Enzyme indigenous milk

As many as 60 indigenous enzymes have been reported in normal bovine milk. With the exception of a-lactalbumin, which is an enzyme modifier in lactose synthesis (Chapter 2) most, if not all, of the indigenous enzymes in milk have no obvious physiological role. They arise from three principal sources ... 

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. 

The distribution of the principal indigenous enzymes in milk and their catalytic activity are listed in Table 8.1. In this chapter, the occurrence, distribution, isolation and characterization of the principal indigenous enzymes will be discussed, with an emphasis on their commercial significance in milk. 

Superoxide dismutase (SOD), an indigenous enzyme in milk, was discussed in section 8.2.10. A low level of exogenous SOD, coupled with catalase, was shown to be a very effective inhibitor of lipid oxidation in dairy products. It has been suggested that SOD may be particularly useful in preserving the flavour of long-life UHT milk which is prone to lipid oxidation. Obviously, the commercial feasibility of using SOD as an antioxidant depends on cost, particularly vis-d-vis chemical methods, if permitted. 

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). 

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). 

Other Indigenous Milk Proteinases. The presence of other minor proteolytic enzymes in milk has been reported, including thrombin and a lysine aminopeptidase (Reimerdes, 1983) and proteinases from leucocytes (Grieve and Kitchen, 1985 Verdi and Barbano, 1991), but they are considered not to be very significant (Grieve and Kitchen, 1985 Grufferty and Fox, 1988). 

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. 

Cathepsin D (EC3.4.23.5). It has been known for more than 20 years that milk also contains an acid proteinase, (optimum pH ss 4.0) which is now known to be cathepsin D, a lysozomal enzyme. It is relatively heat labile (inactivated by 70°C x 10 min). Its activity in milk has not been studied extensively and its significance is unknown. At least some of the indigenous acid proteinase is incorporated into cheese curd its specificity on asl- and / -caseins is quite similar to that of chymosin but it has very poor milk-clotting activity (McSweeney, Fox and Olson, 1995). It may contribute to proteolysis in cheese but its activity is probably normally overshadowed by chymosin, which is present at a much higher level. 

Lipase was first isolated from skim milk and characterized by Fox and Tarassuk in 1967. The enzyme was optimally active at pH 9.2 and 37°C and found to be a serine enzyme (inactivated by organophosphates). A lipoprotein lipase (LPL activated by lipoprotein co-factors) was demonstrated in milk by Korn in 1962 and was isolated by Egelrud and Olivecrona in 1972. LPL is, in fact, the principal indigenous lipase in milk and most recent work has been focused accordingly. The molecule has been characterized at the molecular, genetic, enzymatic and physiological levels (see Olivecrona et al, 1992). 

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