Milk is a highly perishable food, it requires heat treatment for increasing shelf life and safety of consumers. The most common type of heat treatment in many parts of the world is pasteurization, which is performed at a minimum of 72°C for 15 seconds. This is the least heat treatment needed to destroy most pathogenic microorganisms and it also destroys most spoilage organisms. However, a small number of bacteria remain after pasteurization and packaging, and can grow during storage. Such growth is slow at low temperature and consequently pasteurized milk is always kept refrigerated
One way of extending the shelf-life of milk is to heat it at temperatures high enough to destroy almost all microorganisms and then store it in sealed containers without contamination by bacteria. There are two ways in which this can be carried out: in-container sterilization and ultrahigh- temperature (UHT) processing. Both produce a ‘commercially sterile’ product which means the milk does not contain microorganisms which can grow under the normal conditions of storage which, in this case, is room temperatureUHT milk has much less cooked flavor and very little, if any, brown discoloration. The difference in product quality between the two types of sterilized milk was a major driver for the development of UHT processing for producing a shelf-stable product. However, the major driver for its commercialization and widespread adoption was the development of aseptic packing.
UHT milk often appears to have a cooked or heated flavor. Modern UHT technology minimizes the production of this flavor but most consumers can still detect it and it is one reason why many consumers prefer pasteurized milk. The typical flavor of UHT milk is due a combination of flavors, the chief of which are sulphurousflavors caused by volatile Sulphur compounds released from the whey protein, and the proteins in membrane surrounding the milk fat globule. Other contributors are the aliphatic carbonyl compounds formed during heating and compounds formed in the Maillard reaction. Immediately after manufacture, UHT milk has a strong sulphurous smell and taste due to hydrogen sulphide and other volatile sulphur compounds such as methane thiol. These compounds are markedly reduced in the first week, presumably through oxidation.
The initial step in the Maillard reaction is the reaction between lactose and lysine in milk proteins, chiefly whey proteins. In fact, the extent of this reaction is an indication of the intensity of heat given to the milk. In practice, it is measured as furosine, a product formed when the lactose-containing protein is subjected to acid hydrolysis. Another indicator of the heat treatment is lactulose, an isomer of lactose.The whey proteins, particularly -lactoglobulin which forms about 50 percent of these soluble proteins in milk, are denatured by heating over about 70°C so that in UHT milk, a large percentage of the whey proteins are in the denatured state and exist largely as complexes with caseins.The instability of the whey proteins to heat has another consequence during UHT processing. Some whey protein denatures and attaches to the surfaces of the heat exchangers in proteinaceous deposits which obstruct the flow of milk and can eventually cause the plant to be closed down for cleaning. However, this is not the only type of deposit formed during UHT processing. At high temperatures, above about 110°C, calcium phosphate also precipitates on the walls, adding to the ‘fouling’ caused by the whey proteins.UHT process has only a minimal effect on the nutrient value of milk. There is a small decrease in the water-soluble vitamins but virtually no change in the fat-soluble vitamins.
The flavor changes through progress of the Maillard reaction and through oxidation by dissolved oxygen in the milk. The major flavor compounds produced are methyl ketones and aliphatic aldehydes but a large number of flavor compounds are generated. Other flavors which may develop during storage are due to the action of heat-resistant bacterial enzymes which may be present in the raw milk and survive the UHT heat treatment. These include lipases, which break down the fat and form free fatty acids, some of which have strong flavors, and proteases which break down proteins to produce peptides, some of which are bitter.Another change which can be brought about by proteases is what is known as ‘age gelation’ where the milk thickens over storage and eventually turns into a gel akin to a yogurt. This undesirable defect can be caused by the heat-resistant bacterial enzymes but it can be also caused by plasmin, a naturally occurring protease in milk, which is quite heat stable and can remain active in UHT milk. Recently, it has been found to be inactivated by some UHT preheating conditions which is an excellent reason for including a holding time in the preheat section of UHT plants.
Muhammad Ajmal and Muhammad Nadeem, students of Department of Dairy Technology, University of Veterinary and Animal Sciences, Lahore