PASTEURIZATION

 A. Introduction  

Pasteurization is a process of heat treatment used to inactivate enzymes and to kill relatively heat-sensitive microorganisms that cause spoilage with minimal changes in food properties (e.g., sensory and nutritional). It is also defined as mild heat treatment for avoiding microbial and enzymatic spoilage. It is used to extend the shelf life of food at low temperatures (usually 4°C) for several days (e.g., milk) or for several months (e.g., bottled fruit). Heating liquid foods to 100°C is employed to destroy heat-labile spoilage organisms, such as non-spore-forming bacteria, yeast, and molds.

 B. Purpose of Pasteurization  

The primary objective of pasteurization is to free the food of any microorganisms that might cause deterioration or endanger the consumer's health. The severity of the heat treatment and the resulting extension of shelf life are determined mostly by the pH of the food. In low-acid foods (pH > 4.5),  the main purpose is the destruction of pathogenic bacteria, whereas below pH 4.5, destruction of spoilage microorganisms or enzyme inactivation is usually more important.  Pasteurization does not aim at killing spore-bearing organisms, such as the thermophilic Bacillus subtilis, but these organisms and most other spore-bearing bacteria cannot grow in acidic fruit juices, and consequently their presence is of no practical significance. Pasteurization of carbonated juices need only be conducted at such a temperature and for such a time that yeasts and molds are destroyed. Yeast is killed by heating at 60-65°C and the resistance mold spore negative in most cases a temperature of 80 C for 20 minutes. But molds require oxygen for growth and for this reason heavily carbonated juice can be pasteurized safely at 65°C which destroy yeast cells, most still (non-carbonated) juices must be pasteurized at 80°C. Juices of high acidity may be pasteurized at lower temperatures (60-65°C). Processing containers of food that have a naturally low pH (e.g., fruit pieces) or in which the pH is artificially lowered (e.g., pickles) is similar to canning. In acid products like tomatoes, mangoes, bananas, etc. (pH 4.04.4), yeast, molds, and bacteria (both thermophilic and mesophilic) can grow. The main risk of spoilage is from spore-forming species other than Clostridium botulinum, especially B. coagulans among the aerobes and Clostridium pasteurianum and Clostridium thermosaccharolyticum among the anaerobes. In high-acid foods (pH < 3.9) like pineapple juice, spoilage is generally caused by non-spore-forming bacteria (Lactobacillus and Leuconostoc), yeast, or molds [1].  Fruits with a pH lower than 4.5 contains enzyme systems such as catalase, peroxidase, polyphenol oxidase, pectin esterase, etc., in addition to spoilage organisms. Unless inactivated, these enzymes are likely to cause undesirable changes in the canned products. Some of these enzymes, particularly peroxides, have higher heat resistance than the spoilage organisms and have been used in evolving the thermal processing of canned fruits.

 C. Types of Pasteurization  

There are several types of pasteurization:
1. In-package pasteurization: Inside packages, heating to the level of sterility is not required. A gradual change in temperature is preferred in some containers.
2. Pasteurization prior to packaging: Preheating is good for foods that are sensitive to high temperature gradients.
3. Batch pasteurization: This is also called the low temperature, short time process. Here fluid foods like milk are held in a tank where they are heated to 62.8°C for 30 minutes. A batch pasteurizer consists of a steam-jacketed kettle or a tank equipped with steam coils in which the juice or milk is heated to the desired temperature [4].
 4. Continuous pasteurization: This is also called the high temperature, short time process. Foods like milk is subjected to 71.7°C for about 15 seconds or more by flowing through different heat exchangers. In continuous pasteurization generally plate heat exchanger, tubular heat exchanger, scraped surface heat exchanger are used depending on the viscosity of the fluid food material. The heating medium is usually steam or water.

 D. Achieving Desired Pasteurization  

Broadly, pasteurization can be achieved by a combination of time and temperature such as (a) heating foods to a relatively low temperature and maintaining for a longer time, e.g., holding pasteurization and pasteurization by overflow method, or (b) heating foods to a high temperature and holding it for a short time only. Pasteurization can be performed in two ways: (a) by first filling sterile containers with the product and then pasteurizing or (b) by pasteurizing the product first and then filling sterile containers.
 
  E. Pasteurization Equipment  

 1. Pasteurization of Packaged Foods  

In packaged foods like beer and fruit juices, in-container processing is applied. When the container is glass, generally hot water processing is used to reduce any damage due to thermal shock. After processing, the container is cooled to 40°C, which also facilitates evaporation of the surface water. This minimizes external corrosion of metal containers or caps and accelerates the setting of adhesives used in labels.

 Water Bath Pasteurization  

For acidic food products that can be adequately pasteurized at temperatures of 100°C or below, a water bath is one of the simplest methods of heating for pasteurization. The water bath may be either a rectangular steel tank or a vertical retort. The product is packed in retort crates or in racks and immersed in the bath for pasteurization. Cooling may be carried out in the same tank used for heating, or the containers may be moved from the heating tank to a cooling tank. Heating and cooling also may be carried out in steps. Essentially the same procedure is followed for processing of meats, pickles, applesauce, and other acidic food products [2]. The continuous water bath is an improvement over batch operation and is used by both pickle processors and fruit canners for pasteurization where high production rates are required. A conveyor belt moves through the tank at a selected speed to provide adequate time in the bath to accomplish pasteurization. The tank is usually divided into sections, each of which is heated and controlled separately.

 In continuous water bath pasteurizers, the jars and cans must proceed down an incline into the tank and up an incline when they come out of the tank. Since there is considerable hazard in conveying glass containers up or down an incline, plants that pasteurize glass-packed products usually use water spray or steam pasteurizers [2].

Continous Steam or Water Spray Pasteurization  

The continuous water spray pasteurizer is extensively used for pasteurizing beer and acidic food products. In this type of unit, bottles or cans are conveyed through the pasteurizer either by a walking beam or by a continuous belt conveyor. It is common practice to have as many as six different temperature zones or sections throughout the pasteurizer to obtain maximum efficiency. These are first preheat, second preheat, pasteurizing zone, precool, cooling, and final cooling zone. Water spray units are designed so the water in the first preheat zone drains off the jars in the precool zone, and the water that is sprayed in the precool zone is used in the first preheat zone. In this way a considerable amount of heat is recovered and reused and a reduced amount of cooling water is required. Cooling water is also recirculated [2]. Glass containers should not be subjected to excessive thermal shock; when heating products in glass containers it is recommended that the thermal shock temperature difference be kept below 21°C and under no condition to exceed 38°C. When cooling a hot product in a glass container, temperatures are more critical; 10°C is a desirable maximum and under no conditions should the temperature change exceed 21°C.  Several sections are necessary in both continuous steam and water spray pasteurizers to heat glass containers efficiently; however, metal containers may be pasteurized in the same equipment. Through the use of sectionalized equipment, it is possible to have high-temperature heating and low-temperature cooling of glass containers with a minimum amount of thermal shock breakage. The water spray-type unit has been very successful in the pasteurization of beer and similar products where the operation proceeds under ideal conditions.
The steam pasteurizer is simply a tunnel open at both ends with a conveyer along the bottom. Cloth baffles are hung between each section, but these are not adequate to hold the steam in the pasteurizer against strong air currents. The rate of heat transfer from the steam-air mixture to the food container is not constant in the steam pasteurizer, but varies with steam temperature and steam velocity.

Tunnel Pasteurization  

 Hot water sprays are used to heat containers as they pass through the different heating zones of the tunnel and provide an incremental rise in temperature until pasteurization is achieved. Cold water sprays then cool the containers as they continue through the tunnel. Steam tunnels have the disadvantage of faster heating, giving shorter residence time, and smaller equipment. Savings in energy and water are achieved from heat recovery from the hot product and recirculating water. Temperatures in the heating zone are gradually increased by reducing the amount of air in the steam-air mixtures, and cooling takes place using water sprays or by immersion in a water bath.

 2. Pasteurization of Unpackaged Liquids  

 Long-Hold or Vat Pasteurizing  

Vat or tank-type heat exchangers are used for the long-hold method of pasteurization. Here the raw product is pumped into the vat, heated to the pasteurizing temperature, held for the required time, and pumped from the vat through cooling equipment. With most vat pasteurizers circulation of the heating medium can be started as soon as the filling of the vat is begun. In this way some heating of the product takes place during filling so that the heating time can be shortened. With some designs of pasteurizing vats, cold water can be circulated over the outside of the inner liner as soon as the holding period is completed, thus doing part of the cooling in the vat [3].  It is considered good practice with all heat-exchange equipment for dairy products to use a heating medium (hot water or steam vapor) only a few degrees warmer than the milk, resulting in less accumulation of milkstone on heating surfaces and less danger of injury to cream line or flavor.

Advantages: Vat pasteurizers are well suited for small plants and for low-volume products in larger operations. They can handle a variety of products with a wide range of physical characteristics. They are especially well adapted to the processing of cultured products such as buttermilk and sour cream, which, in addition to being pasteurized and cooled, require mixing for the incorporation of starter, several hours of quiescent holding for incubation, agitation for breaking the curd, and final cooling in the tank.

Disadvantages: There are several disadvantages to consider. Vat pasteurization is normally a batch operation and is inherently slow, although the flow can be made continuous by the use of three or more vats (depending upon the holding, heating, filling, and emptying times). The operation may even be made automatic by use of complex and expensive controls. In the great majority of batch operations, manual controls are used, and constant attention must be given by the operator to prevent overheating, overholding, and burning. Another disadvantage is that regenerative heating is not possible in the vat, so both heating and cooling is relatively expensive.

Heat Exchanger Pasteurizer  

 Small-scale batch pasteurization is carried out in open boiling pans or in scrapped surface heat exchangers. Generally less viscous liquids are pasteurized by plate heat exchanger. Some products, such as fruit juices and wines, require deaeration before pasteurization to prevent oxidative changes during storage. This can be achieved by spraying liquids into the vacuum chamber after which dissolved air is removed.  The plate heat exchanger consists of a series of thin vertical stainless steel plates. The plates form parallel channels held tightly together in a metal frame and separated by rubber gaskets to produce a watertight seal. The plates are corrugated to induce turbulence for a high heat transfer rate. The  advantages of heat exchangers over in-bottle processing include (a) more uniform heat treatment, (b) simpler equipment and lower maintenance costs, (c) reduced space requirements and labor costs, (d) greater flexibility for different products, and (e) greater control over pasteurization conditions. A number of systems for pasteurizing milk have been used commercially. The first were batch systems employing holding tanks. Milk was heated in a jacketed tank to a temperature of 63°C and held for 30 minutes. This type of system is now rarely found, but it can be suitable for small opera tions. Improvements on the batch system came with the advent of the continuous-holding or retarding systems. Holding times and temperatures are the same; however, the tanks automatically fill, hold, and empty in a timed cycle. The system of choice in most modern dairies is now the high-temperature, short-time (HTST) process. The heat exchanger has the following advantages over the batch and continuous-holding systems: (a) lower initial cost due to elimination of holding tanks, (b) less labor required as the system incorporates mechanized circulation cleaning, (c) saves space (about 10,000 liters/hr can be pasteurized in 4.5 m2), (d) increased flexibility (capacity of the plant and processing rate are easily controlled), (e) ease of recording and safeguarding pasteurization temperature requirements (milk can be readily diverted if it does not reach minimum safe pasteurized temperatures), and (f) lower operating costs (plant can be almost entirely automatically controlled). The capacity of the equipment varies according to the size and number of platesup to 80,000 liters/hr. Other types of heat exchangers are also used for pasteurization. In particular, the concentric tube heat exchanger is suitable for more viscous food and is used with dairy products, mayonnaise, tomato ketchup, and baby food. It consists of a number of concentric stainless steel coils, each made from double- or triple-walled tube. Food passes through the tube, and heating and cooling water is recirculated through the tube walls. Liquid food is passed from one coil to the next for heating and cooling, and the heat is regenerated to reduce energy costs. Pasteurized food is immediately deposited into cartons or bottles. Care with cleaning and hygiene is therefore necessary.

 High-Temperature, Short-Time Pasteurizers  

HTST pasteurizers are continuous-flow systems using tubular, plate, swept surface, direct steam, in conjunction with a timing pump, a holder, and controls for temperature and flow rate. The great majority of HTST pasteurizers use plate-type heat exchangers with sections for regenerative heating and cooling. This is also referred to as a flash pasteurizer [3]. Continuous pasteurizers assure that all of the product of an entire run receives uniform treatment. HTST pasteurizers employing regenerative heating are much more economical to operate than batch pasteurizers. In the application of controls, the general requirements for flow rate, temperature, and pressure must be considered, for these are the factors that govern proper operation and public health safety. Flow rate through a continuous pasteurizer is regulated by the metering or timing pump. A positive displacement pump of the rotary or piston type is used almost exclusively for milk and milk products. Often variable speed drives are employed so that the flow rate can be changed when desired. A continuous pasteurizer must include synchronization of holding time and flow rate.

Controlling temperature includes maintaining a uniform product temperature at some set value at or above the legal minimum and diverting the flow, directing it back through the system if, at the end of the holder, it is below the legal minimum temperature. Usually, a safety thermal limit-recording controller is used, which keeps continuous record of the temperature. The pressure is especially important in two areas of a continuous pasteurizerthe regenerator and the flow-diversion valve. Where product-to-product regeneration is used, it is necessary, for public health reasons, to maintain at least 7 kPa more pressure on the pasteurized side than on the raw side so that any leakage through the heat exchanger can be identified, thus eliminating the possibility for contaminating the pasteurized product. In order to prevent mixing of air into the product and inefficiency of the pump due to air leakage into the system, the entire system is operated at a positive pressure (above atmospheric pressure). A centrifugal booster pump is employed between the product storage tank and the regenerator. This will ensure that the pasteurized product is always under higher pressure than the raw product.  To facilitate this, it is necessary to (a) size the booster pump correctly to deliver the rated capacity at a predetermined pressure, and (b) equip the booster pump with a pressure-actuated switch located at the outlet of the pasteurized regenerator set so that the pump can run only when the pressure is at least 7 kPa greater than that on the raw-product side. If the cooler section does not produce enough back-pressure on the pasteurized regenerator to satisfy the minimum 7 kPa difference, it may be necessary to install a restrictor in the line. The other pressure requirement is needed on the diverted milk line, since it may affect the holding time during diversion. If, upon testing, the holding time during diverted flow is shorter than that of forward flow, a restricting orifice should be placed in the diverted line.

Flash pasteurization: The process of heating fruit juices for only a short time at a temperature higher than the pasteurization temperature of the juice is called flash pasteurization. In this method the juice is heated rapidly for about 1 minute to a temperature about 5°C higher than the pasteurization temperature, filled into airtight containers under steam to sterilize the steam, and then cooled. This process can also be used for orange juice, apple juice, grape juice, etc. The advantages of this process are that it (a) minimizes flavor loss, (b) aids in the retention of vitamins, (c) effects economy in time and space, (d) helps to keep the juice uniformly cloudy, (e) heats juice uniformly, reducing cooked taste to a minimum, and (f) effects beneficial enzyme inactivation in addition to destruction of viable microorganisms.  Deaerator and flash pasteurizer: Freshly extracted and screened juices contain an appreciable quantity of oxygen, which should be removed before packing. The special equipment used for this purpose is called a deaerator. The deaerated juice is then heated in flash-pasteurization equipment. Commercial deaerators and flash pasteurizers differ greatly in design, construction, and capacity. The deaerator and flash pasteurizer has been used successfully for fruit juices like tomato and pineapple and orange.

Ultra-High-Temperature Pasteurizers  

 The equipment for ultra-high-temperature (UHT) pasteurizers is much the same as for HTST units. The controls are similar, but the operating temperature points are higher. The holder is, of course, much smaller for minimum pasteurizing time. Generally, with a holding time in the order of 3 seconds, it is impossible to determine the holding time accurately by tests like those used with HTST pasteurizers, and calculated holding times are preferred [3].  Where ultra-high-temperature treatment is desired because of its greater bacterial destruction or its beneficial effects on body and texture in ice cream and where confusion exists regarding requirements for UHT pasteurization, a UHT treatment may be given following regular pasteurization. This may be accomplished with a direct-steam heater installed downstream from the flow diversion valve or with steam-vacuum flavor-treating equipment.

 The Vacreator

 The vacreator is a special type of pasteurizing apparatus used particularly in the butter industry. The product is fed into a steam-heated chamber where it is flashed at a temperature of 90-96°C under 13.530 kPa of vacuum. It then passes into a second chamber of higher vacuum (5067 kPa) and is reduced to a temperature of 72-82°C. From there it passes into a third high-vacuum chamber (9195 kPa) and is reduced to a temperature of 38-46°C. This process is claimed to be very effective for pasteurization and at the same time to remove undesirable odors and flavors. It also employs a continuous-type machine and is especially adapted for use on high-viscosity material, although it will also operate on plain fluid milk [3].

 F. Quality of Pasteurized Foods  

 1. Color, Flavor, and Aroma  

The main cause of color deterioration in fruit juices is enzymatic browning by polyphenoloxidase. This is promoted by the presence of oxygen, and fruit juices are therefore routinely deaerated prior to pasteurization. The difference between the color of raw and pasteurized milk is due to homogenization; pasteurization has no measurable effect. Other pigments in plant and animal products are also unaffected by pasteurization [1]. Even a small loss of volatile aroma compounds during pasteurization of juices causes a reduction in quality and may also unmask other cooked flavors. Volatile recovery may be used to produce high-quality juices, but it is not routine. Loss of volatiles from raw milk removes a haylike aroma and produces a blander product.

 2. Nutrients  

 In fruit juices, losses of vitamin C and carotene are minimized by deaeration. Changes to milk are confined to a 5% loss of serum proteins and small changes in the vitamin content. Most pasteurized food products have a low pH because either the natural pH of the system is low or the product has been fermented to produce an acidic environment. Since most heat-labile nutrients are relatively stable in acid conditions, nutrient losses in such products are relatively minor. Although  thermal losses during pasteurization may be small, oxidative losses can be high. Thus pasteurization of liquid foods such as fruit juices, beer, and wine is generally accomplished in indirect heat exchangers like plate or double tube rather than open film-type pasteurizers. Often fluids are deaerated prior to pasteurization. The most important nonacid liquid food is milk. The effect of pasteurization on the nutrients in milk has received considerable attention [5].

 G. Packaging of Pasteurized Foods  

Both bottles and cartons take into account the properties of milk and provide packaging acceptable to consumers worldwide. Glass bottles have the advantage of being easily cleaned, transparent, and rigid, but the great disadvantages of high weight and fragility. Increasingly, milk is also packaged in gable top (PurePak, Elopak) or other cartons (TetraBrik). Even though the equipment may be expensive to install, the advantages include a lower price per unit of milk and a lower risk of contamination from the air during filling [6]. Smaller quantities have been packaged in plastic pouches. A cylindrical milk carton with a reclosable pouring lid has been introduced [7]. While suitable for sterilized milk, glass bottles are a problem for long-life milk. The question of container is therefore of vital interest to the dairy industry, as about 50% of all milk produced is sold in liquid form [7]. Sunlight can destroy riboflavin and vitamin C in milk, producing a taint by the oxidation of fat [7] and protein. This led to the use of brown glass bottles, which hold back the light rays responsible. However, taint is very rare and brown bottles are not very attractive; it has also been found that milk becomes sour faster in brown than in colorless bottles [8].

 1. Returnable Bottles

For economic reasons, the use of the returnable glass bottles has continued over many years. The glass bottle will take a long time to disappear because of its economic advantage, the traditions of the industry, and the attitude of the consumer. Other factors such as transport costs have led to the use of nonreturnable packaging, although more recently green considerations have led to the reintroduction of returnable bottles [7,9]. The advantages of nonreturnable containers are (a) elimination of returned empties, (b) elimination of collecting, sorting, and washing, (c) elimination of foreign object problem, (d) elimination of glass fragment problem, and (e) reduction in transport costs. The disadvantages are (a) possible increases in costs of packages, (b) lack of consumer acceptance, (c) delivery problems resulting in lower total sales, (d) hygiene problems, and (e) environmental considerations. Plastic containers and plastic-coated cartons are nearly sterile by virtue of their method of manufacture. No sterilizing process is necessary for pasteurized milk, but for the aseptic filling of milk, sterilization is essential. So far, TetraBrik has proved most effective. Containers for this purpose must be sterilized immediately before filling.

 2. Glass Bottles

 The traditional glass bottles used for fruit juices and juice beverages provide many advantages. Glass is not susceptible to mold growth and is impermeable to odorous vapors and liquids. Hot-filling and in-bottle pasteurization are generally employed for pure fruit juices or products that do not contain preservatives. Any microbiological contamination on the inner surfaces of the bottle and the closure is destroyed by the hot liquid, and adequate sterility is obtained without heating the container [7]. Glass bottles can also be covered with a polystyrene shield, which enables them to be reduced in weight without risking breakage. Sleeves give protection and graphics can be added easily. Some bottles are shrink-wrapped with plastic sleeves.

 3. The PET Bottle and Other Plastic Containers  

PET bottles are displacing those made from PVC for products such as edible oils and mineral waters as well as glass bottles for carbonated products. Improvements in processing technology have resulted in the appearance of stretch-brown PVC bottles. Other forms of plastic container have also been used [10,11], e.g., the Plastocan, a coextruded plastic container with conventional aluminum easy-open can ends, and the Rigello container, a multilayered polypropylene foil extrusion with a spherical bottom and tear-off cap assembled in a paper-board cylinder. New combinations of materials in can form are also being developed. Coca Cola patented a PET/aluminum can with easy-open top [12]. High-barrier plastics cans that will be recyclable are under investigation. Orange juice has also been packed in clear polypropylene bottles, which provide good oxygen and moisture barrier properties [13]. Tamper-evident pull-tab closures are used on this container. Paperboard basket carriers, plastic clips (on bottle necks), and shrink films are used to provide multipacks holding three, four, or six units.

 4. Cans  

Fruit juices and fruit juice concentrates are frequently distributed in cans [14]. The most common of these are standard tin-plate containers, but specially lacquered and coated cans are also used, especially for high-acid products. Cans are usually hot-filled, but sometimes are aseptically filled. Cold-filling after pasteurization is occasionally employed, but refrigerated or frozen storage is then advisable. Products preserved with benzoic acid can also be filled cold after pasteurizing, but sulfated products are incompatible with cans. The juice tends to deteriorate in the cans due to corrosion and an increasing amount of tin and iron in the product. In the normal hot canning process, the juice is first deaerated to improve its flavor stability and then pasteurized to destroy microorganisms and inactivate enzymes. After hot-filling into the cans, the lids are applied and sealed immediately before cooling, which forms a slight vacuum in the head-space as the liquid contracts. This is desirable as the presence of oxygen encourages corrosion (cold-filling operations usually involve undercover gassing, in which the headspace is replaced by carbon dioxide immediately before sealing the lid). Carbonated beverages are susceptible to metal pick-up and are therefore packaged in lacquered two-piece aluminum cans or three-piece tin-plate with side seams having a special tab design to withstand the internal pressure. Warming the filled cans immediately before packaging is important, otherwise the cans when filled with cold carbonated liquid attract a layer of condensation from the atmosphere and may corrode on the outside. Frozen orange juice concentrate has been distributed in composite paperboard or plastic cartons of approximately 170 ml capacity [10]. There are many pack variations, including cartons with tear-off ends or left unpasteurized to provide maximum freshness of flavor. Spoilage may result if these are not frozen. Beverage cans are also sold in multipacks of four, six, or more [15]. The most common form of overwrap, which assists in handling and distribution, is a plastic ring carrier, which slips underneath the rim of the can and grips tightly throughout distribution. Paperboard multipacks are also popular, as are shrink wraps.

 5. Cartons

 Pasteurized fruit juice and soft drinks can be packaged very successfully in polyethylene-coated cartons or in plastic containers [15]. These products have a limited shelf life when stored in a refrigerator. Materials selected must not absorb flavor components from the juice. In addition, acid diffusion into the plastic material can delaminate the package. Polyethylene is the most common surface-contact material and is regarded as chemically stable to most food products. Packaging materials must also provide the best possible barrier to light, as light affects the color and nutritive value of fruit juices. Aseptic filling of pasteurized fruit juices and other drinks into a TetraPaks and other systems (e.g., Combibloc, PurePak, Elopak) has also become popular, giving the product an extended shelf life. Such products have advantages over hot-filled products or nonaseptically packaged products, which need a chilled distribution chain [18].

 H. Energy Aspects of Pasteurization

Indirect heating through a heat exchanger involves much more energy than direct heating with infusion heating. To reduce the energy input and hence improve the efficiency of the system, a regenerative method is adopted. For example, the total energy difference between the 90 and 80% regenera tion capacity of a system processing milk is about 62,131 kcal/hr. This shows that even a 10% increase in regenerative efficiency can save considerable energy [16]. Regeneration of heating and cooling streams is now an accepted energy-conservation technique. The heat energy consumption for pasteurization will be about 30 MJ per 1000 liters of milk, and correspondingly the cooling energy is about 4 kWh per 1000 liters. Normally, pasteurizers will have the facility of regeneration, with an efficiency of 75-92%. Efficiency can be increased by increasing the number of plates in the regeneration section. But this increases the pumping pressure and hence the electric energy. Hence, the maximum regeneration is about 90%.

Notes

1. P. Fellows, Processing by application of heat, Food Processing Technology: Principles & practice (P. Fellows, ed.), Ellis Horwood, England, 1988, p. 221.
2. I. J. Pflug and W. B. Esselen Heat sterilization, Fundamentals of Food Canning Technology (J. M. Jackson and B. M. Shinn, eds.), Van Nostrand Reinhold, New York, 1979, p. 71.
3. A. W. Farrall, Pasteurizing equipment, Engineering for Diary Food Products (A. W. Farrall, ed.), John Wiley & Sons, New York, 1980, p. 363.
4. N. I. Barclay, J. D. Potter, and A. L. Wiggins, Batch pasteurization of liquid whole egg, J Food Technol. 19:605 (1984).
5. J. E. Ford, J. W. G. Porter, S. Y. Thompson, J. Tootmill, and J. Edwards-Webb, Effects of UHT processing and subsequesnt storage on the vitamin content of milk, J. Dairy Res. 36:447 (1969).
 6. K. J. Burgess, Dairy products, Food Industries Manual, 2nd ed. (M. E. Ranken ed.), Blackie, Glasgow, 1988.
 7. F. A. Paine and H. Y. Paine, Fresh and chilled foods, A Hand Book of Food Packaging, 2nd ed. (F. A. Paine and H. Y. Paine ed.) Chapman & Hall, New York, 1982, p. 224.
 8. G. Stehle, Trends in packaging techniques for milk products and fruit, Neue Verpack 41(10):56 (1988).
 9. Hassen, A dressing down of the bottle freaks, North Eur. Food Dairy J. 55:81 (1989).
 10. A. J. Iversen, Cartons for liquids, Modern Processing, Packaging and Distribution Systems for Food (F. A. Paine, ed.), Blackie, Glasgow, 1987, p. 86.
 11. L. Karjalainen, Packaging of carbonated beverages, Modern Processing, Packaging and Distribution Systems for Food (F. A. Paine ed.), Blackie, Glasgow, 1987.

 12. K. Kimura and G. Mitsu, Discussion on development and innovation of food packaging, Food Policy (Jpn.) 3:65 (1989).
 13. F. A. Paine and H. Y. Paine, Juices, soft drinks and alcoholic beverages, A Hand Book of Food Packaging, 2nd ed. (F. A. Paine and H. Y. Paine ed.) Chapman & Hall, New York, 1982, p. 339.
 14. J. A. G. Rees and J. Bettison, Heat preservation, Food Industries Manual (M. D. Ranken, ed.), Blackie, Glasgow, 1988, p. 477.
 15. J. V. Bousom, Carriers; beverage, The Wiley Encyclopedia of Packaging Technology (M. Bakker, ed.), Wiley, New York, 1986, p. 129.
 16. S. Mardon, Energy saving in European dairy industry, Diary Ind. Int. 47(6):9 (1982)

By M. N. Ramesh (Central Food Technological Research Institute, Mysore, India)  in the book 'Handbook of Food Preservation',  Shafiur Rahman (editor). Marcel Dekker Inc. New York & Basel, 1999,  p. 95-105. Digitized, adapted and illustrated to be posted by Leopoldo Costa.