Milk and derived products contain several essential nutrients and protective agents, such as enzymes and growth factors. Milk produced under normal conditions, and from normal healthy cows, contains a wide variety of enzymes. While normal milk contains substrates for most of these enzymes, very little is known concerning the significance of these enzymes in milk.
Each enzyme has a specific site of action on its target molecule, and optimal conditions (pH and temperature). Indigenous enzymes are milk constituents that originate from four main sources, blood plasma, secretory cell cytoplasm, milk fat globule membrane (MFGM), and somatic cells (leucocytes).
The best characterized enzymes in milk include N-acetyl- β -D-glucosaminidase (NAGase); acid phosphatase (AcP); alkaline phosphatase (AIP); amylase; catalase; γ-glutamyl transferase (γ-GGT); glutathione peroxidase (GSH); lactoperoxidase (LPO); lipoprotein lipase (LPL); lysozyme; plasmin; ribonuclease (RNase); sulphydryl oxidase (SHOx); superoxide dismutase (SOD); and xanthine oxidoreductase (XOR).
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.
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.
Milk enzymes
