Milk and Milk Products Technology

MILK ENZYMES

• Enzymes are a group of proteins that have the ability to catalyze chemical reactions and the speed of such reactions. The action of enzymes is very specific. Milk contains both indigenous and exogenous enzymes. Exogenous enzymes mainly consist of heat-stable enzymes produced by psychrotrophic bacteria: lipases, and proteinases. There are many indigenous enzymes that have been isolated from milk. The most significant group are the hydrolases:
• Enzymes are proteins that have biological functions. Milk enzymes come from several sources: the native milk, airborne bacterial contamination, bacteria that are added intentionally for fermentation, or in somatic cells present in milk.
• Each enzyme has a specific site of action on its target molecule, and optimal conditions (pH and temperature).
• Lipases are enzymes that degrade fats. The major lipase in milk is lipoprotein lipase. It is associated with the casein micelle. Agitation during processing may bring the lipase into contact with the milk fat resulting in fat degradation and off-flavors. Pasteurization will inactivate the lipase in milk and increase shelf life.
• Proteases are enzymes that degrade proteins. The major protease in milk is plasmin. Some proteases are inactivated by heat and some are not. Protein degradation can be undesirable and result in bitter off-flavors, or it may provide a desirable texture to cheese during ripening. Proteases are important in cheese manufacture, and a considerable amount of information is available in the cheese literature.
• Alkaline phosphatase: Alkaline phosphatase is a heat sensitive enzyme in milk that is used as indicator of pasteurization. If milk is properly pasteurized, alkaline phosphatase is inactivated. Phosphatase enzymes are able to split specific phosporic acid esters into phosphoric acid and the related alcohols. Unlike most milk enzymes, it has a pH and temperature optima differing from physiological values; pH of 9.8. The enzyme is destroyed by minimum pasteurization temperatures, therefore, a phosphatase test can be done to ensure proper pasteurization
• Lactoperoxidase is one of the most heat-stable enzymes found in milk. Lactoperoxidase, when combined with hydrogen peroxide and thiocyanate, has antibacterial properties. It is suggested that the presence of lactoperoxidase in raw milk inhibits the disease causing microorganisms (pathogens) present in milk. However, since there is no hydrogen peroxide or thiocyanate present in fresh milk, these compounds would have to be added to milk in order to achieve the antibacterial benefits. Lysozyme is another enzyme that has some antibacterial activities, although the amount of lysozyme present in milk is very small.
• Lactoperoxidase: Lactoperoxidase is present in cowmilk in considerable amount (30mg/ml) but is absent in human milk. This enzymehas been found to be identical with L2 fraction of lactenin. In bovine raw milkthe inhibitory action of this enzyme is due to formation of an anti microbialsystem or LP system under normal conditions.
• The secretion of thiocyanate component in milk is governedby nutrition of the cow. Hydrogen peroxide can be contributed either bypolymorpho-nuclear-leucocytes (PMN) or by hydrogen peroxide producing uddermicro flora Eg.Streptococci
• The LP system is bacteriostatic to MO like Group B and GroupN Streptococci where as it is bacteriocidal for Group A Streptococci, E.coli,S.typhimurium etc. It protects calf against enteric problems and may have somerole in providing defence mechanism to mammary gland against infections.
• Lately attempts have been made to exploit the LP system asa method of preservation (cold sterilization) for raw milk. Unlike in theconventional method of preservation by heat treatment, there is no inactivationof anti microbial substances and other heat labile constituents of raw milk inthe process (like certain vitamins). In these efforts, the two components of LPsystem viz thiocyanate and Hydrogen peroxide are added from out side to attaintheir balanced proportions.
• Lipoprotein lipase (LPL): A lipase enzyme splits fats into glycerol and free fatty acids. This enzyme is found mainly in the plasma in association with casein micelles. The milkfat is protected from its action by the FGM. If the FGM has been damaged, or if certain cofactors (blood serum lipoproteins) are present, the LPL is able to attack the lipoproteins of the FGM. Lipolysis may be caused in this way.
• Plasmin: Plasmin is a proteolytic enzyme; it splits proteins. Plasmin attacks both ß -casein and alpha(s2)-casein. It is very heat stable and responsible for the development of bitterness in pasteurized milk and UHT processed milk. It may also play a role in the ripening and flavour development of certain cheeses, such as Swiss cheese.