Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk) S A Rankin, A Lopez-Hernandez, and A R Rankin, University of Wisconsin–Madison...

239KB Sizes 306 Downloads 657 Views

Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk) S A Rankin, A Lopez-Hernandez, and A R Rankin, University of Wisconsin–Madison, Madison, WI, USA ª 2011 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by S. A. Rankin, Volume 3, pp 1633–1637, ª 2002, Elsevier Ltd.

Introduction The keeping quality or shelf life of fluid milk products is a problem in many of the world’s markets, due to inadequate refrigeration, poor raw material, and/or insufficient processing and filling technologies. Milk pasteurized by traditional high-temperature–short-time (HTST) processing and filling technologies is limited to a shelf life of approximately 1–3 weeks at refrigerated temperatures, thus limiting transportation to regional locations. Until recently, the only solution has been to manufacture ultra-high temperature (UHT) milk with a shelf life of 3–6 months at ambient temperature. However, UHT processing alters the sensory properties of the milk, contributing a pronounced cooked or scorched flavor and associated darker color. In an effort to prevent thermal alteration of the sensory properties, extend the shelf life beyond that of HTST pasteurized milk, and expand the ability to transport milk without spoilage, milk can be processed at temperatures above those used for pasteurization yet below those used for UHT processing. Milk subjected to this treatment is referred to as ultrapasteurized, superpasteurized, extended shelf life (ESL), or extended long-life milk. The terms ‘superpasteurized’ and ‘ESL milk’ will be used interchangeably in this article. Superpasteurized milk is designed to have a shelf life of up to 90 days at refrigeration temperatures and have sensory properties superior to those of UHT milk. However, limited data are available in the scientific literature on the safety, sensory qualities, or shelf life of superpasteurized milk.

Product Stability Shelf Life Product shelf life is generally defined as the time between processing and the point at which the quality of the product falls below an acceptable level. This definition depends on the perception of ‘minimum acceptable quality’. In some applications of pasteurized milk, a microbial count exceeding 107 organisms per ml is associated with a minimum acceptable quality. More practically, the onset

of sensorially defined off-flavors, such as the development of rancidity, denotes the end of shelf life. The shelf life of superpasteurized milk is influenced by raw material quality, processing and packaging conditions, and environmental conditions during distribution and storage. Although chemical reactions resulting from metal- and light-induced oxidation may have dramatic influences, outgrowth of spoilage microorganisms is thought to be the greatest limiting factor for the shelf life of superpasteurized milk.

Flavor Defects of ESL Milk One of the major challenges of producing ESL milk is elimination of spoilage bacteria without compromising flavor, vitamin content, textural character, or appearance of the final product. The reduction of cooked or scorched flavors is perhaps one of the main issues facing processors. Additionally, there are other off-flavors that can be manifest in ESL milk, such as those resulting from lipid oxidation, Maillard reactions, and the adsorption of volatiles from the packaging materials used in milk containers. Cooked flavor (usually related to the formation of sulfur compounds from the decomposition of sulfurcontaining proteins) is a dominant flavor note associated with high-temperature treatment of milk products. The strategies used to reduce the generation of cooked flavors involve the use of high temperatures for shorter periods, thus limiting the off-flavor-producing reaction mechanism. Maillard reactions are very common in heat-treated milk products and can lead to the generation of a variety of off-flavors (described as burnt candy, cabbage, baked potato) and some discoloration. The oxidation of unsaturated fatty acids yields aroma-active aldehydes and ketones capable of inducing cardboard-like flavors. High-temperature processing, exposure to light, irradiation, and the presence of metals (copper and iron) are the factors that can potentiate oxidation. A number of strategies have been proposed to minimize the defects caused by thermal treatment. The main goal of such methods is to use heat treatment sufficient to cause bacterial death, yet limited to avoid undesired

281

282 Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

degradation reactions. Non-thermal approaches such as microfiltration technologies may hold promise for achieving ESL milk with sensory and chemical properties more similar to those of HTST milk. The use of pulsed electric fields to obtain ESL milk has also been reported to be a method that yields no apparent changes in the olfactory or visual characteristics of the product when compared to HTST processing. Texture and Appearance Defects Another quality problem with ESL milk products is related to textural changes that can occur after long periods of storage. The most common textural changes involve the separation of the lipid phase, sedimentation of denatured proteins, and age gelation. Age gelation is an irreversible phenomenon that occurs during the storage of sterilized milk, transforming the product into a gel. Gelation is considered a critical quality problem associated with sterilized milk products and is affected by a multitude of factors such as the severity of heat treatment, proteolysis during storage, milk composition and quality, seasonal milk production factors, and storage temperature. Gelation is regarded as a two-stage process involving the formation of a -lactoglobulin–-casein complex, which cross-links to form a protein network gel. Gelation can be minimized by selecting high-quality milk, inactivating proteinases, increasing the degree of heat treatment, storing the milk at a temperature below or above the optimum range for gelation (25–30  C), and/or adding proteinsolubilizing agents such as citrates or phosphates. Factors That Affect the Shelf Life of ESL Products Storage temperature

Storage temperature of the product after pasteurization is one of the most important factors involved in the extension of the shelf life of pasteurized milk. As a general rule, each 3  C decrease in the storage temperature doubles the shelf life of pasteurized milk. In 1991, Cromie reported that the spoilage level of pasteurized milk, represented by 107 microorganisms per ml and unacceptable flavor scores, was reached at 12  C after 14 days, at 7  C after 25 days, and at 3  C after 50 days of storage. As the temperature falls below the optimum growth temperature of 20–40  C of most spoilage microorganisms, it causes an increase of the lag phase (period of adaptation to new conditions before logarithmic growth ensues) and a decrease in the growth rate of the microorganisms. Microbial spoilage of superpasteurized milk is most commonly associated with inadequate control of postpasteurization storage temperature. It has been reported that in the United States, the overall range of temperature in retail dairy display cases is 2–14  C. However, given

the longer shelf life of superpasteurized milk, it is critical that the storage temperature is held well below 7  C. Psychrotrophic spore-forming microorganisms, which pose a potential spoilage and health risk in ESL milk, grow well at 8–10  C, whereas their activity is suppressed at 2–5  C. Thermal treatment parameters

Typically, superpasteurized milk is processed at approximately 138  C/2 s, which is below the temperature used in UHT processes (145  C/3 s) and above the HTST pasteurization temperature (72–75  C/15 s). Superpasteurized milk processing temperature is intended to extend the shelf life of HTST milk by significantly reducing the microbial load, yet limiting the deterioration of sensory properties caused by extreme thermal treatment. Even though little information is available on the safety and absolute shelf life of ESL milk, there have been some reports that microbial counts for ESL milk may be higher than initially expected. The following explanations for these unexpected reports were linked to the elevated temperature used in ESL processing: (1) Most of the microorganisms are killed; therefore, the influence of competitive microflora is suppressed. As a result, heat-resistant microorganisms, which are found at low levels, and/or post-pasteurization microbial contaminants grow relatively unhindered. (2) Some bacterial spores may be activated by the temperature used in ESL processing. (3) The natural antimicrobial systems in milk may be destroyed. Raw milk quality

In general, raw milk must be free from any impurity or distortion and have low somatic cell and total microbial counts. The quantity, type, and activity of microorganisms present in raw milk prior to processing and packaging are critical for the shelf life and flavor of superpasteurized milk. Heat-resistant microorganisms

Some psychrotrophic spore-forming microorganisms present in raw milk have considerable spoilage potential in ESL milk. Certain thermally resistant bacterial spores not only survive heat treatments used in ESL processing, but are in fact activated by these thermal treatments. Additionally, the elevated temperature used in ESL processing may enhance the growth environment for psychrotrophic spore formers via elimination of competitive microflora and/or the natural antimicrobial systems in milk such as the lactoperoxidase system. Although inhibition of heat-resistant spore-forming psychrotrophic microorganisms is achieved at a storage temperature below 7  C, it must be emphasized that there exists great potential for the product to experience

Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

283

temperature abuse over the course of transportation and the long shelf life period. Some candidate heat-resistant spore-formers include Bacillus circulans and Bacillus cereus. Bacillus circulans is capable of metabolizing lactose to lactic acid, resulting in acid milk. Bacillus cereus is a recognized food-borne pathogen. Dairy products have rarely been implicated in outbreaks related to B. cereus, potentially due to an elevated rate of rancidity resulting from B. cereus enzymes that act on the milk fat globule membrane, thus making the milk unpalatable. Heat-resistant enzymes

Raw milk containing high counts of any Pseudomonas species is not recommended for ESL processing. Pseudomonas species, particularly Pseudomonas fluorescens, produce lipases resistant to heat treatments used in ESL processing. These enzymes, which can remain active during long storage periods at refrigeration temperatures, degrade the fat globule membrane and associated lipids, resulting in rancid off-flavors.

Figure 1 Average consumer acceptance (hedonic rating with 1 ¼ extreme dislike, 9 ¼ extreme like) comparing hightemperature–short-time (HTST; 74  C for 16 s), extended shelf life (ESL; 134  C for 4 s, direct steam injection), and ultra-high temperature (UHT; indirect plate exchange heating) processed milk. Adapted with permission from Blake MR, Weimer BC, McMahon DJ, and Savello PA (1995) Sensory and microbial quality of milk processed for extended shelf life by direct steam heat injection. Journal of Food Protection 58: 1007–1013.

Post-pasteurization contamination

Postpasteurization contamination with Gram-negative psychrotrophs is considered a main cause of spoilage of superpasteurized milk produced from raw material of good quality. Species routinely associated with milk spoilage include Pseudomonas, Enterobacter, Klebsiella, and Flavobacterium. In order for superpasteurized milk to reach the intended shelf life, it is essential to practically eliminate postpasteurization contamination. The absence of competitive microflora in ESL milk may lead to rapid outgrowth of psychrotrophic spoilage organisms. Postprocess contaminants originate from numerous sources, including the atmosphere, milk piping, packaging, and other food contact surfaces. Thus, it is critical that superpasteurization processing utilizes aseptic packaging and filling technologies as well as strictly controlled storage conditions. Sensory and Nutrient Qualities of ESL Milk Relative to UHT milk, the lower temperature of ESL processing results in improved sensory properties. However, HTST milk may still be regarded as having superior flavor to ESL milk. Reflective of the heat treatment, ESL milk achieves intermediate consumer panel ratings relative to HTST and UHT processed milk (Figure 1). Due to the extended duration of retail display, suitable packaging is an important factor in limiting lightinduced degradation of nutritional and sensory qualities of ESL milk. Vitamins A and B2 (riboflavin) can be reduced substantially when exposed to the high-

intensity fluorescent lighting typical of many retail display cases. Light-induced flavor has been demonstrated to strongly diminish consumer preference. Paper cartons allow less than 1% transmission of oxidation-inducing wavelengths of light, while untinted, high-density polyethylene materials have substantially inferior light barrier characteristics. Incorporation of air and the associated oxidative potential are also detrimental to the flavor and shelf life of ESL milk. Removal of absorbed air improves flavor stability, nutrient retention, and container fill uniformity by limiting foaming at the filler. Retention of light-sensitive vitamins, inhibition of lightinduced off-flavor, and minimal external flavor absorption are all achieved by using coated paperboard and multilayer laminates. These laminates are the most suitable materials for packaging ESL milk.

Production of Superpasteurized Milk In general, successful superpasteurized milk processing is the combination of appropriate thermal treatment and aseptic filling technologies. The thermal treatment may be one of a variety of time and temperature combinations designed to achieve the desired microbial lethality and enzymatic destruction. Typical temperature/time combinations for superpasteurized milks fall in the range of 125–145  C for 2–4 s. The US Food and Drug Administration defines ultrapasteurization as a process in which a dairy product is thermally processed at or above 138  C for at least 2 s, either before or after

284 Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

packaging, so as to produce a product that has an ESL under refrigerated conditions. ESL milk affords many advantages including increased flexibility in production and distribution schedules, an increase in allowable shipping distances, and a potential increase in point of sale quality to the consumer.

Thermal Technologies Typical heat transfer devices for ESL milks may include both direct and indirect heating technologies. Direct methods include infusion, where the product is sprayed into a steam atmosphere, and injection, where steam is injected directly into the product. Indirect methods include plate and tubular heat exchangers, which do not involve direct contact between the product and heating medium.

intermediate temperature between 68 and 85  C and then transferred to an infuser or steam injector module. The rapid direct heating allows a high sterilization temperature to be achieved readily with hold times in the 2- to 4-s range. After holding, the product is sent through several cooling stages. The first stage involves ‘flashing’ the product into a vacuum chamber where the residual water gained during the infusion or injection process is removed, thus ensuring that there is no dilution of the finished product. If the product contains milk fat, the discharge from the vacuum chamber is homogenized using an aseptic homogenizer prior to being cooled to container filling temperature. Direct steam systems, particularly infusion, are more complex to operate than indirect systems, making automated control of sterilization, production, and cleaning essential.

Direct heat exchange systems

The primary advantage of direct steam heating systems is the rapid attainment of process temperature. Infusion and injection are the two most common methods for direct heat exchange. The infusion method involves spraying the product into an environment of sufficient culinary quality steam (Figure 2(a)). An infuser creates a milk aerosol, thus dramatically increasing the surface area and rate of heat transfer. Injection involves spraying steam directly into the stream of liquid milk (Figure 2(b)). Steam injectors are operated at pressures to ensure that the steam is fully condensed into the product prior to its entry into the holding tube. Infusion and injection systems follow a similar approach. Incoming raw milk is first heated to an

Indirect heat exchange systems

While limited to processing temperatures and pressures below those of more advanced heat transfer technologies, plate heat exchange systems represent a proven, relatively low-cost means of thermal processing. The high ratio of heat transferred to product volume yields a relatively low residence time. Plate heat exchangers are comprised of a series of parallel, intimately spaced, stainless-steel corrugated plates, which are compressed together in a plate frame (Figure 3(a)). Milk is distributed through narrow passages, producing a turbulent flow and a high rate of heat exchange with minimal contact time. Ports within the plate assembly direct the product and heating/cooling medium to alternate sides of the corrugated plate.

Figure 2 Cursory details of two direct heating systems used for the production of extended shelf life milk: (a) injection port and (b) infuser system. Adapted with permission from Anonymous Long life milk. In: APV Dairy Processing Handbook, p. 224. Lund, Sweden: APV Crepaco, Inc.

Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

285

Figure 3 Cursory details of two indirect heat exchangers: (a) high-temperature–short-time plate system and (b) tube-in-tube module. Adapted with permission from Anonymous Introduction to aseptic/ESL systems. In: APV Aseptic/Extended Shelf Life Processing Handbook, pp. 18, 23. Lund, Sweden: APV Crepaco, Inc. APH292.

A connector grid may be incorporated to allow several independent sections, such as regeneration, heating, and cooling, to be placed on the same frame assembly. Because of the high surface areas and torturous product paths, plate heat exchange systems are more susceptible to fouling than tubular units. There are essentially three different types of tubular heat exchangers: coiled tube, tube-in-tube, and multiple tube-in-tube heat exchangers (Figure 3(b)). In brief, these technologies involve pumping milk through tubes that are surrounded by a heating medium. A tubular process module can be operated at a pressure in excess of 275 bar and at a variety of product velocities.

Equipment Sanitation ESL processing systems typically utilize a cleaned-inplace (CIP) protocol, which involves circulating rinse water/caustic and acid solutions at defined concentrations, temperatures, and time periods. Some processes are given an intermediate clean at a flow rate and temperature identical to those used during normal processing, thus allowing a plant to retain sterility and switch from one product to another. Following cleaning, the entire system is sterilized with hot water to eliminate microorganisms in the aseptic side of the system. Water is heated to a minimum of 132  C and pumped through the holding tube, cooling system, and the filler (or surge tank) before being cooled for recirculation.

Aseptic Packaging and Filling Unlike many traditional HTST processes using rotary filling devices open to the plant atmosphere, ESL milk containers must be filled using aseptic technologies to reduce/eliminate post-process microbial contaminants and to achieve the intended shelf life. Aseptic packaging and filling essentially seals a sterile product into commercially sterile containers. There are several methods of aseptic packaging and filling. Aseptic cartoning systems sterilize a laminated fiberboard material with hydrogen peroxide; a flow of sterile hot air removes residual hydrogen peroxide prior to filling. Aseptic form–fill–seal fillers then heat-form a plastic container within a sterile cabinet under a curtain of sterile laminar-flow air. The container is filled with the product, covered with a sterile foil laminate lid, and sealed. Pre-formed plastic containers can be sterilized with hydrogen peroxide and filled aseptically in a similar fashion. Some systems utilize clean-room technology using a combination of pressurized high-efficiency particulate air (HEPA)-filtered air, hydrogen peroxide mist, superheated air, and/or UV lamps to sterilize the product containers and filling environment; superheated air activates hydrogen peroxide for increased microbial lethality. There are two additional methods for aseptic packaging and filling: bottle-shaped plastic containers and bagin-box systems. Bottle-shaped plastic containers can be formed and sterilized by blow-molding and are filled and capped in an aseptic laminar-flow cabinet. Bag-in-box

286 Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk)

systems are used for larger (>4 l) container applications; milk is filled into laminated plastic bags, which have been sealed and then presterilized with irradiation. The filled bags are held within wood, plastic, or cardboard outer cases or within steel drums. Non-thermal Technologies Several alternatives to thermal treatment have been proposed for the production of ESL milk without compromising the sensory attributes of the product due to heat treatment. Methods such as bactofugation and microfiltration, the use of pulsed electric fields, plasma, or UV lights, or a combination of these with mild thermal treatments have also been evaluated as potential technologies to extend the shelf life of milk. The major challenges for the application of these novel technologies in the elaboration of ESL are the energy costs, the attainment of production rates similar to those of conventional systems, and the validation of the effectiveness in destroying specific pathogenic and spoilage microorganisms and enzymes in milk by recognized authorities. Microfiltration The microfiltration process consists of the removal of bacterial cells and spores from the milk mechanically using membrane processing where milk constituents are separated based on particle size. A main limitation for the application of this technology to ESL processing is that the particle size distribution of bacterial cells and spores is similar to that of milk fat globules, thus limiting the application of this method to non-fat milk. Also, the overlap of the particle size distribution of cells and spores with that of casein micelles requires a compromise in the pore size used. Microfiltration is carried out with a ceramic membrane; membranes with average pore diameters of 0.8–1.4 mm are commonly used commercially. To prevent membrane fouling and preserve a high and constant flux, special circulation systems capable of achieving a spore reduction of about three logarithmic cycles have been proposed. Multi-layered membranes with the same average pore size but a narrower size distribution have enabled a spore reduction of 4–5 log10 steps. The initial content of spores in the milk has a significant influence on the content of spores in the microfiltered milk. Microfiltered milk is marketed in several countries as more ‘pure’ and ‘natural’ than standard heat-treated milk and may have a higher price as a branded product. Pulsed Electric Fields The use of pulsed electric fields (PEFs), a technology consisting of a treatment of foods with very short electric pulses at high electric field intensities and moderate

temperatures, has also been presented as a viable alternative for the production of ESL milk without altering its sensory and nutritional attributes. Milk is reported to be the first electrically pasteurized food product. Systems to process milk electrically involve voltages of 3000–4000 V. In the 1990s, several studies were performed in order to develop different types of equipment for the application of high-intensity PEF for the pasteurization of milk. Some research efforts have demonstrated that PEF technology by itself is able to extend the shelf life of fluid milk stored at refrigeration temperatures for up to 2 weeks without causing changes to the physical or chemical properties of milk or to its sensory attributes. The application of PEF in combination with a mild thermal treatment has been shown to be a more effective preservation strategy, capable of extending the shelf life of fluid milk for up to 4 weeks without compromising its quality. However, the commercial application of PEF technology is yet to be implemented mainly due to lack of regulatory approval, high initial investment, and elevated processing costs. See also: Heat Treatment of Milk: Ultra-High Temperature Treatment (UHT): Aseptic Packaging. Liquid Milk Products: Liquid Milk Products: Flavored Milks; Liquid Milk Products: Membrane-Processed Liquid Milk; Liquid Milk Products: Modified Milks; Liquid Milk Products: Pasteurized Milk; Liquid Milk Products: UHT Sterilized Milks; Pasteurization of Liquid Milk Products: Principles, Public Health Aspects; Recombined and Reconstituted Products.

Further Reading Anonymous (2003) Longer-shelf-life dairy-based beverages: Challenges and opportunities. In: Innovations in Dairy. Rosemont, IL: Dairy Management Inc. Anonymous (1995) Long life milk. In: Teknotext AB APV Dairy Processing Handbook, p. 224. Lund, Sweden: APV Crepaco, Inc. Anonymous (2008) Extended shelf life/ESL. In: APV Technology Update: Long Life Dairy, Food and Beverage Products, pp. 6–10, 22–24. Getzville, NY, APV Nordic, Unit Systems. Anonymous Introduction to aseptic/ESL systems. In: APV Aseptic/ Extended Shelf Life Processing Handbook, pp. 17–19, 31–33, 47. Lund, Sweden: APV Crepaco, Inc. APH292. Anonymous Introduction to aseptic/ESL systems. In: APV Aseptic/ Extended Shelf Life Processing Handbook, pp. 18, 23. Lund, Sweden: APV Crepaco, Inc. APH292. Barbosa-Canovas GV, Gongora-Nieto MM, Pothakamury UR, and Swanson B (1999) Preservation of foods with pulsed electric fields. In: Taylor SL (ed.) Food Science and Technology International Series. San Diego, CA: Academic Press. Bendicho S, Barbosa-Canovas GV, and Martin O (2002) Milk processing by high intensity pulsed electric fields. Trends in Food Science & Technology 13: 195–204. Blake MR, Weimer BC, McMahon DJ, and Savello PA (1995) Sensory and microbial quality of milk processed for extended shelf life by direct steam heat injection. Journal of Food Protection 58: 1007–1013. Bylund G (1995) Pasteurized milk. In: Dairy Processing Handbook, pp. 201–207. Lund, Sweden: Tetra Pak Processing Systems (AB).

Liquid Milk Products | Liquid Milk Products: Super-Pasteurized Milk (Extended Shelf-Life Milk) Cromie SJ (1991) Microbiological aspects of extended shelf life products. Australian Journal of Dairy Technology 46: 101–104. Datta N and Deeth HC (2001) Age gelation of UHT milk – a review. Food and Bioproducts Processing 79: 197–210. Go´ngora-Nieto M, Sepulveda D, Pedrow P, Barbosa-Ca´novas GV, and Swanson B (2002) Food processing by pulsed electric fields: Treatment delivery, inactivation level, and regulatory aspects. Lebensmittel-Wissenschaft und Technologie 35: 375–388. Hoffmann W, Kiesner C, Clawin-Ra¨decker I, et al. (2006) Processing of extended shelf life milk using microfiltration. International Journal of Dairy Technology 59: 229–235. Kessler HG (1997) Engineering aspects of currently available technological processes. Bulletin of the International Dairy Federation 320: 16–25. Larsen PH (1992) Mikrofiltration zur Entfernung von Bakterien und Sporen. Deutsche Milchwirtschaft 43: 46–50. Mans J (2000) Long live the quik!. Dairy Foods 101: 37–42.

287

Olesen N and Jensen F (1989) Microfiltration: The influence of operation parameters on the process. Milchwissenschaft 44: 476–479. Qin BL, Pothakamury UR, Vega H, Martin O, Barbosa-Canovas GV, and Swanson BG (1995) Food pasteurization using high-intensity pulsed electric fields. Food Technology 49: 55–60. Rysstad G and Kolstad J (2006) Extended shelf life milk – advances in technology. International Journal of Dairy Technology 59: 85–96. Sepulveda DR, Go´ngora-Nieto MM, Guerrero JA, and BarbosaCa´novas GV (2005) Production of extended-shelf life milk by processing pasteurized milk with pulsed electric fields. Journal of Food Engineering 67: 81–86. Vatne KB and Castberg HB (1991) Processing and packaging aspects of extended shelf life products. Australian Journal of Dairy Technology 46: 98–100. Westhoff DC (1978) Heating milk for microbial destruction: A historical outline and update. Journal of Food Protection 41: 122–130.