Milk: Processing of Milk RS Chavan, R Sehrawat, and V Mishra, National Institute of Food Technology & Entrepreneurship Management (NIFTEM), Kundli, India S Bhatt, Anand Agricultural University, Anand, India ã 2016
Elsevier Ltd. All rights reserved.
Introduction Milk is nutritious, appetizing, and nature’s perfect food (except vitamin C, copper, and iron) and is recommended by nutritionists for development of sound body and consumed by all sectors of people. People suffering from lactose intolerance and old age are unable unable to digest digest milk, but can easily easily digest digest fermented foods like yogurt (as lactic acid bacteria converts lactose into lactic acid). Demand for milk was greater in developed countries as compared to developing countries, but the gap gap has has narr narrow owed ed due due to an incre increas asee in urba urbani niza zati tion on,, population, and consumption. Total production of milk up to 20 July 2014 was 5339.4 million liters, which is exhibiting an increasing trend from the previous year. Pasteurized milk and ultra-high temperature (UHT) milk market share in 2014 increased to 83.7% and 6.9% (in volume), respectively, as compared to 81.9% and 5.2% in 2013. India is the largest producer of milk with a total volume of 121.8 million tons followed by the United States and China. One of the disadvantages associated with heating of milk is destruction of nutrients like water-soluble vitamins, but technological advancement and research have made it possible to prepare fortified milk or milk with added supplements. Milk is a perishable commodity, and being rich in nutrients it acts as the perfect substrate for growth of microflora, and to reduce this, different thermal and nonthermal techniques are implemented. Thermal treatments are the common techniques used for extending the shelf life of milk, for example pasteurization, sterilization, and UHT, but loss of nutrients is a concern associated with these treatments. Nonthermal treatments like highpressure processing (HPP), pulse electric field, ultrasonication, and irradiation are also explored to process milk to minimize the loss loss of nutrie nutrients nts as compar compared ed to therma thermall treatm treatment ent.. Postprocess contamination is a major factor that can affect the shelf life of milk, and to maintain the product, safe packaging aging plays plays an import important ant role role when when themilk and milk milk pro produc ducts ts are stored stored at refrigerat refrigeration ion or ambient ambient temperatu temperature. re. Aseptic Aseptic packaging packaging of milk when used in combination combination with high temperature can considerably extend shelf life of milk for more than 6 months at room temperature.
Types of Products With judicious preservation technique and by harnessing the potent potential ial of milk milk into into differ different ent types types of pro produc ducts, ts, milk milk could could be made available year-round in those sectors where it is not being produced or during the lean season. Different types of liquid milk include pasteurized milk, sterilized milk, homogenized nized milk, milk, and specia speciall milk. milk. Specia Speciall milk milk includ includes es whole whole milk, milk, standardized milk, soft curd milk, flavored milk, vitaminized/
irradiated milk, fermented milk (natural buttermilk, cultured buttermilk, acidophilus milk, kumis, and kefir), reconstituted/ rehydrated milk, recombined milk, soya milk, toned, double toned milk, filled milk, and imitation milk that are generally Figure re 1). Standardiz pasteurize pasteurized d or UHT treated (Figu Standardized ed milk, toned and double toned are those milks in which standardization of fat and solid not fat (SNF) content are done by addition or removal of cream. Fermented milks are categorized depending on inoculation of different cultures of bacteria for example Lactobacillus bulgaricus, Streptococcus thermophilus, and Streptococcus lactis. Flavored milk is prepared by incorporated flavors flavors like chocolate, chocolate, strawberry, strawberry, mango, mango, buttersco butterscotch, tch, and vanilla along with fruit pulps, cocoa powder, sugar, permitted color, and stabilizers followed by sterilization/UHT treatment. Milk products available in the market include cream, butter, butter oil, ice cream, cheese, powdered milk, and condensed milk. Manufacturing of milk products varies worldwide as a consequenc consequencee of traditional traditional practices, practices, different different dietary dietary habits, habits, and social social,, cultur cultural, al, and religi religious ous dissim dissimila ilarit rities ies.. Market Market milk milk is available in different packaging materials including glass containers, PET (polyethylene terephthalate) bottles, tetra packs, and high-density polyethylene (HDPE) laminate pouches. In the dairy market, liquid milk (54%) occupies the largest segment followed by ghee, yogurt, and whiteners 14%, 8%, and 4%, respectively.
Collection and Storage of Raw Milk Previously milk was collected by small-scale dairies directly from nearby farms so there was no critical critical requiremen requirementt of chilling chilling centers. centers. Howeve However, r, with an increase increase in distance distance between between collection and processing, large dairy processing units required chilling centers, as on many occasions it took 3 days for transportation from distant places. Milk can be collected directly from producers, through cooperative societies, or by the chilling centers. If the processing plant or dairy is near a village/ town, then farmers individually can contact processors on a daily basis. Sometimes a middleman is also involved for transporting raw milk from villagers to the dairy using cans on rickshaw, bike, and so on. A cooperative society is formed by a group of persons at the village level. They do not ensure to take milk on daily basis and can deny on various grounds. The milk is procured procured either on a weight or a volumebasis, and payme paymentis ntis madeon madeon thebasis thebasis ofpercent ofpercentageof ageof fat,SNF,and fat,SNF,and other other quality attributes. Generally cans, insulated tankers, trucks, or rail rail tanke tankers rs are used used as a means means to transp transpor ortt milk milk to a pro proce cessi ssing ng plant. At a chilling center the milk is usually stored at a temperature ature below below 5 C in insulate insulated d tanks/si tanks/silos,and los,and the coolingof coolingof milk can be achieved by implementing techniques like can immersion, in-can cooling, rotor freeze, bulk milk cooler, plate chiller, tubular tubu lar cooler, cooler, brine brine cooling cooling,, and ice cooling. cooling. Milk storage storage
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Milk: Processing of Milk
Yoghurt Fermented products Fermented milk products /Beverage
Heat desiccated products
Concentrated milk Condensed milk
Stirred yoghurt Low-fat yoghurt Sweetened or flavored Strained yoghurt Kumiss Kefir Acidophilus milk Bulgarian buttermilk Khoa
Burfi, Peda, Gulabjamun
Sweetened condensed milk Sweetened condensed skim milk Unsweetened condensed milk Unsweetened condensed skim milk
Clotted cream
Cheese
On basis of manufacture
Coagulated products Whole milk
On basis of texture
Clarified butter fat products
Chhana Butter oil Butter Butter milk
Frozen products
Ice cream Frozen yoghurt
Dried products
Figure 1
Pressed Cheeses (Mimolette) Semi-Pressed Cheeses (Cheddar) Pressed and Cooked Cheeses (Gruyère) Hard (Cheddar) Medium-hard (Gouda) Soft (Port Salut)
Whole milk powder Skim milk powder Butter milk powder Whey powder Butter powder Ice cream mix powder Cheese powder Infant milk powder
Milk and milk products.
temperature is generally kept low, as it will minimize the bacterial growth andthereby avoidmilk spoilage, whichis very high if the milk is stored at 30–40 C, as it is the optimum growth temperature for most of the microorganisms present in milk. Quality assessment is the foremost requirement as soon as the milk is received. It includes a platform test like organoleptic (taste, smell, and visual), temperature, clot on boiling (COB), sediment test, neutralizer test, and alcohol-alizarin test, which areperformed on thedock itself and are indicators of thequality of the milk. Based on the results of platform tests, a decision is taken to accept or reject the milk. After acceptance of the milk, tests are further conducted in a laboratory that will include determination of fat, SNF, acidity, methylene blue reduction test (MBRT), microbial counts (direct microscopic, standard plate count, and presumptive coliform test), and tests for adulterants that ensure the quality of raw milk. Milk is further pumped into large storage insulated tanks (silos) in which milk is stored at temperature less than 4 C until it is further pasteurized or sterilized, depending on the requirement.
Standardization Standardization involves adjustment of the fat/SNF content of milk, or a milk product, by addition of cream or skim milk as appropriate to obtain a given fat/SNF content. Generally raw milk is standardized before processing, and it also depends on regulatory authorities, as standardization is not allowed in some countries. Depending on scale of automation in the dairy, either direct inline or batch standardization can be carried out. Milk products that are utilized for adjusting fatin milk include cream, butter oil, and whole milk powder, and for increasing the SNF content skim milk powder is often used.
Preservation Techniques Because milk is a rich source of nutrients, it is susceptible to growth of microorganisms if not processed or stored at a low temperature. Second, to meet the demand of the urban population and to supply the milk in the lean season, milk needs to
Milk: Processing of Milk
be dried and stored in the flush season. With the growing demand for milk and milk products and changing lifestyles, it is necessary to increase the shelf life of the milk not only by days or months but up to years while maintaining the nutrients present. Extension of shelf life can be achieved by implementing one of these techniques: lowering the temperature (chilling, freezing), heat treatment (pasteurization, sterilization, UHT), removal of moisture by concentrating (evaporation, drying, sweetening, or salting), modified atmosphere (removal of oxygen), or microfluidization and cold pasteurization techniques (HPP, pulse electric field, oscillating magnetic field and irradiation; Figure 2).
Pasteurization Louis Pasteur in 1860 demonstrated that heat treatment (50–60 C) killed most of the spoilage microorganisms in fruit juices. In the case of milk, pasteurization means treating every particle of milk with a time–temperature combination effective to destroy bacteria ( Mycobacterium tuberculosis, Coxiellaburnetii and Listeria monocytogenes as indicator) without altering nutritional status, as well as color and flavor. At the industrial level batch pasteurization or low temperature for longer time (LTLT) and high temperature for short time (HTST) are carried out at 63 C for 30 min and 72 C for 15 s, respectively, which is immediately followed by cooling
to 5 C or below. LTLT involves a water jacketed vat, waterspray, or a coil vat pasteurizer. LTLT has benefit of low equipment cost, but the energy and labor requirement is high compared to HTST. LTLT impairs creaming properties due to excessive agitation that leads to churning. The HTST pasteurizer provides a continuous flow of milk, occupies less floor area, uses automation (clean-in-place [CIP] cleaning), and requires low operating cost (effective regeneration). Other time–temperature combinations are 89, 90, 94, and100 C for 1, 0.5, 0.1, and 0.01 s, respectively (US code of federal regulation). Regeneration allows heating of incoming milk and cooling of outgoing milk, where 80% of pasteurization temperature is achieved and 94–95% of heat from the outgoing milk is recovered. In HTST, plates are generally used for exchanging the heat and are made of stainless steel (SS type 320) of 0.8–2.0 gauge with a gap of 2.5–4.0 mm between two consecutive plates. Apart from the fact that copper has an excellent heat transferring capacity of 218 BTU h1 ft 1 F1 as compared to stainless steel (10 BTU h1 ft 1 F1), it is not used in fabrication of plates as it may produce off flavor by catalyzing the oxidation of milk fat. The heating medium used for heating of milk may be steam or hot water, and for cooling of milk, glycol, brine, water, or ice water can be used. Capacity of the dairy plant using plate heat exchanger can be as high as 1 00 000 L h 1, which can be adjusted by increasing or decreasing the size of
Receiving raw milk Cooling to 5 C
Preheating (35 − 40 C)
Filtration/Clarification Cooling to 5 C
Standardization and storage (5 C)
Preheating (60 C)
Homogenization (2500 Psi) (60 C)
Ultra-high temperature (135 − 150 C/1 − 2 s)
Filling and capping
Pasteurization (LTLT/63 C for 15 min)
Pasteurization (HTST /72 C for 15 s)
Aseptic Packing
Sterilization (108 − 121 C/25 − 30 min)
Cooling
Cooling
UHT milk
Sterilized milk
Pasteurized milk in finished tank
Pasteurized milk in finished tank
Cooling
Cooling
Packing
Packing
Storage /Distribution
Storage /Distribution
Storage/Distribution
Storage/Distribution
Figure 2
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A flowsheet for the manufacture of pasteurized liquid milk, sterilized milk, and UHT milk.
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the plate and the number of plates. In the plate heat exchanger, the heat of the pasteurized milk is utilized to warm the incoming cold milk that is simultaneously precooled. This in turn saves the cost of heating and refrigeration, and this capacity of the plate exchanger is called regenerative heat exchange or, more commonly, heat recovery. The regeneration section is divided into different subsections with the help of a block, and these blocks are further connected to processes like homogenization, clarification, microfiltration, and separation. Shell and tube heat exchangers are used for pasteurization of highly viscous products like cream, and an added advantage of shell and tube heat exchanger over plate heat exchanger is of low maintenance.
UHT UHT processing of milk involves heating milk in a continuous process to temperatures higher than 135 C for a few seconds, cooling rapidly, and aseptically packaging in the milk into sterile containers. UHT treatment must be sufficient to produce a commercially sterile product in which bacterial growth will not occur under normal storage conditions, but not severe enough to cause chemical changes that result in an unacceptable flavor, color, and nutritive loss. In general, the heating should be equivalent to a minimum of 9 log reduction of thermophilic spores (referred to as a B* value, >1) and a maximum reduction of 3% in the level of thiamine (referred to as a C* value, <1). UHT was invented in 1960, and UHT-treated milk was commercially available in 1970 with an objective to reduce nutritional loss caused by in-container sterilization. UHT-treated milk is generally packed in paperboard made of six different layers. A polyethylene layer is used for sealing in the milk and also acts as a moisture barrier; aluminum foil acts as an oxygen barrier and protects from off-flavors and light. UHT-treated and aseptically packed milk can have a long shelf life extending up to 6 or even 9 months without any preservatives at ambient temperature. The disad vantage associated with UHT treatment of milk is that in many instances the milk appears to have a cooked taste with a slight to dark brown color that may be due to Maillard browning. Development of sulfurous flavor is also associated with UHT milk, which is developed on thermal treatment as a result of unfolding of whey proteins. Milk with a high preprocessing microbial count is more susceptible to gel formation than milk with a low count. Microorganisms that produce heat-stable enzymes cause the most serious gelation problems. Longer refrigeration times prior to sterilization allow increased growth of psychotropic microorganisms and concomitant production of heat-stable enzymes, especially proteinases and lipases. Two main types of UHT processes are used commercially: direct heating, in which milk comes into directcontact with the heating medium, that is, steam; and indirect heating, in which the heating medium, steam or superheated water, is separated from the milk by a stainless steel plate or wall of a tube. In direct systems, heating to the specified temperature and cooling from the high temperature is very fast due to transfer of the latent heats of condensation and evaporation, respectively, between the steam and liquid milk. A plate heat exchanger used in UHT processing is able to withstand high temperature and internal pressure as compared to a pasteurizer. A plate heat
exchanger provides more surface area for heat transfer, low cost, and simple design. Tubular heat exchangers are more commonly used for thick liquids; do not need to acquire an aseptic homogenizer, as they can withstand 200–300 bar pressure required during homogenization; and by employing homogenizer valve will serve the purpose as a source of contamination in a homogenization pump thus eliminating the requirement of a pump. The rate of fouling of tubular heat exchangers is less as compared to a plate heat exchanger that has narrow channels for milk flow. Cooling to around 20 C is achieved in a vacuum chamber. Direct heating requires sterilization temperatures 3–4 C higher than indirect heating to achieve an equal sterilization effect because of the greater heat input during the heat-up phase of indirect heating. As compared to direct heating systems, indirect systems have a low heat transfer rate and a higher rate of deposition over the surface that requires frequent cleaning, but less processing and operational cost (less pumps and accessories, regeneration of energy) and require less controls. To exploit the benefits of direct as well as indirect processes, systems using a combination of direct and indirect heating are available commercially (e.g., the High-Heat Infusion system of APV88 and the Tetra Therm Aseptic Plus Two system of Tetra Pak). These achieve better energy economy than conventional direct systems (due to greater heat regeneration) and cause less chemical damage, especially flavor change, than conventional indirect UHT systems. Some of the adversely affected nutrient components in milk after UHT treatment are ascorbic acid, folacin, vitamin B12, vitamin B6, thiamine, and riboflavin up to an extent of 15–25%, 10–20%, 0–30%, 0–10%, <10–18%, 0–10%, respectively. Fat-soluble vitamins, proteins, and amino acids are affected to a much lesser extent.
Sterilization In-can sterilization of milk is usually carried out above 100 C with a time–temperature combination of 121 C for 15 min at 15 psi. The sterilization process may be a batch (autoclaves) or continuous process (vertical hydrostatic towers or horizontal sterilizers) and can be carried out in either cans or glass bottles, which are prefilled and hermetically sealed. Sterilization effect is achieved by using hot water or saturated steam, which transfers the heat to milk and milk products through the walls of the container. The main objective of sterilization is to kill vegetative cells and endospores or reduction of 12 log cycles (12 D reduction) of Clostridium botulinum by using a suitable time–temperature combination. The fact that sterilization takes place after bottling eliminates the need for aseptic handling, but on the other hand, heat-resistant packaging materials must be used. Sterilized milk usually has a creamy appearance and an extended shelf life of more than 6 months as compared to a few days for pasteurized milk. Rendering milk to high temperature with a long holding time often results in burnt flavor and brown color, which may be a result of the Maillard reaction. Popularity and consumption of sterilized milk declined after introduction of UHT processed milk. Nutrient loss is a major concern associated with sterilization, as it is reported by various scientists that almost 50% of vitamin C and 30–40% of thiamine and vitamin B12 are lost, but
Milk: Processing of Milk
components like vitamin A, riboflavin, carotene, and nicotinic acid remain unaffected.
Homogenization and Microfluidization Homogenization of milk has become a standard process that is applied as a means of stabilizing the fat emulsion against gravity separation. Gaulin, who invented the process in 1899, described it in French as ‘fixer la composition des liquides.’ Homogenization is carried out by using a homogenizer that pumps the milk under pressure through an orifice of 0.1 nm to disintegrate the fat globules to less than 2 m m. The disintegration of the milk fat globules is achieved by a combination of contributing factors such as turbulence and cavitation that do not allow visible cream separation in milk and also increase the mouthfeel, color, and digestibility. Homogenizers used in the milk industry are either single-stage or two-stage, depending on the number of homogenizing devices. Single-stage homogenization is used to homogenize products with a high viscosity, whereas two-stage homogenization is applied for products with a high fat content or where high homogenization efficiency is desired. Homogenization temperatures normally applied are 60–70 C, and homogenization pressure is between 10 and 25 MPa (100–250 bar), depending on the product. In a two-stage homogenization, pressure of 2000 psi and 500 psi is applied at the first stage and second stage, respectively. Pressure at the first stage reduces fat globules size and increases viscosity, whereas a second stage is used to prevent fat cluster reformation. The purpose of homogenization of milk is to stabilize the milk fat emulsion, as during storage and distribution a creamy layer appears at the top surface as a result of separation under gravitational force. Homogenized milk cannot be efficiently separated and also on many occasions it leads to development of sunlight flavor. Moreover, the homogenized milk is also reported to have low heat stability, and also the milk is not suitable for production of semihard and hard variants of cheese, as the coagulum formed will be too soft, which will make it vulnerable during dewatering. Homogenized milk is more sensitive to light and enzymatic activity of lipase, so pre-heat treatment is recommended before homogenization of milk to inactivate lipase. Microfluidization is a novel technique, using high pressure up to 241 MPa by combining high velocity, shear, turbulence, and collision inside the chamber for a short duration of 5 s to obtain high-quality products by breaking down large particles. It is a continuous process that has a chamber for fluid flow, in which pressure stream is forcibly directed to flow into two different fine orifices. At the point of intersection, both splited microstreams clash into each other at high speed, resulting in the reduction of large-size particles to very small sizes. Breakdown into fine particles inside the interaction chamber occurs due to high speed, high shear, cavitation, air compression, and coerces of the microstreams. The milk obtained after microfluidization is more homogenized and viscous when compared with commercially available homogenized milk using a two-stage homogenizer. Microfluidized milk, when used for cheese preparation, can improve textural and melting characteristics and also lower the microbial population. Yogurt prepared from homogenized milk was comparable to low-fat (1.5%) microfluidized yogurt in terms of texture profile and
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water retention property. Some of the characteristics associated with microfluidization include chances of contamination, hard to scale up the production rate, loss of product, and encapsulated material structure due to high shear and striking coerces during microfluidization.
Novel Nonthermal Milk Processing Techniques Consumers prefer products that are minimally processed; have natural aroma, fresh taste, and no additives; are stable; are convenient to use; and are microbiologically safe. Although thermal processing (heating or cooling operation) maintains safety of food, it adversely affects nutritional value. However irradiation, pulse electric field, and HPP are alternatives to traditional thermal processing. Technologies such as ohmic heating, dielectric heating (which includes microwave heating and radio-frequency heating), and inductive heating have been developed that can replace, at least partially, the traditional heating methods that rely essentially on conductive, convective, andradiativeheat transfer. They allhave a common feature: heat is generated directly inside the food and this has direct implications in terms of both energetic and heating efficiency. Among these nonthermal techniques, HPP of milk is well studied although other techniques has attracted little attention and commercial application but has considerable potent future.
•
High Pressure Processing
HPP is a technique that involves the application of pressure in the range of 100–900 MPa, which is instantaneously and uniformly transmitted throughout the product irrespective of its size and shape. The principle of HPP is based on Le Chatelier’s principle (with increase in pressure, decrease in volume occurs) and isostatic transfer (uniform transmission of pressure in all sides). Pressure of 300–600 MPa breaks ionic and electrostatic bonds but does not disrupt covalent bonds. HPP processing of milk usually has least effect on color, flavor, and nutritional value, but is effective enough to reduce microbial load as well to enhance rennet or acid coagulation of milk. Milk processed at a pressure of 600 MPa was found to enhance the shelf life of milk by reducing the microbial population, and factors that govern the microbial reduction are duration of exposure and target species, as well as composition. Microbial quality of HPP (400–600) treated raw milk was found to be comparable with pasteurized milk (72 C for 15 s), and treatment at 400 MPa for 15 min and 600 MPa for 3 min is effective enough to increase the shelf life at 10 and 20 C, respectively, for 10 days.
•
Pulse Electric Field (PEF)
PEF has proven to have a potential for increasing the milk shelf life, as it is highly applicable to liquids. PEF had shown minimal effects on the nutritional, flavor, and functional characteristics of milk and also has an ability to inactivate microorganisms. PEF is based on the application of pulses of high voltage (typically 20–80 kV cm-1) delivered to theproduct placed between a set of electrodes that confine the treatment gap of the PEF chamber. The process can be conducted at ambient temperature for a fraction of second, and energy loss due to the heating of foods is minimized. Preservation of milk and fluid dairy products seems to be one of the main market
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Milk: Processing of Milk
niches for PEF technology because it is mainly intended for preservation of pumpable fluid or semifluid foods. When self-inoculated (Salmonella) milk was treated with PEF (36.7 kV cm1 and 40 pulses, 25 min) and stored at 7–9 C for 8 days, the level of Salmonella was below detection limit. PEF-treated cheese milk has not shown any significant changes on physicochemical characteristics and quality of cheese. No adverse effect of PEF on water-soluble vitamins in milk (riboflavin, thiamin, and ascorbic acid) and fat-soluble vitamins (cholecalciferol and tocopherol), after applying treatments of up to 400 m s at field strengths from 18.3 to 27.1 kV cm1, was found except for ascorbic acid. As compared to LTLT- and HTST-treated milk, PEF-treated milk retained more ascorbic acid after a 400 m s treatment at 22.6 kV cm 1 (93.4%) than either a LTLT (49.7%) or HTST (86.7% retained).
• Other Techniques Milk irradiation is a process of exposing milk to ionizing radiation (gamma rays, beta-particles), emitted by a source (cesium and cobalt) to destroy the spoilage-causing microorganisms. Treatment of milk with 2–5 kGy reduced bacterial population by around 99%, but to achieve complete sterilization of milk, a dosage of 10–20 kGy is required. A hurdle in irradiation of milk is development of off-flavor and color change. Irradiation can be effectively used for sterilization of packaging material used for packing of milk and milk products. Ultrasound waves are similar to sound waves buthave a frequency that is above 16 kHz. It produces very rapid localized changes in pressure and temperature that cause shear disruption, ‘cavitation’ (creation of bubbles in liquid foods), thinning of cell membranes, localized heating, and free radical production that have a lethal effect on microorganisms. A combined heat and ultrasound treatment under pressure was termed ‘manothermosonication’ (MTS) and can be used for enhancing the shelf life of milk. Unfortunately, ultrasound does not inactivate alkaline phosphatase or lactoperoxidase enzymes and hence cannot be used to indicate a successful ultrasonic treatment. If ultrasonication is to be used as an alternative to thermal pasteurization, a need exists to find a quick and efficient method to indicate whether ultrasonication was sufficient in terms of ensuring a microbiologically safe product.
Packaging, Storage, and Distribution Packaging is a medium between product manufacturers and consumers aiding the maintenance of original quality of the packaged products and providing information and characteristics of the packaged products to the consumers. The functions of packaging are to enable efficient food distribution, maintain product hygiene, protect nutrients and flavor, and reducespoilage and waste. Immediately after heat treatment, milk products need to be packaged to avoid postprocess contamination that will affect the shelf life. In earlier days returnable glass bottles were used for packaging and distribution of milk, but with technological advancement plastic bottles, tetra packs, laminates, and sachet are now preferred over glass bottles. Before a packaging material is selected for milk and milk products it is important to know the type, composition, compatibility, conditions of storage, and transportation
(light, temperature, humidity, mechanical stress during transportation, and storage). The ideal packaging material for milk shouldbe an inert,nontoxic, safe, andhygienicdesign and should not directly or indirectly impart smell. Packaging material should be well labeled with manufacturing, expiration date, storage conditions, and ingredients. Single-use packages are used for packaging of fluid milk that is predominately plastic pouch (PE, LDPE, HDPE, polyvinylchloride) that has a good moisture barrier properties but poor recyclability and disposal complications. Cartons are generally used as a secondary storageof milk; paper andpaperboardsmaterials include laminates consisting of outer kraft paper with PE coatings and aluminum foil. The advantage of single-use packaging material is that they decrease the use of detergents, which is required for washing returnable glass bottles. Apart from sterilizing the milk and milk products and also the packaging material, sterilization of equipment, line, and filling area can enhance the shelf life of milk. Aseptic packaging is being utilized in developing countries for presterilized packages under sterile environment into sterile packaging material. For storage and distribution of milk, insulated tanks, cans, bottles, tetrahedral package, paperboard cartons, and automatic vending machines are used. The most important factor to be considered during the storage and distribution of milk is the temperature of storage, which should be kept at 5 C. Hygienic conditions must be maintained throughout the supply chain so that the milk products are not cross contaminated and are also free from defects like off flavor due to exposure to chemicals.
Quality Assurance and Quality Control Quality assurance is an important element for safe milk and milk products from any of the contaminants that can render the product unsafe for consumers. Addition of water by deceitful middlemen is one of the challenges faced by the dairy industry in developing countries. To provide a quality milk and milk product it is of utmost importance to start the process by procuring good-quality raw milk and maintaining strict quality measures at different checkpoints (collection, transportation, receiving dock, after processing, and during dispatch), including transportation and storage. Quality testing includes a platform test at the receiving dock and physicochemical cum microbiological analysis in the processing (before–after) industry and should follow up the standards as laid out by regulatory agencies/bodies. To ensure the quality, dairy industries should follow good manufacturing policies (GMP), hazard analysis critical control points (HACCP), and ISO:22000 (2005) based programs, which will help in controlling the quality by identifying and reducing the hazards within a safe limit for consumers. Other key factors that control quality are hygiene, safety, adequate thermal treatment during manufacturing, packaging, and postprocess contamination.
Steps Where Preventive Measures Are Required to Maintain Quality
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Good and hygienic milking practices must be carried out at the farm level that can minimize the proliferation of microorganisms from environment, dirt, soil, feces, human beings, and equipment into milk.
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•
Utensils and equipment used for milking the animals must be well sanitized and preferably manufactured using stainless steel. • Milk mulched from mulching animals suffering from mastitis and other diseases must not be mixed with a batch of good raw milk. • Longer storage temperatures of rawmilk must be avoided as it allows few psychrotrophic organisms (L. monocytogenes and Pseudomonas species) to grow that in return can secrete enzymes like proteases and lipases, which produces defects like sweet curdling in UHT-treated milk. • Monitoring the milk for adulteration with adulterants including urea, sugar, neutralizers, and nondairy proteins. • Routine monitoring must be done for critical control points (time–temperature) for pasteurization and sterilization that can be evaluated by conducting an alkaline phosphatase test (pasteurization) and turbidity test (sterilized). • Monitoring the sanitation and CIP programs before processing and after processing to ensure that the residual milk in pipeline/equipment is removed and they are in sanitized condition, which will not support the further growth of spoilage bacteria. • Hygiene conditions must be maintained during processing and also after packaging of milk to avoid postprocess contamination. Usage of unsterilized packaging material for packing of • milk and milk products must be avoided. • Temperature fluctuation during the storage and distribution of milk (collection center, transportation, storage distribution) must be avoided to maintain the product safety. Good farming practice, animal health management, control of feed, hygienic milking operations, inspection, quality analysis, sanitation, and hygiene during manufacturing as well as packaging and effectively monitoring every parameter during processing will help in reducing microbial load to render products safe for end consumers.
See also: Acidophilus Milk; Cheese: Chemistry and Microbiology ;
Fermented Foods: Composition and Health Effects ; Fermented Foods:
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Origins and Applications ; Fermented Foods: Use of Starter Cultures ; Functional Foods; Lactic Acid Bacteria.
Further Reading Antonio JT, Marta C, Jordi S, Ramon G, and Buenaventura G (2002) Applications of high-hydrostatic pressure on milk and dairy products: A review. Innovative Food Science and Emerging Technologies 3: 295–307. Chavan R, Kumar A, Mishra V, and Nema PK (2014) Effect of microfluidization on mango flavoured yoghurt: Rheological properties and pH parameter. International Journal of Food and Nutritional Sciences 3: 84–90. Datta N and Deeth HC (1999) High pressure processing of milk and dairy products. Australian Journal of Dairy Technology 54: 41–48. Holsinger VH, Rajkowski KT, and Stabel JR (1997) Milk pasteurisation and safety: A brief history and update. Revue Scientifique et Technique (International Office of Epizootics) 16: 441–451. Jafari SM, He YH, and Bhandari B (2006) Nanoemulsion production by sonication and microfluidization – A comparison. International Journal of Food Properties 9: 475–485. McCrae CH (1994) Homogenization of milk emulsions – Use of microfluidizer. Journal of the Society of Dairy Technology 47: 28–31. Naik L, Sharma R, Rajput YS, and Manju G (2013) Application of high pressure processing technology for dairy food preservation – future perspective: A review. Journal of Animal Product Advances 3: 232–241. Rattray W, Gallmann P, and Jelen P (1997) Nutritional, sensory and physico-chemical characterization of protein-standardized UHT milk. Lait 77: 279–296. Sı´lvia B, Barbosa-Ca´ novas GV, and Olga M (2002) Milk processing by high intensity pulsed electric fields. Trends in Food Science and Technology 13: 195–204.
Relevant Websites http://www.dairyco.org.uk/ . http://www.dairyco.org.uk/market-information/dairy-sales-consumption/liquid-milkmarket/liquid-milk-market/#.VFmq3TSUflE. http://www.fao.org/docrep/004/T0045E/T0045E02.html. http://www.fil-idf.org/Public/PublicationsPage.php?ID¼27121&highlight¼true. http://www.fonterra.com/global/en. http://www.idmc.coop/ . http://www.tetrapak.com/in/products-and-services/processing-equipment/dairyequipment.
