand the rigors of distribution without failure. Completeness means that there are no gaps, holes, tears, etc. in the material or the seal. A seal or material can be strong yet incomplete or can be complete yet have insufficient strength for distribution. In some cases, flexible materials can contain minute pinholes which allow entry to microorganisms yet still not show signs of leakage (Chen et aL, 1991). Flexible packaging can also develop pin holes during shipping. Strength and integrity have become important issues because foods are transported further and stored for extended periods. The canning industry has had considerable experience with double seam closure for metal cans, making a container sealed in this way one of the safest available. The lack of such long-term experience with the heat seal as a means of closure has raised safety concerns. Flexible materials are also more prone to failure during transport and storage. As the change to polymeric packaging has occurred, concern for the integrity of the container has increased. Research has been undertaken in an attempt to improve the testing of integrity of polymer-based packages. Gnanasekharan and Floros (1994) have reviewed methodology for detecting leaks in flexible food packaging. No currently available method is entirely satisfactory for all situations. Increased potential for chemical contamination has become a concern because polymers are permeable to organic vapors, and foods which are hermetically sealed in polymer-based containers can absorb environmental
contaminants. The transfer to foods of potentially toxic compounds used to preserve wooden shipping pallets and wooden container floors has been reported and is exacerbated by the increase in long-distance shipment of foods (Whitfield et al, 1994). In addition to toxicological concerns, many environmental organic compounds which permeate films impart undesirable odors to foods. As the pace in the use of recycled materials has gained momentum, concern over microbiological contamination of fiber based packaging materials has also increased (Klungness et al., 1990). The paper making process inactivates most vegetative cells but does not inactivate microbial spores. Foods packaged in recycled materials have the potential to have acquired high spore loads from the packaging (Vaisanen et al, 1994). It is likely that in some cases potentially pathogenic spores could be transferred to foods from recycled materials. Packaging also protects the nutritional and organoleptic quality of foods. While not directly safety issues, foods which have lost their nutritional attributes or are not consumed because of poor taste or appearance, become a health issue. Nutrients and organoleptic properties can be adversely affected in several ways. For example, nutrients can be destroyed when oxygen or light enters a package or when the product is exposed to excessive heat. The loss 'of vitamin C in orange juice when stored in low barrier packaging is a prominent example. Sorption of nutrients by the packaging material is also a mechanism of loss. 11.3.2 Prevention of migration The second major safety function of packaging is to limit the transfer (i.e., migration) of packaging components to foods. Considerable research has been conducted into the migration of packaging materials to foods (Crosby, 1981). Migrants include inorganic toxicants, primarily lead from soldered cans, as well as organic toxicants such as vinyl chloride monomer which is a known human carcinogen. Both the theoretical and empirical aspects of migration have been studied in detail and in most cases the process is scientifically understood. Concern over migration has been recently heightened because of the use of recycled materials or refillable containers for food and beverage packaging (Begley and Hollifield, 1993). At least two potential problems exist. One is that non-food grade plastics which may contain additives or monomers that are not intended for human food use will enter the recycle stream. These additives or monomers could then migrate to foods. The second problem is the potential contamination of food grade polymeric packages by consumers (gasoline and pesticides are commonly mentioned as potential contaminants). These contaminants could then migrate to foods packaged in containers made from recycled materials.
Several solutions to the post-consumer contamination of recyclable plastics have been proposed. First is the use of equipment to detect contaminated containers prior to refilling. These are commonly referred to as 'sniffers' and are designed to sample the air inside the container and determine if volatile organic compounds such as might be found in gasoline are present. Commercial sniffers are available and in use. The second solution is to chemically break down the polymeric structure and subsequently reform the basic polymer. Any contaminants would be removed during this processing. The third solution is to construct containers in which the recycled polymer is separated from the product by a functional barrier. Such functional barriers are intended to prevent the migration of contaminants from recycled polymers to products. Combining virgin and recycled PET by co-extrusion into a PET bottle has been commercially undertaken. The virgin PET is expected to be a functional barrier to potential contaminants in the recycled layer. The major question is, How effective is this virgin layer at preventing migration? Other treatments of polymers such as cross-linking can retard migration. Migration is also affected by the chemical and physical nature of the migrant. Active packaging materials which can minimize or eliminate migration would be of substantial interest as the concern over the environmental cost of packaging increases. A second recent packaging migration safety concern has resulted from the use of polymer-based packaging as containers for food during heating such as in processing low acid foods in a retort or heating foods in microwave ovens. Initially, food polymer-based containers were designed to store products at ambient or sub-ambient temperatures. Thus, most laboratory work on migration was undertaken at room temperature or lower over extended periods. The advent of the microwave oven has meant that foods are heated in plastic vessels and on plastic surfaces. Heating plastics has two effects on migration. First, migration in general follows Arrhenius-type kinetics and increasing temperature increases migration rates in an exponential fashion. The second effect is that elevated temperature can cause degradation of the polymer and additives which can result in migration of the breakdown products. Each of these issues has been addressed by several regulatory agencies in the USA and Europe. 11.4 Active safety interactions Active packaging systems face similar barrier and migration safety issues as conventional packaging, as well as some additional issues. While there is concern that some active packaging systems will detract from safety there also is the possibility that new active systems can enhance safety. Materials and containers are being developed specifically to reduce food safety risks.
11.4.1 Emitters and sorbers One the earliest and most successful active packaging concepts was to incorporate a material which either absorbed or emitted vapors or gases inside a package after closure. This might be as simple as water vapor absorbers which are designed to control relative humidity, or more complex substances which absorb ethylene from produce, absorb undesirable odors from foods, or emit ethanol to control molds in bakery products. Particularly desirable types of sorbers are those that remove both residual and ingress oxygen after the package has been sealed (Rooney, 1994). Oxygen absorbers which remove oxygen from the headspace of bottled beer, for example, have been successfully tested commercially. Initially, absorbers/emitters were contained inside packets which were added to the package along with the product. More recent technologies have incorporated the sorber/emitter into the film or container wall. This reduces the likelihood of accidental ingestion. These absorbers are a form of modified atmosphere packaging and can change the microbiology of foods. The safety implications of such changes are the same as those for conventional MAP, as discussed below. There is also the concern that the components making up the absorber/ emitter will migrate to the food. 11.4.2 Active packaging and migration Many active packaging systems incorporate functional additives in food contact materials. These may be as simple as iron oxides which absorb O2 or as complex as systems which react with singlet oxygen (Rooney, 1994). In each case, the potential for and consequences of migration need to be assessed. For example, there has been some reluctance in some parts of the world to allow the use of ethanol emitters in foods which will be consumed without further cooking or processing. The residual ethanol might be considered a food additive and thus be required to undergo the rigors of complete toxicological testing. For those active packaging systems which indirectly add components to foods, the governmental regulatory and health issues will be similar to those related to migration of residual monomers or other polymer components (Crosby, 1981). Laboratory investigations will be required to determine the potential for migration and to quantify the amount of migration. If the amount of additive migrating is considered of potential significance, toxicological testing may be required. In some cases, active packaging systems may involve migrants for which there is little concern in food systems. For example, approved antimycotic agents such as sorbic, benzoic, or propionic acids, would likely be of little regulatory or safety concern if incorporated into antimicrobial films (Giese, 1994). However, in most cases active components and additives will not be common food additives and potential toxicological concerns will need to be addressed. The addition of antimicrobial metal ions to food contact surfaces
is likely to result in the migration of small amounts of the metals to foods (Ishitani, 1994). While these metal ions may be of low toxicity, the metals may be classified as food additives and require rigorous toxicological testing. The regulatory consequences of intentional addition of even low amounts of metals will need addressing. In some cases, the use of functional barriers to prevent migration of the active components will be required. Incorporating absorbers or scavengers into the adhesives used to bind layers of inert film as a means of 'burying' the additive is an example. 11.4.3 Barrier to contamination In addition to migration, active packaging systems must fulfill the safety requirement of acting as a barrier to microbial and chemical contamination. The addition of active ingredients to films could decrease their mechanical properties resulting in a higher failure rate during transport. Such failures become safety concerns if they allow for contamination by pathogenic microorganisms or toxic chemicals. For example, the addition of inorganic compounds such as metal-coated zeolites, desiccants, or oxygen scavengers will likely reduce the mechanical properties of films raising the possibility that the contamination barrier will be reduced. The addition of packets or sachets to packages of food raises concern that they will be inadvertently ingested. While sachets and packets have been in use for several years without apparent problems, caution about adding nonedible items to packages should be taken. Incorporation of active ingredients directly into the packaging rather than as sachets seems prudent. 11.4.4 Indirect effects on safety Active packaging often has indirect as well as direct effects on food safety. For example, packaging which absorbs oxygen from inside a package with the goal of reducing deteriorative affects will affect both the types and growth rate of the microorganisms in products. The inclusion of antimicrobial agents in the contact layer of a packaging material may have similar effects. This will result in a change in the microbial ecology of the food. The type of microorganisms present on a product will thus be different from the same product packaged in a conventional manner. This change in microbiology will indirectly influence safety. In some cases, safety may be enhanced such as when carbon dioxide is added to high pH cheeses such as cottage cheese (Chen and Hotchkiss, 1993). In other cases, safety may be compromised as when the growth of Clostridium botulinum is favored. The effect of such changes in the microbial ecology of foods has not been investigated in detail and only a few reports on changes in microbial ecology have been published (Reddy et al.9 1992). Smith and co-workers have
investigated the effects of MAP on the microbiology and toxin production by Clostridium botulinum in meats (Lambert et al9 1991). Somewhat surprisingly, toxin was produced most rapidly in samples packaged in air (i.e., 20% O2). This confirms an earlier observation we had made in cooked beef inoculated with both Pseudomonas and Clostridia spp. It is likely that this occurred because the aerobic Pseudomonas grew rapidly and consumed the oxygen rapidly leaving a highly anaerobic environment for the Clostridia. MAP in high carbon dioxide atmospheres inhibited the Pseudomonas inoculum and left traces of unconsumed O2 which inhibited the Clostridia. These results point out that large changes in microbial populations can result indirectly from altering the gases inside a package. These changes can both detract from safety but can also improve safety. 11.4.5 Indicators of safety/spoilage In addition to decreasing the safety of food, active packaging holds the promise of reducing risks from certain foods compared to conventional packaging. One example is the use of packaging which shows or in some way indicates the condition or history of a product. One currently available technology is time-temperature indicators. These devices integrate the time and temperature history of a product and give a visual indication if the combination has exceeded some standard or desirable amount (Taoukis et al9 1991). Shelf-life is related not only to how long a product is stored, but just as importantly, to the conditions, such as temperature, under which the product is stored. For example, pasteurized milk will last weeks at O0C but only a few hours at 35°C. Time-temperature indicators can be used on individual packages to warn consumers that a product has been exposed to a combination of time and temperature which may compromise safety or they may be used on shipping cartons to alert store personnel of potential quality/safety problems or allow stock rotations based on both time and temperature. Such devices would be especially useful when combined with other shelf-life technologies such as MAP or sub-sterilization radiation. The next generation of safety/quality indicators may be more specific than integrating time and temperature. In the future it may be possible to directly detect the presence of specific toxins in packaged foods using biosensors. Immunologically based sensors coupled to packaging could find applications in food safety, food processing, and detection of adulteration (Deshpande, 1994). Such sensors could, for example, detect the presence of bacterial toxins in packaged foods. They could also be used to determine if a food had been properly pasteurized or contained enzyme activity. Biosensors which combine electronics with biological specificity and sensitivity may find use in packaging as monitors of safety and quality (Deshpande and Rocco, 1994). In time, it may be possible to incorporate these or similar biosensors
into food packaging systems for which the risk of toxin formation exists. Reportedly, methods to quantify the presence of microorganisms on fresh meats are near commercialization (Bsat et #/., 1994). Such systems could eventually be incorporated directly into food packaging. It may likewise be possible to detect the presence of toxic chemicals using similar technologies. The presence of specific pesticides or other environmental contaminants could be detected with immunological-based systems (Deshpande, 1994). Lastly, packaging should provide a margin of safety against tampering. Tamper-indicating packaging has been discussed in detail since several malicious incidents of tampering with drugs and foods have occurred (Hotchkiss, 1983). Several simple and complex tamper-evident packaging systems have been developed and a few implemented for foods. 11.4.6 Direct inhibition of microbial growth Microbial contamination and growth are the major factors in food spoilage and responsible for food-borne disease outbreaks. Two general approaches, heat sterilization and direct addition of antimicrobial additives, have been used to eliminate or minimize microbial growth. In conventional thermal processing, foods are sealed in a package and the combined productpackage thermally processed. This is the basis of the canning industry. More recently, the process of the package and the product being sterilized separately then filled and sealed aseptically has been used. This is known as aseptic packaging. Foods can also be dried to reduce microbial growth. Another method to reduce microbial growth is to add antimicrobial additives directly to foods. This approach usually does not inhibit all growth but is selective for certain types of microbial growth, molds for example. The use of these additives is regulated and their use, in most cases, must be stated on the label. 11.4.7 Modified atmosphere packaging Recently, the development of alternative methods of inhibiting microbial growth has resulted from a consumer desire for fresher and more natural foods. The most successful alternative to canning or the direct addition of antimicrobial agents has been modified atmosphere packaging (MAP). The number and type of microorganisms present on a food is governed by five general variables: time, temperature, substrate (food) composition, microbial load (type and number), and gas atmosphere. For a given food product which must be held above freezing, alteration of the gas atmosphere surrounding the product is the most accessible method of inhibiting microbial growth. However, inhibition is not uniform for all types of microbes. In general (although there are exceptions) Gram-negative rods are
inhibited by a modified atmosphere containing more than 10% CO2 while Gram-positive organisms are not inhibited and their growth can be promoted. The major goal of MAP is to reduce the growth rate of microorganisms which cause the product to become organoleptically unacceptable. However, organisms which cause disease (i.e., pathogens) do not, in many cases, cause organoleptic changes in foods. Increasing the shelf-life by suppressing spoilage organisms might allow for the growth of pathogenic organisms without development of the normal organoleptic cues of spoilage that warn consumers that a product may not be wholesome (Farber et ai, 1990). Thus, the major safety concern with MAP and controlled atmosphere packaging or other technologies which selectively change the microbiology of a food is that suppression of organoleptic spoilage (i.e., extension of shelf-life) will decrease competitive growth pressure and provide sufficient time for slow growing pathogenic organisms to become toxic or reach infectious numbers (Hotchkiss and Banco, 1992). The knowledge gained over the last decade about pathogenic microorganisms which are capable of surviving and growing at common refrigeration temperatures increases concern about the safety of refrigerated extended shelf-life foods (Gormley and Zeuthen, 1990; Farber, 1991). The effect that this change in microbiology might have on the risk of food-borne disease has been debated (Gormley and Zeuthen, 1990). There are several methods of creating a modified atmosphere inside a package. One is the use of selective or engineered barriers which are used for respiring products such as fruits and vegetables. The combination of product respiration rate (i.e., rate OfCO2 formation and O2 consumption) and CO2 egress and O2 ingress results in the formation of an equilibrium concentration of gases which, if properly designed, will reduce senescence and extend shelf-life. Alternatively, a specific gas mixture can be directly introduced into the package after removal of the air and before sealing. A third method is to use an additional material contained in a sachet or incorporated into the film which will alter the gas composition after sealing. In each case, the change in atmosphere will affect both the growth rate and type of microorganisms present. However, temperature will affect the respiration rate to a much greater extent than the permeability. If the product is stored at an elevated temperature, respiration rates will increase and the O2 content of the package may approach zero. At the same time the growth rate of pathogenic microorganisms substantially increases with the increase in temperature. This could allow for the growth of anaerobic pathogens such as Clostridium botulinum. For example, Lambert et al (1991) have shown that toxigenesis occurs more rapidly in aerobically packaged pork samples compared to anaerobically packaged samples when Pseudomonas spp. were present along with the Clostridium botulinum inoculum. It was presumed that the Pseudomonas
rapidly consumed the oxygen allowing the C. botulinum to become toxic. These results agreed with earlier results of Hintlian and Hotchkiss (1987) who made a similar observation. 11.4.8 Antimicrobial films Packaging may directly affect the microbiology of foods in ways other than changing atmosphere. In solid or semi-solid foods, microbial growth occurs primarily at the surface. Surface treatment by spraying or dusting with antimicrobial agents for products such as cheeses, fruits, and vegetables is widely practised. Antimycotic agents are commonly incorporated into waxes and other edible coatings used for produce items (Peleg, 1985). More recently, the idea of incorporating antimicrobial agents directly into packaging films which would come into contact with the surface of the food has been developed. Antimicrobial films can be divided into two types: those containing an antimicrobial agent which migrates to the surface of the food and those that are effective against surface growth without migration of the active agent(s) to the food. Several commercial antimicrobial films have been introduced, primarily in Japan. One widely discussed product is a synthetic zeolite which has had a portion of its sodium ions replaced with silver ions. Silver can be antimicrobial under certain situations. This zeolite is incorporated directly into a food-contact film. The purpose of the zeolite apparently is allow for the slow release of silver ions to the food. Only a few scientific descriptions of the effectiveness of this material have appeared and the regulatory status of the deliberate addition of silver to foods has not been clarified in the US or in Europe. Several other synthetic and naturally occurring compounds have been proposed and/or tested for antimicrobial activity in packaging (Table 11.2). For example, the antimycotic (i.e., antifungal) agent, imazalil, is effective when incorporated into LDPE for wrapping fruits and vegetables (Miller et Table 11.2 Some antimicrobial agents of potential use in food packaging Class
Examples
Organic acids Bacteriocins Spice extracts Thiosulfinates Enzymes Proteins Isothiocyanates Antibiotics Fungicides Chelating agents Metals Parabens
Propionic, benzoic, sorbic Nisin Thymol, p-cymene Allicin Peroxidase, lysozyme Conalbumin Allylisothiocyanate Imazalil Benomyl EDTA Silver Heptylparaben
al, 1984; Hale et al.> 1986). We have demonstrated that the same compound is effective at preventing mold growth on cheese surfaces when incorporated into LDPE films (Weng and Hotchkiss, 1992). Although imazalil is not approved for cheese, this work established that antimycotic films could be effective for control of surface molds in foods. Halek and Garg (1989) chemically coupled the antifungal agent benomyl, which is commonly used as a fungicide, to ionomer film and demonstrated inhibition of microbial growth in defined media. While not directly addressed by the authors, the method used to determine inhibition of growth indicated that the benomyl migrated from the film to the growth media. It is unlikely that benomyl would be approved for food use for toxicological reasons. Reports have appeared which demonstrate the effectiveness of adding common food-grade antimycotic agents to cellulose-based edible films (Vojdani and Torres, 1990). Films were constructed of cellose derivatives and fatty acids in order to control the release of sorbic acid and potassium sorbate. These films would seem to have the greatest application as fruit and vegetable coatings. Cellulose films are not heat sealable are not good barriers in high humidity situations. We have spectroscopically demonstrated that propionic acid, which is a common approved food antimycotic agent, could be coupled to ionomeric films but that antimycotic activity could not be demonstrated on rigorous testing (Weng, 1992). Direct addition of simple antimycotic acids such as propionic, benzoic, and sorbic acids to polymers such as LDPE was unsuccessful because of lack of compatibility between the acid and the nonpolar film. This incompatibility is likely to be due to differences in polarity. We have solved this problem by first forming the anhydride of the acid which removes the ionized acid function and decreases polarity (Weng and Hotchkiss, 1993). Anhydrides are stable when dry and relatively thermally stable yet become hydrolysed in aqueous environments such as foods. Hydrolysis leads to formation of the free acid which in turn leads to migration from the surface of the polymer to the food where the free acids can be effective antimycotics. This is an example of 'switched on' packaging; the active ingredient remains in the film until the film comes into contact with a food. The activity is initiated by the moisture in the food. Future work in antimicrobial films may focus on the use of biologically derived antimicrobial materials that are bound or incorporated into films and do not need to migrate to the food to be effective. For example, a group of substances known as bacteriocins, which are proteins derived from microorganisms in much the same way as penicillin is derived from mold, have been described in the literature (Hoover and Steenson, 1993). Bacteriocins are effective against organisms such as Clostridium botulinum and one such compound, nisin, has been approved for food use. These peptides could, theoretically, be attached to the surface of food-contact films. Whether or not such bound bacteriocins would be effective remains unclear.
Antimicrobial enzymes might also be bound to the inner surface of foodcontact films. These enzymes would produce microbial toxins. Several such enzymes exist, such as glucose oxidase which forms hydrogen peroxide. A third possibility for antmicrobial films is to incorporate radiationemitting materials into films. Reportedly, the Japanese have developed a material which emits long-wavelengt IR. This is thought to be effective against microorganisms without the risks associated with higher energy radiation. However, little direct evidence for the efficacy of this technology has been published in the scientific literature. In general, several questions, including those dealing with safety, should be considered in developing antmicrobial films: •
•
• • •
What is the spectrum of organisms against which the film will be effective? Films which may inhibit spoilage without affecting the growth of pathogens will raise safety questions similar to those of technologies such as a MAP. What is the effect of the antimicrobial additives on the mechanical and physical properties of the film? It is likely, for example, that effective levels of antimicrobial agents will reduce seal strength. This may adversely affect safety. Is the antimicrobial activity a reduction in growth rate (yet still a positive increase in cell numbers) or does it cause cell death (decline in cell numbers)? To what extent does the antimicrobial agent migrate to the food and what, if any, are the toxicological and regulatory concerns? What is the effect of food product composition? Some antimicrobial agents, for example, are effective only at acid pH while others might require certain product compositions (e.g. aw9 protein, glucose, etc.) to be effective.
Each of these questions need addressing before the safety consequences of antimicrobial packaging can be understood. 11.4.9 Rational functional barriers As pointed out, one major safety function of packaging is to act as a chemical and biological barrier. Films are frequently selected for food use based on the highest degree of oxygen and/or water vapor barrier at the lowest cost. More recently, the concept of rational design or engineering of film permeability has evolved. These so-called 'smart' films have barrier properties which are designed to adapt or change permeabilities according to conditions such as a change in gas composition or temperature. These engineered barriers have at least two important safety-related applications. The first is to act as a barrier to permeation of contaminants. Packaged foods can be exposed to contaminants from environmental sources
or from the use of recycled plastics in food packaging. Common environmental sources of toxic permeants include chemicals used to treat shipping containers and pollutants. Chlorinated wood preservatives readily permeate through common films and cause taint in foods (Whitfield et al, 1991). Common environmental pollutants such as diesel exhaust and industrial solvents used in printing also permeate many common food-packaging films. The second use of engineered barriers is in passive MAP systems in which an equilibrium in the gas mixture is achieved through the combination of product respiration and package permeability. This equilibrium results from the consumption of O2 and evolution of CO2 by the food product at the same time that O2 is permeating into the package and CO2 is permeating out at a given temperature. At some point these respiration and permeation rates will reach an equilibrium concentration. Selection of a film with the proper permeability will result in the desired gas mixture. Several mathematical models have been developed which predict the proper permeation rates given by a specific product respiration rate (Mannapperuma et ai9 1991). There are two difficulties with this concept. The first is that respiration rates of most produce items vary widely, even within the same type of item. Thus, permeation rates will have to be tailored for each individual product item. The second problem is that CO2 permeation rates for common packaging films is 2-A fold or more higher than for O2. This means that CO2 may egress at a faster rate than O2 will enter, making the atmosphere anaerobic. Engineered films which can independently select CO2 and O2 permeation rates as well as films that change permeation at the same rate that fruits and vegetables change respiration rate with temperature, would be desirable. As pointed out above, the rate of gas change will determine the type of microorganisms on the products and, probably, the safety of such foods. We have recently devised equations for achieving optimum atmosphere concentrations for extending the shelf-life of fresh corn on the cob and head lettuce, each of which illustrates some of the problems with engineered barriers. Head lettuce respires relatively slowly and films which will allow a passive modified atmosphere to be established are commercially available. However, about 90 hours are required for establishment of a suitable atmosphere (Morales-Castro et al, 1994a). During this time considerable deterioration can occur. Sweet corn, on the other hand, respires rapidly and the establishment of a desirable steady-state atmosphere is not possible with normal films because the permeabilities are too low even for very low barrier films (Morales-Castro et aL, 1994b). MAP products such as lettuce and corn or other vegetables could become safety concerns if the atmosphere were to become anaerobic. This might occur if products were stored at a higher than expected temperature. This would cause an increase in respiration beyond that expected and the oxygen
might be substantially depleted. This would leave open the possibility of growth of anaerobic pathogens such as Clostridium botulinum. 11.4.10 Combined systems The most successful active packaging materials are likely to combine different technologies. A few examples of such combinations have appeared in the literature. For example, Fu and Labuza (1992) have suggested that MAP might be combined with time-temperature indicators as a means of extending the shelf-life of perishable foods while at the same time minimizing food-borne disease risks. MAP would reduce the deterioration of food while the time-temperature indicator would insure that the product was stored and handled within the time and temperature window for which the product was designed, to insure safety. Labuza et al (1992) have suggested that predictive microbiology should be used to evaluate the safety of MAP foods. Low dose irradiation and MAP have been combined to extend shelf-life (Thayer, 1993). Irradiation reduces the numbers of spoilage and pathogenic vegetative organisms while a modified atmosphere reduces the likelihood that those not destroyed will grow significantly in number. Lambert et al (1992) have demonstrated a substantial increase in the shelf-life of fresh pork treated with both irradiation and MAP. Other combinations such as antimicrobial films combined with MAP or oxygen absorbers combined with antimicrobial films may find commercial uses. Zeitoun and Debevere (1991) have suggested that combining a simple lactic acid dip combined with MAP would enhance the shelf-life of fresh poultry. 11.5 Conclusions It can be expected that safety will continue to be an important attribute for foods. New packaging technologies which improve quality, usefulness, or reduce environmental impact will also be required to maintain a high level of safety. Active packaging systems will not be an exception. Those active packaging systems which reduce the risks associated with foods may find niche markets for products at the highest risk of deterioration. MAP of non-sterile foods is one example where additional safety measures such as use of microbial inhibitors or indicators of temperature abuse would be useful. Recycled materials for food packaging is another. Active packaging systems which provide benefits for foods will have to adhere to governmental regulatory standards in most of the world. This will inhibit the introduction of some active systems. Antimicrobial films are a
prime example. Developers of such materials should understand the safety and regulatory implications of their work early in the process if they expect to be successful.
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Ishitani, T. (1994) Active Packaging for Foods in Japan. International Symposium, Interaction: Foods - Food Packaging Material, Programme, Information, Participants, Abstracts, sponsored by The Lund Institute of Technology, Lund University and SIK, The Swedish Institute for Food Research, Gothenburg. Klungness, J.H., Lin, CH. and Rowlands, R.E. (1990) Contaminant removal from recycled wastepaper pulps. Pulping Conference Proceedings, 1, 8-12. Labuza, T.P. and Breene, W. (1989) Application of active packaging technologies for the improvement of shelf-life and nutritional quality of fresh and extended shelf-life foods. Journal of Food Processing Preservation, 13(1), 1-69. Labuza, T.P., Fu, B. and Taoukis, P.S. (1992) Prediction for shelf-life and safety of minimally processed CAP/MAP chilled foods: A review. Journal of Food Protection, 55(9), 741-50. Lambert, A.D., Smith, J.P. and Dodds, K.L. (1991) Effect of initial O2 and CO2 and low-dose irradiation on toxin production by Clostridium botulinum in MAP fresh pork. Journal of Food Protection, 54(12), 939-44. Lambert, A.D., Smith, J.P., Dodds, K.L. et al. (1992) Microbiological changes and shelf-life of MAP, irradiated fresh pork. Food Microbiology, 9(3), 231-44. Mannapperuma, J.D., Singh, R.P. and Montero, M.E. (1991) Simultaneous gas diffusion and chemical reaction in foods stored in modified atmosphere. Journal of Food Engineering, 14(3), 167-83. Miller, W.R., Spalding, D.H., Risse, L.A. et al (1984) The effects of an imazalil-impregnated film with chlorine and imazalil to control decay of bell peppers. Proc. Florida State Horticultural Society, 97, 108-11. Morales-Castro, J., Rao, M.A., Hotchkiss, J.H. et al. (1994a) Modified atmosphere packaging of sweet corn on cob. Journal of Food Processing and Preservation, 18, 279-93. Morales-Castro, J., Rao, M.A., Hotchkiss, J.H. et al. (1994b) Modified atmosphere packaging of head lettuce. Journal of Food Processing and Preservation, 18, 295-304. Peleg, K. (1985) Produce Handling, Packaging and Distribution, AVI Publishing Company, Inc., Westport, CT. Reddy, N.R., Armstrong, D.J., Rhodehamel, EJ. et al (1992) Shelf-life extension and safety concerns about fresh fishery products packaged under modified atmospheres: A review. Journal of Food Safety, 12(2), 87-118. Rooney, M.L. (1994) Oxygen-Scavenging Plastics Activated for Fresh and Processed Foods. IFT Annual Meeting Technical Program: Book of Abstracts, Abs. No. 21-5, p. 52. Taoukis, P.S., Fu, B. and Labuza, T.P. (1991) Time-temperature indicators. Food Technology, 45(10), 70-82. Thayer, D.W. (1993) Extending shelf-life of poultry and red meat by irradiation processing. Journal of Food Protection, 56(10), 831-3, 846. Vaisanen, O.M., Nurmiaho-Lassila, EX., Marmo, S.A. et al. (1994) Structure and composition of biological slimes on paper and board machines. Applied and Environmental Microbiology, 60(2), 641-53. Vojdani, F. and Torres, J.A. (1990) Potassium sorbate permeability of methylcellulose and hydroxypropyl methylcellulose coatings: Effect of fatty acids. Journal of Food Science, 55(3), 841-6. Weng, Y.-M. (1992) Development and Application of Food Packaging Films Containing Antimicrobial Agents. PhD dissertation, Cornell University, Ithaca, NY. Weng, Y.-H. and Hotchkiss, J.H. (1992) Inhibition of surface molds on cheese by polyethylene film containing the antimycotic Imazalil. Journal of Food Protection, 9, 29-37. Weng, Y.-H. and Hotchkiss, J.H. (1993) Anhydrides as antimycotic agents added to polyethylene films for food packaging. Packaging Technology and Science, 6, 123-8. Whitfield, F.B., Ly-Nguyen T.H. and Last, J.H. (1991) Effect of relative humidity and chlorophenol content on the fungal conversion of chlorophenols to chloroanisoles in fibreboard cartons containing dried fruits. Journal of the Science of Food and Agriculture, 54(4), 595-604. Whitfield, F.B., Shaw, K.J., Lambert, D.E. et al (1994) Freight containers: Major sources of chloroanisoles and chlorophenols in foodstuffs. Developments in Food Science, 35, 401-7.
Wolf, I.D. (1992) Critical issues in food safety, 1991-2000. Food Technology, 46(1), 64-70. Zeitoun, A.A.M. and Debevere, J.M. (1991) Inhibition, survival and growth of Listeria monocytogenes on poultry as influenced by buffered lactic acid treatment and modified atmosphere packaging. International Journal of Food Microbiology, 14(2), 161-9.
Index Index terms
Links
A absorbers food constituents
99
100
100
213
free oxygen see oxygen absorbents odours and taints activation energy permeation
68
respiration model
65
active packaging chemical
21
composite
8
definition
1
do-it-yourself
17
economic benefit
76
future potential
32
history
4
horticultural
9
limitations
31
literature
10
multiple effects
9
origins
3
physical
20
reasons for
24 2
143
203
252
3
regulatory considerations
33
reviews
10
252
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256
257
Index terms
Links
active packaging (Continued) scope
12
terminology whole packages
1
10
29
active packaging plastics and safety
243
combined effect
82
commercial use
106
effects on foods
74
environmental considerations
107
migration from
243
regulations
106
106
Ageless and aw
150
capacity
150
chemistry
149
types
149
aldehydes
100
almonds rancidity and oxygen absorbent amines
155 100
anaerobic respiration antimicrobials
68 243
enzymic
250
silver zeolite
248
antimycotics
244
248
249
ascorbic acid and oxygen absorbent Aspergillus spp
154 159
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258
Index terms
Links
B bacteriocins
249
bakery products and oxygen absorbent
156
ethanol releasing sachets
166
169
mould growth
159
160
barriers functional
250
beer commercial oxygen scavenger closures
198
oxygen concentration in
194
oxygen ingress
195
blueberry
65
broccoli
65
196
69
C calcium carbonate as filler
69
cans tinplate
238
carbon dioxide permeability
123
carnauba wax
132
carrots packaging condensation control cauliflower
98 65
casein films sorbic acid in
128
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259
Index terms
Links
cheese antimycotic films for
249
Clostridium botulinum
244
Clostridium sporogenes effect of Ageless
163
closure beer bottles
193
benefits
199
beverage bottles
193
oxygen scavenging with ascorbic acid
198
with sodium sulfite
168
composite films sorbic acid barrier consumer resistance to sachets
130 162
contamination barriers to controlled atmosphere (CA)
240
244
55
corn gas atmospheres for
251
crusty rolls and oxygen absorbents
159
D deoxidisers see oxygen scavengers desiccant retorting
75
79
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260
Index terms
Links
E edible coatings see edible films edible films as active packaging
113
as food
112
bilayer
117
composite
111
food surface modification
126
formation
112
gas exchange with
121
moisture barrier
114
multilayer
117
permeability
114
plasticisation
120
energy transfer
90
environment
117
107
enzymes active packaging
176
antimicrobial
186
binding
105
function
174
history
178
oxygen scavenging
174
release
105
equilibrium modified atmosphere (EMA)
247
177
251
see also passive modified atmosphere packaging Clostridium botulinum in
247
generation
56
packaging
2
Pseudomonas spp in
9
55
247
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66
261
Index terms ethanol oxidase
Links 186
ethanol vapour absorption
167
antimicrobial effects
165
and aw
165
in bakery products
165
and food spoilage
171
and food poisoning
171
171
generators advantages
171
aw effects
166
disadvantages
171
required sizes
166
types
166
uses
168
ethylene adsorption
40
chemistry
38
degradation
39
effects
41
interaction with other gases
44
sources
45
46
synthesis
38
39
ethylene scavengers activated carbon
47
activated earth
48
novel approaches
50
potassium permanganate
46
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262
Index terms
Links
F films antimicrobial
248
ceramic filled
70
condensation control
98
controlled diameter holes to humidity buffering
96
for MAP
67
microporous
68
moisture control
94
perforated
68
temperature-compensating
70
with oxygen absorbents
92
153
fish products and oxygen absorbents flavour scalping
157 99
foods oxygen absorbents for
155
Freshilizer and aw
151
reaction speed
151
types
151
FreshMax
212
Fresh Pax and aw
151
reaction speed
152
types
152
uses
211
fruits edible coatings gas atmosphere for
126 57
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263
Index terms functional barriers
Links 250
G gelatin films sorbic acid retention
128
tannic acid crosslinked
128
tocopherol retention
128
glucose oxidase
181
gluten films with beeswax green pepper
124 65
H hydrogen oxidation
84
hydrogen peroxide
86
hydroxypropyl methylcellulose
130
I IMF and Staphylococcus aureus
134
casein-coated
132
microbiological stability
132
papaya cubes
132
with sorbic acid
132
indicator bacterial toxins oxygen concentration
245 2
safety
245
spoilage
245
temperature
209
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264
Index terms
Links
indicator (Continued) time-temperature see Time-Temperature Indicators interactive packaging see active packaging intermediate moisture food see IMF
L lettuce gas atmosphere for limonin
251 99
Listeria monocytogenes
163
ethanol vapour effect
171
M mathematical modelling
251
limitations
64
parameters
71
steady state
66
unsteady state
65
variables
66
70
meat colour and oxygen absorbents
160
meat packaging oxygen scavenging methylcellulose with palmitic acid
158 130 132
microbiological stability carrageenan effect
134
lactic acid effect
133
surface pH effect
133
microencapsulation
85
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265
Index terms microwave susceptors
Links 203
field intensifiers
206
reflectors
206
microwaveable bottle
209
migration from active packaging
243
from heated plastics
242
from recycled plastics
242
of preservatives
127
prevention of
241
modified atmosphere packaging (MAP) active
55
and Clostridium botulinum
245
definition
143
and ethanol vapour
170
experimentation for
60
film selection
66
gas concentration boundaries
58
gas tolerance limits
61
feasibility study
59
films for
67
flow chart
56
and irradiation
57
optimisation
60
passive
55
pathogens in
246
and Pseudomonas
247
regulation of
67
252
literature review
quality criteria
247
59 252
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266
Index terms
Links
modified atmosphere packaging (MAP) (Continued) and Time-Temperature Indicators
252
moisture transfer ice cream-wafer
121
mould growth on bakery products
75
on cheese
75
mozarella cheese and oxygen absorbents mushrooms
160 69
O odours from oxygen scavengers organic acids Ox-Bar
82
91
243 82
oxygen concentration measurement
193
permeability of closure liners
195
194
oxygen absorbents see also oxygen scavengers advantages
161
and aflatoxins
164
and mould growth
159
applications
148
choice of
152
classification
145
CO2 producing
146
definition
144
disadvantages
161
dual effect
146
150
151
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267
Index terms
Links
oxygen absorbents (Continued) function history
148 4
in Japan
144
153
in USA
155
159
market statistics
144
reaction speed
146
reactions in
145
requirements for
145
research with
159
oxygen absorbers see oxygen absorbents; oxygen scavengers oxygen permeation aw effect chemical barrier edible films oxygen scavengers
124 92
185
123 1
see also oxygen absorbents Advanced Oxygen Technologies, Inc.
198
Ageless
211
Aquanautics Corp.
198
ascorbic acid
85
ascorbic acid plus sodium sulfite
86
beer bottle closures future
200
beer flavour and
199
bottle closures
213
chemistry of
83
closure liner theory
197
composite systems
8
erythorbic acid
198
198
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268
Index terms
Links
oxygen scavengers (Continued) fatty acids
82
food applications
212
FreshMax
212
Fresh Pax
211
history
91
4
hydrogen/catalyst
84
iron see oxygen absorbents Mitsubishi Gas and Chemical Co
211
Multiform Desiccants Inc.
211
novel designs
7
Ox-Bar
83
OxyBan
181
patent applications
5
plastics vs. sachets
81
PureSeal release of carbon dioxide rubbers and odours SmartCap sulfite oxidation
199 89
90
82
91
199 85
86
198
oxygen scavenging activation see triggering and aflatoxin production
164
and microbial growth
163
by autoxidation enzymic
91 179
peroxide formation
86
purposes
75
oxygen scavenging packaging forms of
76
81
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269
Index terms oxygen scavenging plastics
Links 77
forms of
76
history
80
light-energised
80
light triggered
91
metal-catalysed photosensitised
91
MXD-6 nylon
83
permeability effects
99
photoreduction-reoxidation
92
photosensitised
87
potential applications
75
singlet oxygen mechanism
87
81
oxygen scavenging sachets see oxygen absorbents Oya Stone
209
broccoli packaging and
210
Evert-Fresh Bag
209
P package integrity
240
packaging active constituent impact on antimicrobial
27 243
244
246
intelligent
2
215
216
interactive
2
modelling
64
modified atmosphere
2
safety problems
239
tamper evident
246
with oxygen scavengers
92
246
153
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270
Index terms
Links
patents oxygen scavenging
5
pectin films
128
Penicillium spp
159
81
perforated films computer simulation
69
permeability activation energy
68
composite films
118
edible films
114
effect on oxygen scavenging
78
humidity effect
79
modification ratio
89
105 67
71
125 116 169
smoke
105
table
68 123
78 124
temperature effect
120
130
to ethanol
169
to water vapour
115
peroxide value and oxygen absorbent
154
photosensitisation
87
Pichit
97
pizzas and oxygen absorbents
160
plastics definition
74
for oxygen scavenging
77
functional barrier
242
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117
271
Index terms
Links
plastics (Continued) iron in
78
recycled
242
poly(1,2-butadiene)
77
poly(1,3-butadiene)
89
polydimethylbutadiene
89
80
91
46
55
209
252
polyethylene antimycotics in polyisoprene potassium sorbate
248 89 127
processed foods and oxygen absorbents produce packaging
158 9
propionic acid
249
propionic anhydride
249
Q Q10
62
R radiation far infra-red regulation
70
250
106
242
243
179
181
release antimicrobial agents
102
antioxidants
103
butylated hydroxytoluene
104
enzymes
105
ethanol vapour
165
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272
Index terms
Links
release (Continued) flavours
103
food ingredients
102
fumigants
104
hinokitiol
102
Maillard reaction products
104
permethrin
104
silver ions
248
sorbic acid
126
removal aldehydes
100
amines
100
cholesterol
190
lactose
189
styrene
101
respiration rate circulation
62
closed system
63
flow-through
61
measurement
62
temperature effect
61
respiratory quotient
61
retortable packaging
3
63
79
S Saccharomyces cerevisiae ethanol vapour effect
170
sachets combined effect
146
consumer resistance
162
150
151
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152
273
Index terms
Links
sachets (Continued) ethanol releasing ethylene-removing
165 46
oxygen scavenging see oxygen absorbents and public health
163
safety with
244
safety active interactions
242
and packaging
238
indirect effects on
244
passive interactions
240
scalping
99
sheets drip-absorbent
95
shelf life with oxygen absorbents
153
silica as filler
69
silicone film
69
smart films
1
sodium chloride sorbic acid
98 126
diffusivity
129
permeability
129
Staphylococcus aureus
250
163
strawberries
69
sulfites
79
superabsorbent polymers
96
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274
Index terms
Links
T taints
99
251
see also odours Time-Temperature Indicator (TTI) activation
228
bar code
222
classification of
233
consumer attitudes
229
definitions
215
enzymic
188
operating principles
217
reasons for
216
requirements of
217
specifications
229
thermal process validation with
230
validation tests
224
toxicology
245
243
triggering chain reaction of propionic acid
91 249
oxygen scavenging plastics
83
photoreduction
92
V vegetables gas atmospheres for
59
vitamin C see ascorbic acid
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275
Index terms
Links
W water activity (aw) and ethanol generators
68
Y yeast growth ethanol effect
4
oxygen absorber
4
170
Z zein films lactic acid retention
128 134
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