Milk enzymes

Enzymes are delicate organic catalysts produced by living cells, either animal or plant, but work independently from the cells that produce them.

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

Lipoprotein lipase

Lipases are enzymes that degrade fats. The major lipase in milk is lipoprotein lipase. It is associated with the casein micelle. Lipoprotein lipase activity has been found in the milks from several species where it is assumed to result from leakage from the mammary gland into milk.

The LPL gene provides instructions for making an enzyme called lipoprotein lipase. This enzyme is found primarily on the surface of cells that line tiny blood vessels (capillaries) within muscles and in fatty (adipose) tissue.

The function of the enzyme in the gland is apparently to assist in the transfer of blood lipoprotein triacylglycerol fatty acids into milk triacylglycerols. Bovine skim milk is one of the richest sources of lipoprotein lipase.

The total lipase activity in raw milk is sufficient to cause rapid hydrolysis of a large proportion of the fat. However, in reality this does not happen, because the lipase is prevented from accessing the fat by the milkfat globule membrane. The lipase is unable to hydrolyze the triglycerides of cream and would be destroyed in the digestive tract in any case.

In cheese manufacturing, lipase is utilized to separate milk fat and gives desirable flavors to the cheese. Lipase is mostly used in cheese maturing for the improvement of flavors. Lactase is usually applied to hydrolyze lactose to glucose and galactose sugars and increase the solubility and sweet flavor in different dairy items.
Lipoprotein lipase

Enzymes in milk

Milk contains more than 60 different indigenous enzymes. Indigenous milk enzymes are found in or associated with a virus casein micelles, milk fat globule membrane, milk serum or somatic cells and may originate from blood, somatic ells, the MFGM or the cell cytoplasm.

Many enzymes found commonly in milk are not intrinsic milk enzymes, but are in milk because of the presence of microorganisms.

Pasteurization eliminates most natural enzyme activity in milk.

The two groups of enzymes of major economic importance are proteinases and lipases, including phospholipases, which are produced extracellularly and can act on micellar casein or on milk fat globules.

Besides microbial enzymes, milk also contains a natural proteinase and lipase. The milk lipase is inactivated by pasteurization, while the alkaline milk proteinase is heat stable and survives low-temperature short-time pasteurization conditions.

Other milk enzymes are stable; plasmin, for example, loses only 60% of its activity after 15 sec at 100 °C. Plasmin is the predominant indigenous proteinase in milk. It is part of a complex protease system in milk, consisting of its inactive, precursor, plasminogen, plasminogen activators, which catalyze the conversion of plasminogen to plasmin, plus inhibitors of plasmin and plasminogen activators.

Plasmin and plasminogen originate from the mammal’s blood and are predominantly associated with the casein micelle in milk.
Enzymes in milk

Fat in Milk

Fat in Milk
Fat and associated lipid materials are present in milk as an emulsion of small individual globules in an aqueous phase. These fat globules are predominantly in the liquid state at 37 degree. The fat globules vary in size from about 0.1 to 10 um in diameter.

Surrounding each milk fat globule is a membrane composed of cholesterol, phospholipids, proteins, and other surface-active molecules. The fat globules membrane appears to have two layers: an inner layer that contains a phospholipids-protein complex, and an outer layer that contains enzymes originating in the mammary gland.

The membrane serves to prevent individual fat globules from combining with other to form a larger globule. Mechanical agitation can lead to a disruption of the membrane and thus cause the individual fat globules to coalesce to form larger globules and eventually to form butter. Disruption of the globule membrane may also contribute to the development of rancid flavor in milk.

Milk fat is made up of approximately 12.5% glycerol and 85.5% fatty acids by weight. The predominant glycerides are triglycerides (8-99%); diglycerides account for 0.05%; and monoglycerides for 0.04% of the total. These glycerides contain more than 16 different fatty acids combined with glycerol in a number of different ways.

The principal fatty acids contain an even number of carbon atoms. Milk fat is unique in that it is the only natural food fat that contains butyric acid in its glycerides structure. This fatty acid has a strong characteristics odor. Their release from triglycerides during lipolysis is the cause of rancid odor and flavor in milk.

Milk fat contains a number of minor components. Among these are free fatty acids, cholesterol, phospholipids, and the fat soluble vitamins.
Fat in Milk