Applications of Spray-drying in Microencapsulation of Food Ingredients- An Overview

Food Research International International 40 (2007) 1107–1121 www.elsevier.com/locate/foodres

Review

Applications of spray-drying in microencapsulation of food ingredients: An overview Adem Gharsallaoui *, Gae¨ lle Roudaut, Odile Chambin, Andre´ e Voilley, Re´ mi Saurel ´ cules Actives, Macromole´ cules, ´ cules, Activite´ (EMMA), ´ (EMMA), ENSBANA, Universite´ de ´ de Bourgogne, 1 Esplanade Erasme, 21000 Dijon, France Eau, Mole´ cules

Received 2 March 2007; accepted 21 July 2007

Abstract

Spray-drying process has been used for decades to encapsulate food ingredients such as flavors, lipids, and carotenoids. During this drying process, the evaporation of solvent, that is most often water, is rapid and the entrapment of the interest compound occurs quasiinstantaneously. This required property imposes a strict screening of the encapsulating materials to be used in addition to an optimization of the operating operating condition conditions. s. Likewise, Likewise, if the encapsulated encapsulated compound compound is of hydrophob hydrophobic ic nature, nature, the stability stability of the feed emulsion emulsion before drying should also be considered. Thus, spray-drying microencapsulation process must rather be considered as an art than a science because of the many factors to optimize and the complexity of the heat and mass transfer phenomena that take place during the microcapsule formation. This paper reports the main process engineering information that are considered useful to the success of a microencapsulation operation by spray-drying. Besides, a summary of the most commonly used wall materials and the main encapsulated food compounds are presented.  2007

Elsevier Ltd. All rights reserved.

Keywords: Spray-drying; Microencapsulation; Core; Wall material; Food applications

Contents

1. 2.

3.

4.

*

Intr Introd oduc ucti tion on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1108 1108 Spray Spray-d -dry ryin ing: g: summ summar ary y of some some tech techni nica call cons consid ider erat atio ions. ns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 11099 2.1. 2.1. Atom Atomiz izat atio ion n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1109 1109 2.2. 2.2. Drop Drople lett – hot hot air air cont contac actt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1109 1109 2.3. 2.3. Evap Evapor orat atio ion n of dro dropl plet et wat water er . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 11099 2.4. 2.4. Dry Dry produ product ct-h -hum umid id air air sep separ arat atio ion n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 11100 Spray Spray-d -dry ryin ingg as a proc proces esss for mic micro roen enca caps psul ulat atio ion. n. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 11100 3.1. 3.1. Micr Microe oenc ncap apsu sula lati tion on pro proce cess ss step steps. s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 11111 3.2. 3.2. Oper Operat atin ingg cond condit itio ions ns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1111 1111 Micr Microe oenc ncap apsu sula lati tion on by by spray spray-d -dry ryin ing: g: whi which ch wall wall mus mustt be used used?? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 11122 4.1. 4.1. Wa Wall ll mat mater eria iall requ requir ired ed pro prope pert rtie ies. s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 11122 4.2. 4.2. Wa Wall ll mat mater eria iall sele select ctio ion n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1113 1113 4.3. 4.3. Most Most com commo monl nlyy used used wal walll mate materi rial als. s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 11133 4.3. 4.3.1. 1. Carb Carboh ohyd ydra rate tes. s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1114 1114 4.3. 4.3.2. 2. Gums. Gums. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115 1115 4.3. 4.3.3. 3. Prot Protei eins ns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115 1115

Corresponding author. E-mail address: [email protected] [email protected] (A. Gharsallaoui).

0963-9969/$ - see front matter 0963-9969/$ matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodres.2007.07.004

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5.

6.

A. Gharsallaoui et al. / Food Research International 40 (2007) 1107–1121

Some examples of food ingredients microencapsulated by spray-drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116 5.1. Flavors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1116 5.2. Lipids and oleoresins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1117 5.3. Other food ingredients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1117 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1118 Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1118 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1118

1. Introduction

Thanks to microencapsulated ingredients, many products that were considered technically unfeasible are now possible. Such ingredients are totally enveloped in a coating material, thereby conferring useful or eliminating useless properties to or from the original ingredient. Microencapsulation is defined as a process in which tiny particles or droplets are surrounded by a coating, or embedded in a homogeneous or heterogeneous matrix, to give small capsules with many useful properties. Microencapsulation can provide a physical barrier between the core compound and the other components of the product. More especially, in the food field, microencapsulation is a technique by which liquid droplets, solid particles or gas compounds are entrapped into thin films of a food grade microencapsulating agent. The core may be composed of just one or several ingredients and the wall may be single or double-layered. The retention of these cores is governed by their chemical functionality, solubility, polarity and volatility. Shahidi and Han (1993) proposed six reasons for applying microencapsulation in food industry: to reduce the core reactivity with environmental factors; to decrease the transfer rate of the core material to the outside environment; to promote easier handling; to control the release of the core material; to mask the core taste; and finally to dilute the core material when it should be used in only very small amounts. In its simplest form, a microcapsule is a small sphere with a uniform wall around it. The material inside the microcapsule is referred to as the core, internal phase, or fill , whereas the wall is sometimes called shell , coating , wall material , or membrane. Practically, the core may be a crystalline material, a jagged adsorbent particle, an emulsion, a suspension of solids, or a suspension of smaller microcapsules. The microcapsule may even have multiple walls. In this review, only ‘‘core’’ and ‘‘wall ’’ will be used to refer to the encapsulated ingredient and encapsulating agent, respectively. Most microcapsules are small spheres with diameters comprised between a few micrometers and a few millimeters. However many of these microcapsules bear little resemblance to these simple spheres. In fact, both the size and shape of formed microparticles depend on the materials and methods used to prepare them. The different types of microcapsules and microspheres are produced from a wide range of wall materials (monomers and/or polymers)

and by a large number of different microencapsulation processes such as: spray-drying, spray-cooling, spray-chilling, air suspension coating, extrusion, centrifugal extrusion, freeze-drying, coacervation, rotational suspension separation, co-crystallization, liposome entrapment, interfacial polymerization, molecular inclusion, etc. (Desai & Park, 2005; Gibbs, Kermasha, Alli, & Mulligan, 1999; Gouin, 2004; King, 1995; Shahidi & Han, 1993). Depending on the physico-chemical properties of the core, the wall composition, and the used microencapsulation technique, different types of particles can be obtained (Fig. 1): simple sphere surrounded by a coating of uniform thickness; particle containing an irregular shape core; several core particles embedded in a continuous matrix of wall material; several distinct cores within the same capsule and multiwalled microcapsules. Although most often considered as a dehydration process, spray-drying can be used to encapsulate active material within a protective matrix formed from a polymer or melt (Dziezak, 1988). Although many techniques have been developed to microencapsulate food ingredients, spray-drying is the most common technology used in food industry due to low cost and available equipment. Microencapsulation by spray-drying has been successfully used in the food

Fig. 1. Morphology of different types of microcapsules (Gibbs et al., 1999).


industry for several decades (Gouin, 2004), and this process is one of the oldest encapsulation methods used since the 1930s to prepare the first encapsulated flavors using gum acacia as wall material (Shahidi & Han, 1993). The objective of this paper is to review the state of the art of microencapsulation of food ingredients by spray-drying and present necessary theoretical and practical information on this process. Thus, the present paper discusses the uses of spray-drying for microencapsulation ends from four perspectives. First, it focuses on some theoretical aspects of the spray-drying process. Next, the paper discusses the application of spray-drying in microencapsulation of food ingredients. The third section presents criteria required for encapsulating agents and describes several wall materials that have proved good encapsulation efficiency. The final part summarizes important recent applications concerning the microencapsulation of food ingredients by spray-drying. 2. Spray-drying: summary of some technical considerations

Spray-drying is a unit operation by which a liquid product is atomized in a hot gas current to instantaneously obtain a powder. The gas generally used is air or more rarely an inert gas as nitrogen. The initial liquid feeding the sprayer can be a solution, an emulsion or a suspension. Spray-drying produces, depending on the starting feed material and operating conditions, a very fine powder (10–50 l m) or large size particles (2–3 mm). Water removal by spray-drying solutions is a common engineering practice. By decreasing water content and water activity, spray-drying is generally used in food industry to ensure a microbiological stability of products, avoid the risk of chemical and/or biological degradations, reduce the storage and transport costs, and finally obtain a product with specific properties like instantaneous solubility for example. The spray-drying process has been developed in connection with the manufacture of dried milk. However, when milk is spray-dried, the process can be considered as a microencapsulation one; milk fat is being the core material that is protected against oxidation by a wall material consisting of a mix of lactose and milk proteins. In this mix, the carbohydrates provide structure through glass formation whereas the proteins provide emulsification and film forming properties. We present in this part some fundamental information concerning the process of spray-drying and we strongly think that the knowledge of these considerations is very important to carry out the process of microencapsulation by spray-drying. 2.1. Atomization

Liquid atomization in small droplets can be carried out by pressure or centrifugal energy. Used atomizers include pneumatic atomizer, pressure nozzle, spinning disk configurations and recently two fluid nozzle and sonic nozzle

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(Masters, 1968). The goal of this stage is to create a maximum heat-transferring surface between the dry air and the liquid in order to optimize heat and mass transfers. The choice upon the atomizer configuration depends on the nature and viscosity of feed and the desired characteristics of dried product. The higher the provided energy is, the finer are the formed droplets. For the same energy amount, the size of formed particles increases with increasing feed rate. However, the size of particles increases when both viscosity and surface tension of the initial liquid are high. Several types of atomizers such as centrifugal, steam, and homogenizing ones have been described by Bowen (1938). 2.2. Droplet – hot air contact

This contact takes place during atomization and initiates the drying stage. According to the atomizer emplacement compared to the hot air spreader, one can distinguish cocurrent drying and counter-current one. In co-current process the liquid is sprayed in the same direction as the flow of hot air through the apparatus, hot air inlet temperature is typically 150–220  C, evaporation occurs instantaneously (Fleming, 1921) and for which dry powders will be exposed to moderate temperatures (typically 50–80  C) which limits thermal degradations. Whereas, during counter-current drying, the liquid is sprayed in the opposite direction of the flow of hot air and for which the dry product is exposed to high temperatures which limits the applications of this process to thermo-sensitive products. However, the main advantage of the counter-current process is that it is considered as more economic in term of consumed energy. 2.3. Evaporation of droplet water

At the time of droplets – hot air contact, balances of temperature and vapor partial pressure are established between liquid and gas phases. Thus, heat transfer is carried out from air towards the product as a result of temperature difference whereas water transfer is carried out in the opposite direction due to the vapor pressure difference. Based on the fundamental theory of drying, three successive steps can be distinguished. Just after the hot air – liquid contact, heat transfer principally causes the increase of the droplets temperature up to a constant value. This value is defined as the air drying humid thermometer temperature; after that, the evaporation of droplets water is carried out at constant temperature and water vapor partial pressure. The rate of water diffusion from the droplet core to its surface is usually considered constant and equal to the surface evaporation rate. Finally, when the droplet water content reaches a critical value, a dry crust is formed at the droplet surface and the drying rate rapidly decreases with the drying front progression and becomes dependent on the water diffusion rate through this crust. Drying is theoretically finished when the particle temperature becomes equal to that of the air.

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These three steps have different durations depending on both product nature and air inlet temperature. In fact, if the air inlet temperature is high, the dry crust is rapidly formed because of the high water evaporation rate. Due to the large surface to volume ratio of the atomized droplets, drying of formed droplets in the hot atmosphere is a very rapid process. In a first stage, the hot gas causes an increase of the droplet temperature, which promotes liquid evaporation from the droplet surface and a corresponding shrinkage of the droplet. The rapid migration of the water to the droplet surface maintains a constant evaporation rate. At some point the suspended particles form a continuous network. Originally the use of spray-drying was advocated as a rapid method for drying compounds which showed unacceptable levels of degradation when dried in the slower classic drying processes. Usually, drying times are of the order of 5–100 s ( Corrigan, 1995). However, in a well-designed system 15–30 s is a fair time for the passage of the sprayed particle through the drying zone (Fogler & Kleinschmidt, 1938). The general spraydrying process mechanism description proposed by Fleming (1921) is still valid. In fact, at the moment of the mixing of the air with the atomized liquid, the drying takes place almost instantaneously and an intensive evaporation is taking place at the surface of each droplet. The evaporation is so rapid that the droplet remains kept cool until the dry state is reached; this is due to the absorption of heat in vaporizing the liquid. After the evaporation has ceased, the temperature of the particle rises to the general temperature of the drying chamber (Papadakis & King, 1988a). 2.4. Dry product-humid air separation

This separation is often done through a cyclone placed outside the dryer which reduces product losses in the atmosphere: most dense particles are recovered at the base of the drying chamber while the finest ones pass through the cyclone to be separated from the humid air. In addition to cyclones, spray-dryers are commonly equipped with both filters, called ‘‘bag houses’’ that are used to remove the finest powder, and chemical scrubbers to remove the remaining powder or any volatile pollutants (e.g. flavorings). The obtained powder is made of particles which originate from spherical drops after shrinking. Depending on the composition, the water and gas content of the drop, these particles can be compact or hollow (Bimbenet, Bonazzi, & Dumoulin, 2002). The use of multi-stage spray-dryers makes it possible to increase the residence time and to decrease the drying temperature limiting thus thermal denaturation and improving thermal effectiveness (kJ needed per kg of dry product) (Schuck, 2002). Moreover, integration of a fluidized bed on the drier outlet side makes it possible to better control particle size and to manufacture powders with very low

water contents (Turchiuli et al., 2005). Mass and heat balances of a multistage spray-dryer were experimentally established and the energy specific consumption was studied (Bimbenet et al., 2002). The morphological modifications of the particles during spray-drying were described by Alamilla-Beltran, Chanona-Pe´rez, Jimeńez-Aparicio, and Guite´rrez-Lopez (2005). Changes of particle size and morphology during spray-drying are related to moisture content and drying temperature. These changes are usually described by classical theory of drying. However, a method based on image analysis of maltodextrin particles at various vertical distances from the atomizing nozzle has been proposed (Alamilla-Beltran et al., 2005). Technical properties of spray-dried microcapsules depend mainly on particle– particle interactions for flowability, and particle–liquid interactions for wettability and re-dispersibility. Because of the complexity of the various spray-drying steps, spray-drier design has traditionally been based on experience, trial-and-error, and pilot scale work. However, new development in Computational Fluid Dynamics (CFD) and drop size measurement have important effects on device design as well as residence-time distribution function method (Paris, Ross, Dastur, & Morris, 1971). CFD was applied in spray-drying of food ingredients (Langrish & Fletcher, 2001; Straatsma, Van Houwelingen, Steenbergen, & De Jong, 1999) in order to predict thermal degradation, aroma loss, wall deposition, and particle stickiness. Moreover, modeling of droplet morphology (Lin & Gentry, 1999), particle formation (Lin & Gentry, 1997; Teunou & Poncelet, 2005), droplet shell formation (Lin, Zhang, & Gentry, 2000), as well as air temperature and humidity profiles (Papadakis & King, 1988b) during spray-drying were reported. Mathematical modeling of spray-drying process is difficult because of the great number of parameters that intervene during the process such as, droplets exhibiting evaporation, collision, breakups, agglomeration, heat and mass exchange between the droplets and the drying medium, among other factors (Negiz, Lagergren, & C ¸ inar, 1995).

3. Spray-drying as a process for microencapsulation

Spray-drying is the most common and cheapest technique to produce microencapsulated food materials. Equipment is readily available and production costs are lower than most other methods. Compared to freeze-drying, the cost of spray-drying method is 30–50 times cheaper (Desobry, Netto, & Labuza, 1997). Spray-drying has been considered as a solution for conventional drying problems because the process has usually proved not only efficient but also economic (Masters, 1968). The economics of spray-drying were discussed by Quinn (1965). However, Spray-drying is considered as an energy wasting operation because it is impossible to utilize all the heat going through the drying chamber.


3.1. Microencapsulation process steps

The application of spray-drying process in microencapsulation involves three basic steps (Dziezak, 1988): preparation of the dispersion or emulsion to be processed; homogenization of the dispersion; and atomization of the mass into the drying chamber. However, and more detailing the third stage of the process, Shahidi and Han (1993) suggested that the microencapsulation by spray-drying involves four stages: preparation of the dispersion or emulsion; homogenization of the dispersion; atomization of the infeed emulsion; and dehydration of the atomized particles. The first stage is the formation of a fine and stable emulsion of the core material in the wall solution. The mixture to be atomized is prepared by dispersing the core material, which is usually of hydrophobic nature, into a solution of the coating agent with which it is immiscible. The dispersion must be heated and homogenized, with or without the addition of an emulsifier depending on the emulsifying properties of the coating materials because some of them have themselves interfacial activities. In the spray-drying process, the initial emulsion droplets are in the order of diameter 1–100 l m. Before the spray-drying step, the formed emulsion must be stable over a certain period of time (Liu et al., 2001), oil droplets should be rather small and viscosity should be low enough to prevent air inclusion in the particle (Drusch, 2006). Emulsion viscosity and particle size distribution have significant effects on microencapsulation by spray-drying. High viscosities interfere with the atomization process and lead to the formation of elongated and large droplets that adversely affect the drying rate (Rosenberg, Kopelman, & Talmon, 1990). The core material retention during microencapsulation by spray-drying is affected by the composition and the properties of the emulsion and by the drying conditions. The obtained oil-in-water emulsion is then atomized into a heated air stream supplied to the drying chamber and the evaporation of the solvent, usually water, consequently leads to the formation of microcapsules. As the sprayed particles fall through the gaseous medium, they assume a spherical shape with the oil encased in the aqueous phase (Dziezak, 1988). The short time exposition and the rapid evaporation of water keep the core temperature below 40  C, in spite of the high temperatures generally used in the process (Dubernet & Benoit, 1986). 3.2. Operating conditions

In order to obtain good microencapsulation efficiency and even if the wall material is suitable, optimal spray-drying conditions must be used. The main factors in spray-drying that must be optimized are feed temperature, air inlet temperature, and air outlet temperature (Liu, Zhou, Zeng, & Ouyang, 2004). In fact, feed temperature modifies the viscosity of the emulsion, its fluidity and thus, its capacity to be homogenously sprayed. When the feed temperature is

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increased, viscosity and droplets size should be decreased but high temperatures can cause volatilization or degradation of some heat-sensitive ingredients. The rate of feed delivered to the atomizer is adjusted to ensure that each sprayed droplet reaches the desired drying level before it comes in contact with the surface of the drying chamber. Moreover, appropriate adjustment of the air inlet temperature and flow rate is important (Zbicinski, Delag, Strumillo, & Adamiec, 2002). In fact, air inlet temperature is directly proportional to the microcapsule drying rate and the final water content. When the air inlet temperature is low, the low evaporation rate causes the formation of microcapsules with high density membranes, high water content, poor fluidity, and easiness of agglomeration. However, a high air inlet temperature causes an excessive evaporation and results in cracks in the membrane inducing subsequent premature release and a degradation of encapsulated ingredient or also a loss of volatiles (Zakarian & King, 1982). The air inlet temperature is usually determined by two factors: the temperature which can safely be used without damaging the product or creating operating hazards and the comparative cost of heat sources ( Fogler & Kleinschmidt, 1938). The temperature at the end of the drying zone, also called in literature exhaust temperature or air outlet temperature, obtained under given conditions can be considered as the control index of the dryer. It is quite difficult to predict this outlet temperature in advance for a given product, since it depends on the drying characteristics of the material. Contrary to the air inlet temperature, the air outlet one cannot be directly controlled since it depends on the air inlet temperature, and the ideal air outlet temperature for the microencapsulation of food ingredients such as flavors has been reported to be 50–80  C. Table 1 summarizes experimental conditions that have been recently optimized for the encapsulation of different food ingredients by spray-drying. The best spray-drying conditions are a compromise between high air temperature, high solid concentration of the solution, and easy pulverization and drying without expansion and cracks of final particles (Bimbenet et al., 2002). Reineccius (1988) reported that the greatest loss of the volatiles during microencapsulation by spray-drying takes place at early stages of drying, prior to the formation of a dry crust at the surface of the drying particles. However, the use of some specific compounds can modify the drying properties of microcapsules. Indeed, the addition of lactose to the whey protein-based system appeared to enhance crust formation by improving the drying properties of the wall. This positive effect of lactose has been attributed to the formation of a continuous glass phase of lactose in which the protein chains are dispersed (Rosenberg & Sheu, 1996). In addition, the lactose glass phase increases the hydrophilic nature of the wall matrix and limits the diffusion of the solvent through the wall (Moreau & Rosenberg, 1996). It was expected also that heat denaturation of whey proteins influences emulsification characteristics and thus

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Table 1 Experimental conditions recently optimized for the encapsulation of some different food ingredients by spray-drying Encapsulated ingredient

Wall material

Feed temperature (C)

Anhydrous milk fat Ethyl butyrate ethyl caprylate

Whey proteins/lactose Whey proteins/lactose

50 5

Oregano, citronella and marjoram flavors Soya oil Calcium citrate calcium lactate Lycopene Fish oil Cardamom essential oil

Whey proteins/milk proteins

Arachidonyl L -ascorbate Cardamom oleoresin Bixin D-Limonene L-Menthol

Black pepper oleoresin Cumin oleoresin Fish oil Caraway essential oil Short chain fatty acid

Air inlet temperature (C)

Air outlet temperature (C)

160 160

80 80

NR

185–195

85–95

Sodium caseinate/carbohydrates Cellulose derivatives/ polymethacrylic acid Gelatin/sucrose Starch derivatives/glucose syrup Mesquite gum

NR NR

180 120–170

95 91–95

55 NR Room T

190 170 195–205

52 70 105–115

Maltodextrin/gum arabic/soybean polysaccharides Gum arabic/modified starch/ maltodextrin Gum arabic/maltodextrin/sucrose Gum arabic/maltodextrin/modified starch Gum arabic/modified starch

NR

200

100–110

NR

176–180

115–125

Room T NR

180 200

130 100–120

NR

180

95–105

Gum arabic/modified starch Gum arabic/maltodextrin/modified starch Sugar beet pectin/glucose syrup Milk proteins/whey proteins/ maltodextrin Maltodextrin/gum arabic

NR NR

176–180 158–162

105–115 115–125

NR NR

170 175–185

70 85–95

NR

180

90

References

Young et al. (1993) Rosenberg and Sheu (1996) Baranauskiene´ et al. (2006) Hogan et al. (2001) Oneda and Re´ (2003) Shu et al. (2006) Drusch et al. (2006) Beristain et al. (2001) Watanabe et al. (2004) Krishnan et al. (2005) Barbosa et al. (2005) Soottitantawat et al. (2005a) Soottitantawat et al. (2005b) Shaikh et al. (2006) Kanakdande et al. (2007) Drusch (2006) Bylaite´ et al. (2001) Teixeira et al. (2004)

NR: not reported.

microencapsulating properties (Rosenberg & Sheu, 1996). Microencapsulation efficiency can be increased by increasing wall solution solids concentration which can be related to the effect of wall solids concentration on the formation of surface core prior to the formation of crust around the drying droplets (Young, Sarda, & Rosenberg, 1993). The main limitation of the spray-drying technique in microencapsulation is the limited number of wall materials available and that must have a good solubility in water. Another disadvantage for spray-drying that should be considered is that it produces a fine microcapsules powder which needs further processing such as agglomeration.

4. Microencapsulation by spray-drying: which wall must be used?

The choice of a wall material for microencapsulation by spray-drying is very important for encapsulation efficiency and microcapsule stability. The criteria for selecting a wall material are mainly based on the physico-chemical properties such as solubility; molecular weight; glass/melting transition; crystallinity; diffusibility; film forming and emulsifying properties. Moreover, the costs should be also considered. Thus, judicious choice of encapsulating mate-

rial according to the desired application is an important task. 4.1. Wall material required properties

The wall system is designed to protect core material from factors that may cause its deterioration, to prevent a premature interaction between the core material and other ingredients, to limit volatile losses, and also to allow controlled or sustained release under desired conditions (Shahidi & Han, 1993). Depending on the core material and the characteristics desired in the final product, wall materials can be selected from a wide variety of natural and synthetic polymers. Since almost all spray-drying processes in the food industry are carried out from aqueous feed formulation, the wall material must be soluble in water at an acceptable level (Gouin, 2004). In addition to its high solubility, a wall material for microencapsulation by spray-drying should possess good properties of emulsification, film forming, and drying and the wall concentrated solutions should have low viscosity (Reineccius, 1988). Many available wall materials possess these properties but the number of materials approved for food uses is limited (Dziezak, 1988). Many biopolymers have been used in microencapsulation of var-


ious food ingredients by spray-drying. The microencapsulation of food ingredients is often achieved with biopolymers of various sources, such as natural gums (gum arabic, alginates, carragenans, etc.), proteins (milk or whey proteins, gelatin, etc.), maltodextrins with different dextrose equivalence, waxes and their blends. However, typical wall materials for microencapsulation by spray-drying are low molecular weight carbohydrates, milk or soy proteins, gelatin and hydrocolloids like acacia gum (Reineccius, Ward, Whorten, & Andon, 1995; Thevenet, 1995) and more recently local materials, such as mesquite gum (Beristain & Vernon-Carter, 1994; Beristain, Garcıá, & VernonCarter, 2001) have been used to overcome the expensive cost of some commonly used materials. Unusual novel microencapsulation methods based on interfacial engineering technology have recently been developed in order to improve quality of manufactured powder. Among these methods, a process of electrostatic layer-by-layer deposition was successfully developed to prepare spray-dried microcapsules containing tuna oil (Klinkesorn, Sophanodora, Chinachoti, Decker, & McClements, 2006). These microcapsules were reported to be unaffected by air inlet temperature probably because of the robustness of the wall barrier. 4.2. Wall material selection

An important step in developing microcapsules is the selection of a wall material that meets required criteria (mechanical strength, compatibility with the food product, appropriate thermal or dissolution release, appropriate particle size, etc. (Brazel, 1999). The selection of wall materials for microencapsulation by spray-drying has traditionally involved trial-and-error procedures in which the microcapsules are formed. The latter are then evaluated for encapsulation efficiency, stability under different storage conditions, degree of protection provided to the core material, surface observation by scanning microscopy, among other evaluations (Pe´rez-Alonso, Baez-Gonzalez, Beristain, Vernon-Carter, & Vizcarra-Mendoza, 2003). An important method that can be useful to determine suitable composition of a wall material blend for lipid encapsulation has been proposed by Matsuno & Adachi (1993). A suitable material that can be used in this method should possess a high emulsifying activity, a high stability, a tendency to form a fine and dense network during drying and should not permit lipid separation from the emulsion during dehydration. This method is based on the measurement of the drying rate of an emulsion as a function of moisture content. Because the isothermal drying rate is governed by the diffusion rate of water during drying, the drying rate may reflect the characteristics of the sample matrix: the finer and denser the matrix, the lower the drying rate (Imagi, Yamanouchi, Okada, Tanimoto, & Matsuno, 1992). A characteristic curve of drying rate has been presented as a function of moisture content for four groups of wall materials (Fig. 2) (Matsuno & Adachi,

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Fig. 2. Schematic characteristic isothermal drying curves of various wall materials. See text for details. (Matsuno & Adachi, 1993).

1993). This figure has been interpreted in term of ability of the wall material to form a dense network. In fact, drying rate of materials that gives type 1 curves decreases rapidly as the water content decreases, that means a rapid formation of a dense skin and a good protection of core ingredient against oxygen transfer and possible deterioration. Such a curve corresponds to maltodextrin, pullulan, gum arabic and gelatin. These materials were thus considered as the most suitable for microencapsulation using a drying process. Characteristic type 2 materials are substances of high molecular weight and with a three-dimensional structure such as sodium caseinate and albumin, type 3 materials are low molecular weight saccharides like glucose, and type 4 materials are those that easily crystallize upon dehydration such as mannitol (Matsuno & Adachi, 1993). According to this interpretation, type 2, 3, and 4 materials cannot efficiently protect encapsulated lipids because they do not form a dense skin at an early stage of drying, and consequently are not suitable for this purpose. This method has been criticized for its lack of precision and no-discrimination between materials showing similarly shaped drying curves (Pe´rez-Alonso et al., 2003). Moreover, in order to screen the most suitable wall material for lipid encapsulation, these authors proposed a quantitative method based on estimation of the activation energy of carbohydrate polymers blends dried isothermally. This method provides a quantitative discriminating parameter and requires the knowledge of the drop volume shrinkage of every conceivable blend.

4.3. Most commonly used wall materials

It is obvious that the chemical functionality, the solubility and the diffusion through the forming matrix determine the retention degree of core compounds during the preparation of microcapsules by spray-drying. Therefore,

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microencapsulation efficiency and microcapsules stability during storage are largely dependent on wall material composition. Carbohydrates such as starches, maltodextrins and corn syrup solids are usually used in microencapsulation of food ingredients. However, wall materials that are based on these compounds have poor interfacial properties and must be chemically modified in order to improve their surface activity. In contrast, proteins have an amphiphilic character that offer physicochemical and functional properties required to encapsulate hydrophobic core materials. Moreover, protein compounds such as sodium caseinate, soy protein isolate, and whey protein concentrates and isolates, could also be expected to have good microencapsulating properties. 4.3.1. Carbohydrates

Carbohydrates such as starches, corn syrup solids and maltodextrins have been usually used as encapsulating agents (DeZarn, 1995; Kenyon, 1995). These materials are considered as good encapsulating agents because they exhibit low viscosities at high solids contents and good solubility, but most of them lack the interfacial properties required for high microencapsulation efficiency and generally associated with other encapsulating materials such as proteins or gums. Soybean soluble polysaccharide was also found to be a superior emulsifier over gum arabic to retain microencapsulated ethyl butyrate during spray-drying (Yoshii et al., 2001). In addition, it is known that polysaccharides having gelling properties could stabilize emulsions towards flocculation and coalescence (Dalgleish, 2006). A novel approach to improve encapsulating properties of common wall materials consists on chemical modifications of carbohydrates. For example, some modified starches have surface active properties and are widely used in the process of microencapsulation by spray-drying. Hydrolyzed starch products are hydrophilic compounds, thus have little affinity for hydrophobic flavors (Shaikh, Bhosale, & Singhal, 2006). Their hydrophilic nature can be modified by linking hydrophobic side chains like octenyl ones (Drusch & Schwarz, 2006). Maltodextrins provide good oxidative stability to encapsulated oil but exhibit poor emulsifying capacity, emulsion stability and low oil retention (Kenyon, 1995). However, the retention of emulsified ethyl butyrate during spray-drying was shown to be dependent on the maltodextrin concentration and the type of emulsifier (Yoshii et al., 2001). A maltodextrins characterization study published by Raja, Sankarikutty, Sreekumar, Jayalekshmy, & Narayanan (1989) showed that maltodextrins with dextrose equivalence between 10 and 20 fit in for using as wall material. Those maltodextrin samples show the highest retention of flavor because they could be dispersed in water up to 35.5% of the solution without haze formation. Sucrose, glucose and starch were reported to be not suitable for spray-drying of sumac flavor due to their caramelization properties, adherence to the surface of the spray-dryer, and heterogeneous form that caused the clog-

ging of the nozzle (Bayram, Bayram, & Tekin, 2005). As for trehalose, in its glassy state this disaccharide decreases lipid oxidation and is therefore a suitable wall material for microencapsulation purposes. However, a rapid oxidation of microencapsulated fish oil was observed upon crystallization of trehalose, which limits the range of applications to products to be stored at low humidity (Drusch, Serfert, Van Den Heuvel, & Schwarz, 2006). These same authors proposed to use polymers, mixtures of carbohydrates and salts that are known to modify the kinetics of sugar crystallization in order to improve stability of microencapsulated oils. Pectin is a polymer able to produce stable emulsions at low concentration. The emulsifying properties of pectin are due to the protein residues present within the pectin chain and its chemical composition characterized by a higher content of acetyl groups (Leroux, Langendorff, Schick, Vaishnav, & Mazoyer, 2003). A pectin content of 1–2% is considered to be sufficient for the preparation of a stable emulsion for spray-drying (Drusch, 2006). Sugar beet pectin could be considered as a suitable wall material for microencapsulation of lipophilic food ingredients by spray-drying. Fish oil was successfully encapsulated in a wall system containing sugar beet pectin as coating agent and glucose syrup as bulk one ( Drusch, 2006). It was also shown that spray-drying had no effect on the majority of functional properties of pectin (Monsoor, 2005). Likewise, dry emulsions of oil dispersed in aqueous solution of hydroxypropylmethyl-cellulose (HPMC) were prepared by spray-drying (Christensen, Pedersen, & Kristensen, 2001a). The obtained powders were cohesive, but the flow properties were improved by adding sucrose (Christensen, Pedersen, & Kristensen, 2001b). In another research paper, the same authors suggested that the re-crystallization of amorphous sucrose obtained by spray-drying could be avoided by storage below the glass transition temperature under dry conditions (Christensen, Pedersen, & Kristensen, 2002). The use of low molecular weight carbohydrates in microencapsulation is usually associated with problems of caking, collapse and re-crystallization of the amorphous carbohydrate upon storage. Caking can be explained by the formation of inter-particle bonds between adjacent particles when surface viscosity reached a critical value (Le Meste, Champion, & Roudaut, 2002). In addition, the presence of fine particles and the distribution of grain size are particularly important for control and prevention of sugar caking (Mathlouthi & Roge´, 2003). Spray-drying technique is known to be unsuitable for producing powders of sugar and acid-rich foods due to their stickiness. In fact, the process of spray-drying may result in: disordering the sugar crystal lattice, formation of polymorphic or pseudo-polymorphic phase, elimination of crystallinity, and general in alteration in the energy of the dried solids (Corrigan, 1995). The rapid removal of water that occurs by spray-drying produces amorphous materials and in this context surface stickiness of droplet could be predicted


during spray-drying by many approaches such as glass transition temperature (Adhikari, Howes, Lecomte, & Bhandari, 2005). 4.3.2. Gums

Gums are used in microencapsulation for both their film forming and emulsion stabilization properties. Among all gums, acacia gum, generally called gum arabic, stands out due to its excellent emulsification properties and thus is widely used. Gum arabic is a polymer consisting of D-glucuronic acid, L-rhamnose, D-galactose, and L-arabinose, with approximately 2% protein (Dickinson, 2003). The emulsification properties of the gum arabic are attributed to the presence of this protein fraction (Dickinson, 2003). Gum arabic was found to be a better wall material for encapsulation of cardamom oleoresin than maltodextrins and modified starch and the obtained microcapsules exhibited a free flowing character (Krishnan et al., 2005). It was also recently reported that gum arabic is good wall material for encapsulation of cumin oleoresin by spray-drying (Kanakdande, Bhosale, & Singhal, 2007). Gum arabic is usually preferred because it produces stable emulsions with most oils over a wide pH range, and it also forms a visible film at the oil interface. Because of this emulsifying efficiency, gum arabic has been usually used to encapsulate lipids (Kenyon, 1995). Typically the ratio of oil/wall material, when gum arabic is used, is lower than 0.15. Gum arabic is ideally suitable for the microencapsulation of lipids because of both its surface activity and its film forming properties. The partial replacement (50%) of gum arabic with glucose was reported to increase microencapsulation efficiency of soy oil from 74% to 92% (McNamee, White, O’Riordan, & O’Sullivan, 2001). However maltodextrin DE 18.5 was considered as the most suitable partial replacer for gum arabic because the obtained wall material presents a good solubility and a rapid reconstitution of the emulsion in water. However, another study showed that gum arabic was not efficient as a wall material for the encapsulation of five different monoterpenes (citral, linalool, b-myrcene, limonene, and b-pinene) (Bertolini, Siani, & Grosso, 2001). Indeed, gum arabic-based obtained capsules show a limited barrier capacity against oxidation because they act as semipermeable membranes and their permeability to oxygen is a preponderant factor in the shelf life of the core material. It was reported also that gum arabic is not effective as orange oil microencapsulant when it is compared to whey and soy protein isolates. Moreover, this gum produced the smallest orange oil encapsulating particles and the least stable ones (Kim & Morr, 1996). Properties of gum arabic and other hydrocolloids at interfaces were well reviewed by Dickinson (2003). Likewise, expensive cost, limited supply, and quality variations have restricted the use of gum arabic for encapsulation purpose and turned researchers to seek alternative microencapsulating materials. Since Beristain & VernonCarter (1994) reported that mesquite gum can be consid-

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ered as an alternative inexpensive encapsulating polymer that could entrap orange peel oil with a high efficiency. Later, this research group suggested that mesquite gum can be used in the preparation of oil-in-water emulsions, over a wide range of pH values. Cardamom-based oil microcapsules were also successfully produced by spraydrying, and high efficiency (83.6%) was reached when mesquite gum/oil ratio equal to 4 was used (Beristain et al., 2001). 4.3.3. Proteins

The excellent functional properties of proteins allow them to be a good coating material for the microencapsulation by spray-drying. In addition, proteins possess high binding properties for the flavor compounds (Landy, Druaux, & Voilley, 1995). The most commonly used proteins for encapsulating food ingredients by spray-drying are milk (or whey) proteins and gelatin. Because they possess functional properties required for microcapsule forming wall material (Rosenberg & Sheu, 1996), whey proteins have been successfully used as a wall system to encapsulate anhydrous milk fat by spray-drying and encapsulation yield greater than 90% was obtained (Young et al., 1993). According to the same authors, microencapsulation efficiency can be improved by partial (50%) replacement of whey proteins by lactose. In fact, the incorporation of lactose in the whey protein-based wall system can limit the diffusion of apolar substances through this wall. Lactose in its amorphous state acts as a hydrophilic sealant that significantly limits diffusion of the hydrophobic core through the wall and thus leads to high microencapsulation efficiency values. Spray-drying was used to study microencapsulation of ethyl butyrate and ethyl caprylate in wall systems consisting of whey protein isolate or a mixture of whey protein isolate and lactose (ratio 1:1) (Rosenberg & Sheu, 1996). The authors reported that retention of these two volatiles was affected by wall solids concentration, initial ester load, and by ester and wall type. It has later been shown that caraway essential oil could be encapsulated in a wall system consisting of milk proteins which should provide it an effective protection against oxidation (Bylaite, Nylander, Venskutonis, & Jonsson, 2001; Bylaite, Venskutonis, & Mapdbieriene, 2001). It has been shown that sodium caseinate has better encapsulation properties than micellar casein (Vega, Kim, Chen, & Roos, 2005). This result was explained according to the molecular conformation, the high diffusivity, and the strong amphiphilic characteristics of the individual caseins that allow for a better distribution around the encapsulated fat globule surface than micellar caseins. The reversible depletion–flocculation of emulsions stabilized with caseinate and containing excess protein can be inhibited by the addition of calcium ions concentration well below that required to precipitate the protein (Dickinson, Radford, & Golding, 2003). Microencapsulation by spray-drying of soy oil in sodium caseinate as wall material has been studied

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(Hogan et al., 2001). It was shown that oil/protein ratio should not exceed 1 or the formed emulsion could be destabilized during spray-drying. The effect of spray-drying on the physico-chemical properties of oil-in-water emulsions stabilized by milk proteins was studied (Sliwinski et al., 2003). It was shown that spray-drying resulted in a denaturation and aggregation of b-lactoglobulin. Heat-treatments of whey proteins were shown to affect the functional properties of spray-dried powder probably by protein denaturation (Millqvist-Fureby, Elofsson, & Bergensta˚hl, 2001). During spray-drying the temperature of the drying droplet increases slightly, while its water content decreases at the same time. Protein denaturation, especially globular proteins, can occur only when two parameters are combined: high temperature and high water activity of the drying droplet. As a result, it is very difficult to predict the effect of spray-drying process on the stability of wall proteins. Gelatin is a water-soluble material with wall-forming ability in spray-drying (Lee, Kim, & Kim, 1999) and it was reported that the characteristics and morphology of gelatin microparticles could be improved by addition of mannitol (Bruschi, Cardoso, Lucchesi, & Gremiao, 2003). While based on the drying characteristic curves, Imagi et al. (1992) had showed that, compared to maltodextrin, pullulan, glucose, maltose and mannitol, gelatin had all the properties of an effective entrapping agent: high emulsifying activity, high stabilizing activity, and a tendency to form a fine dense network upon drying. The addition of a small amount (1% (w/w)) of gelatin could increase the retention of ethyl butyrate when gum arabic is used as emulsifier and according to Yoshii et al. (2001), the early formation of the surface crust due to the presence of gelatin, prevented the loss of ethyl butyrate emerged from the unstable ethyl butyrate emulsion. In Iwami, Hattori, Yasumi, & Ibuki (1988) obtained spherical microcapsules of gliadin containing polyunsaturated lipid. It was reported that these spray-dried microparticles were resistant to oxidative deterioration during a long-term storage at various water activity values. In this study spray-drying has proven to raise the antioxidant effect of gliadin without impairing its digestibility. Spraydrying is known to induce protein denaturation as shown above, but in the case of gluten hydrolysates, it was reported that spray-drying process can only modify interfacial protein properties (Linare`s, Larre´, & Popineau, 2001). Recently, Pierucci, Andrade, Baptista, Volpato, & RochaLeaô (2006) reported that pea protein can be considered as a good coating agent for the microencapsulation of ascorbic acid. In all cases, it should be noticed that there are sometimes some issues in the use of proteins as encapsulating agents. For example, losses of aldehyde based flavor component, labelling, allergy and precipitation when protein based microcapsules are added to food products having pH near their isoelectric point. In addition, some religious and social (halal, kosher, vegetarian food choices) issues must be considered.

5. Some examples of food ingredients microencapsulated by spray-drying

Actually, essential interests are attributed to encapsulation of flavors, lipids, and carotenoids among other ingredients. Because a single encapsulating agent can not possess all ideal wall material properties, recent researches have focused on mixtures of carbohydrates, gums, and proteins. This section focuses on the most important food ingredients that were recently encapsulated by spray-drying. 5.1. Flavors

Most of the flavoring compounds which give foods their characteristic aroma are highly volatile with respect to water. Therefore they are easily lost during spray-drying operation. Several methods have been reported for microencapsulation of flavors, but the most common technique employed is spray-drying. Microencapsulation of hydrophobic flavors is of great importance in the flavoring and food industries, since solid or liquid microencapsulated food flavor exhibit a good chemical stability and a controlled release. Spray-drying is generally used to produce flavor powders in a short time. Many studies have been carried out on the influence of wall materials compositions and the operating conditions on the retention and controlled release of encapsulated flavors (Madene, Jacquot, Scher, & Desobry, 2006). L -Menthol, a cyclic terpene alcohol normally available in the form of crystals or granules with a melting point 41-43  C presents a high volatility and a whisker growth (Soottitantawat et al., 2005b). These two problems that limit its application and shelf life storage were overcome by spray-drying microencapsulation and it was reported that the high retention of L -menthol in a wall system of maltodextrin and gum arabic was observed only at high solid content. Thus, it was shown that the optimal Lmenthol concentration in the feed emulsion, which should have a high retention of flavor and low flavor residue on the surface, was L -menthol/wall materials of 1/4. Recently, flavors of oregano, citronella and marjoram were successfully encapsulated by spray-drying in wall systems of skimmed milk powder and whey protein concentrate (Baranauskiene´, Venskutonis, Dewettinck, & Verhe´, 2006). The major problem reported during the microencapsulation of these flavors is that the global molecular composition of microencapsulated flavors can be modified by drying. Oxidative stability of encapsulated D-Limonene in a matrix of gum arabic, soybean water-soluble polysaccharides and modified starch, blended with maltodextrin, and prepared by spray-drying, was also studied (Soottitantawat et al., 2005a, 2004). Essential oils, especially those rich in monoterpenes are usually used as flavor ingredients. The greater polarity, that leads to a greater solubility of the encapsulated compound in an aqueous medium, results in a greater capacity of diffusion through the matrix during spray-drying, and consequently to greater losses during the formation of the


capsules (Rosenberg et al., 1990; Voilley, 1995). Diffusion of volatiles through the wall matrix during spray-drying can be explained by a mechanism of selective permeability. This assumes mainly that the retention of the compound is a function of the core molar volume. It was shown later, that it could be the ‘‘molecular diameter’’ and not the molar volume that determines the diffusion of the compounds through the matrix during the drying process (Bertolini et al., 2001). In fact, monoterpenic molecules having the same molecular weight (C10H16) and similar solubilities showed different retention yields in the drying process. These differences were explained by the molecular structures of the monoterpenes isomers (Bertolini et al., 2001). In another study, sumac flavor has been successfully encapsulated by spray-drying in sodium chloride as wall material (Bayram et al., 2005) but the salty property of this carrier and the acidic property of sumac limit the applications of the obtained particles to salted cookies, salads, crackers, etc. 5.2. Lipids and oleoresins

In general, lipids are difficult to disperse in food products, in addition and especially polyunsaturated fatty acids are susceptible to auto-oxidation, which results in off-flavors and toxic compounds. Lipids can be used as solvents in which one can solubilize hydrophobic substances such as volatile aromatic compounds. Matsuno & Adachi (1993) enumerated five advantages of lipid encapsulation: retarding auto-oxidation; enhancing stability; controlling lipid-soluble flavor release; masking bitter taste of lipid-soluble substances; and protecting dissolved substances against enzyme hydrolysis. Spray-drying is quite suitable in the encapsulation of oils and oleoresins. Microencapsulation of cardamom oleoresin by spray-drying using gum arabic, maltodextrin, and modified starch as wall materials has been reported (Krishnan et al., 2005). This study showed that the stability of cardamom oleoresin decreased as the quantity of gum arabic decreased in its blends with maltodextrin and modified starch, and the blend (gum arabic: maltodextrin: modified starch) of (4/6:1/6:1/6) was considered as suitable. This ternary blend proved to be also efficient for the encapsulation of cumin oleoresin (Kanakdande et al., 2007). Stability of black pepper oleoresin encapsulated in gum arabic and emulsifying modified starches by spray-drying has been reported (Shaikh et al., 2006). Sodium caseinate/carbohydrate blends were used to encapsulate soya oil by spray-drying and the obtained results showed that microencapsulation efficiency can be improved by increasing the dextrose equivalence of the carbohydrates (Hogan et al., 2001). Study of spray-dried powders with 50% butteroil encapsulated in sucrose and double encapsulated in a matrix of vegetable waxes shows that double encapsulation method could improve capsule resistance to moisture sorption, but also decreases powder flow properties (Onwulata, Konstance, & Holsinger, 1998). In a previous study, this research groupsuggested that encapsulatedspray-dried powders con-

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taining 40% butteroil had shown lower flow characteristics and where highly cohesive. These results had been explained by the fat content and the propensity of bridging, but flow properties were enhanced by adding a commercial anti-caking flow agent (Konstance, Onwulata, & Holsinger, 1995). The oxidative and thermal stabilities of crude squid oil have been effectively enhanced by spray-drying microencapsulation (Lin, Lin, & Hwang, 1995). Used wall materials were gelatin, sodium caseinate, and maltodextrin. The addition of lecithin and carboxymethyl cellulose was reported to improve encapsulating effectiveness and oxidative and thermal stabilities. The most effective formulation that shows the best thermal stability was (oil/gelatin/caseinate/maltodextrin/lecithin/carboxymethyl cellulose) of (30/20/20/20/ 4/1). In general, microencapsulation of polyunsaturated acids or their acylglycerols limits or delays its oxidation. In fact, the esterification of polyunsaturated fatty acids with L-ascorbic acid and subsequent microencapsulation of the ester would be a useful technology for suppressing or retarding the oxidation of the polyunsaturated fatty acid. So that, 6- O-arachidonoyl L-ascorbate was microencapsulated in various wall materials by spray-drying and the oxidation of arachidonic acid under 37  C and 12% relative humidity was well retarded by its microencapsulation in gum arabic or a soluble soybean polysaccharide (Watanabe, Fang, Adachi, Fukami, & Matsuno, 2004). Morover, it was reported that saturated acyl L -ascorbate can be used to stabilize against oxidation linoleic acid encapsulated using whey protein concentrate (Jimenez, Garcia, & Beristain, 2004), maltodextrin or gum arabic by spray-drying (Watanabe, Fang, Minemoto, Adachi, & Matsuno, 2002). Fat properties, such as crystal forms and habit, in spraydried microcapsules were also investigated as a function of storage time (Millqvist-Fureby, 2003). 5.3. Other food ingredients

Spray-drying was used in formulating calcium microparticles using cellulose derivatives and polymethacrylic acid as wall systems (Oneda & Re´, 2003). Results showed that the size and morphology of spray dried microparticles were found to be affected by the type of polymer and its initial concentration. Lycopene was successfully microencapsulated by spray-drying using a wall system consisting of gelatin and sucrose. The optimal ratio of gelatin/sucrose was 3/7 and that of core/wall material was 1/4 (Shu, Yu, Zhao, & Liu, 2006). In this work, and although lycopene is a thermo-sensitive compound, feed temperature was 55  C, inlet temperature was 190  C, homogenization pressure was 40 MPa, and lycopene purity was more than 52%. Spray-drying was also considered as the most effective technique for encapsulating iodine and the best results were obtained with dextrin as an encapsulating agent ( Diosady, Alberti, & Venkatesh Mannar, 2002). The obtained microparticles containing up to 1% iodine were stable for periods up to 12 months under a temperature of 40  C and a high relative humidity. The stabilization of carotenoids in foods

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and foodstuffs against isomerization or oxidation is very important because their degradation lowers the final product quality in terms of nutritional properties as well as coloration. Maltodextrins were found to be effective in protecting the carotenoids of paprika oleoresin (Beatus, Raziel, Rosenberg, & Kopelman, 1985). Light stability of spray-dried bixin encapsulated with gum arabic and maltodextrin was studied, and results prove that bixin microencapsulated in gum arabic was more stable to photo-degradation than that in maltodextrin + Tween 80 (Barbosa, Borsarelli, & Mercadante, 2005). This result was interpreted by the highly branched structure of the gum arabic that is a film-forming and acts as an excellent emulsifier for non-polar substances (Dickinson, 2003). Spray-drying has been considered as an excellent means of preservation of nutritive value of vitamins (Hartman, Akeson, & Stahmann, 1967) and this technique could consequently be suitable to encapsulate all vitamin groups. 6. Conclusion

In spite of the recent developments of the spray-drying technique, the process remains far from completely being controlled. Especially, the use of spray-drying for microencapsulation ends is complex because of the multitude of factors to optimize. The drying step in itself is not difficult to succeed and can be optimized by trial-and-error procedure but distinct improvements should be made on the choice of encapsulation materials as well as the study of the various types of molecular interactions: water/wall, water/core, and wall/core. The majority of current studies relate to the formulation of emulsions before drying, the microcapsules size distribution, their morphology, as well as the release rate measurements of the encapsulated compound. However, a great work remains to be made concerning the choice of encapsulating materials according to the physico-chemical properties of food ingredient to encapsulate and the desired properties of microcapsules. In the same way, it is necessary to characterize the various encapsulation polymers especially from emulsifying ability, viscosity in aqueous solutions, and drying properties in order to implement new efficient encapsulating blends. Also, the screening of new wall materials which could ensure both good encapsulation effectiveness and modulated release of the core ingredient becomes essential to overcome expensive costs and unavailability of some commonly materials. Acknowledgement

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