Milk enzymes

Many indigenous milk enzymes are technologically significant from five viewpoints ... 

With a few exceptions (e.g. lysozyme and lactoperoxidase), the indigenous milk enzymes do not have a beneficial effect on the nutritional or organoleptic attributes of milk, and hence their destruction by heat is one of the objectives of many dairy processes. 

Andrews, A.T. (1993) Indigenous milk enzymes phosphatases, in Advanced Dairy Chemistry, Vol. 1 Proteins (ed. P.F. Fox), Elsevier Applied Science, London, pp. 322-31. 

Figure 9.11 Time-temperature combinations required for which milk must be heated to a certain temperature to inactivate some indigenous milk enzymes (from Walstra and Jenness,...

The thermal denaturation of indigenous milk enzymes is important from two major viewpoints ... 

Since sulphur compounds are important in the off-flavour of UHT milk, attempts to improve its flavour have focused on reducing the concentration of these, e.g. by adding thiosulphonates, thiosulphates or cystine (which react with mercaptans) or sulphydryl oxidase, an indigenous milk enzyme (which oxidizes sulphydryls to disulphides Chapter 8). 

Egelrud and Olivecrona (1973) studied the catalytic activity against several substrates of bovine milk enzyme preparations that had been purified about 7000-fold to a purity higher than 80% (Egelrud and Olivecrona 1972). The enzyme catalyzed the hydrolysis of emulsified trioleate, trioctanoate, monooleate, Tween 20 (polyoxyethylene sorbitan monolaurate), and p-nitrophenyl acetate. It was concluded that the enzyme had rather low substrate specificity and that the presence of activating serum factors is not needed for catalysis to occur. 

Jenness, R. and Patton, S. 1959. Milk enzymes. In Principles of Dairy Chemistry. John Wiley Sons, New York., pp. 182-202. 

Tsugo, T. and Hayashi, T. 1962. The effect of irradiation by ionizing radiation on milk enzymes. III. Effect of irradiation on lipase and xanthine-oxydase activities in milk. Jap. J. Zootech. Sci. 33, 125-129. 

The major studies in this field are the work of one group102 using mainly in vitro techniques. The action of xanthine oxidase (XO), a milk enzyme, converts purine through hypoxanthine and xanthine to uric acid, in which the order of oxidation is seen to be the same as for base substitution of 2,6,8-trichloropurine. It was first held that the nucleophilic reagent responsible was water, which added across the double bonds and resulted in dihydropurines which then lost hydrogen, so restoring the aromaticity of the system.103 This view has now been modified and hydroxyl-ions are assumed to be the nucleophiles.104 As purine contains no substituents, directive influences are supplied by the dipolar forms extant at the time of reaction, of which 59 represents... 

The enzymes responsible for the detrimental effects of lipolysis are of two main types those indigenous to milk, and those of microbial origin. The major indigenous milk enzyme is lipoprotein lipase. It is active on the fat in natural milk fat globules only after their disruption by physical treatments or if certain blood serum lipoproteins are present. The major microbial lipases are produced by psychrotrophic bacteria. Many of these enzymes are heat stable and are particularly significant in stored products. 

Hydrolytic rancidity results from the hydrolytic degradation of milk lipids. The hydrolysis is catalyzed by lipases and produces free fatty acids (FFAs), some of which have a low flavor threshold and can cause unpleasant flavors in milk and milk products. These flavors are variously described as rancid, butyric, bitter, unclean, soapy or astringent. The lipases involved are of two types indigenous milk enzyme(s) and enzymes of microbial origin. 

Farkye, N.Y., Imafidon, G.I. 1995. Thermal denaturation of indigenous milk enzymes. In Heat Induced Changes in Milk (P.F. Fox, ed.), pp. 331-348, International Dairy Federation, Brussels. 

Sulphydryl oxidase, an indigenous milk enzyme, has been proposed for the oxidation of thiols in UHT milk to reduce cooked flavor and also thereby to serve as an antioxidant, in conjunction with lactoperoxidase (to destroy the resultant H2O2), by obviating pro-oxidants resulting from autoxidation of thiols (Swaisgood and Abraham, 1980). 

Emulsion A suspension of small globules of one liquid in a second liquid with which the first will not mix (e.g., an emulsion of oil and water). Any liquid preparation of a color and consistency resembling milk. Enzymes Proteins or conjugated proteins produced by living organisms that ftinction as catalysts in specific biochemical reactions. 

The bovine milk enzyme is still active in the transfer of hexoses not too different from D-galactose to appropriate substrates. The donor is then the corresponding uridine diphosphate hexose nucleotide (for a review see Nishida et al. 1993). 

Short time exposure to high pressure was reported to enhance activity of lipoprotein lipase and glntamyl transferase in milk. However, long exposure time (100 min) did not bring abont any inactivation of lipase, while glutamyl transferase followed the first-order inactivation kinetics (Pandey and Ramaswamy, 2004). Milk enzymes... 

Although precise information is lacking, it is our opinion that indigenous milk enzymes are not major causes of variability in cheese quality, although some contribute to cheese ripening and may contribute to the superior quality of raw milk cheese, a possibility that warrants investigation. 

Milk Enzyme Time for MUk Gastric juice Time for... 

Alkaline phosphatase 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 and therefore, a phosphatase test can be done to ensure proper pasteurization. 

Urea Milk Enzyme urease+ NH3 gas electrode Bacterial cell + NH neutral carrier electrode... 

Xanthine Oxidase. One of the most thoroughly studied flavoproteins is xanthine oxidase. This enzyme has been purified from milk and from liver. Recently the milk enzyme, for many years known as the Schardin-ger enzyme, has been crystallized in a highly purified state. For many years the absorption spectrum of the purified enzyme was a source of concern, because instead of the two distinct characteristic flavin peaks, an end absorption was always found to obscure the flavin spectrum. This has now been found to be the true spectrum of the enzyme. In addition to FAD, the enzyme contains iron and molybdenum in the ratio, 8 iron 2 FAD 2 Mo per mole of enzyme. Molybdenum does not affect... 

Xanthine oxidase has been partially purified from liver.The purified preparations catalyze the oxidation of aldehydes, purines, and DPNH2, much as does the milk enzyme. Although flavoprotein was present in the purified preparations, it was not shown to be directly related to the enzyme activity. 

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