OTHER CONSTITUENTS OF MILK

Enzymes

A number of enzymes have been reported to be present in milk.These enzymes may be grouped into following categories:

Lipases - Lipases are fat splitting enzymes

Proteases - Proteases are protein decomposing enzymes

Amylases - Amylases are starch hydrolyzing enzymes

Catalases - Catalases are hydrogen peroxide decomposing enzymes

Phosphatases - Phosphatases decompose phosphoric acid

Lactases - Lactases hydrolyze lactose in to lactic acid

Peroxidases - They cause oxidation, responsible for peroxidase test.

Pigments

The vegetable colouring pigments present in milk include carotene and xanthophill, grouped together as carotenoids. Carotene, the most important fat soluble pigment of milk, gives yellow colour to milk and acts as an antioxidant and is also the precursor for vitamin A. Milk fat contains 0.3 mg carotene per 100 g.

Non Protein Nitrogen (NPN)

These are nitrogenous substances of milk, which are not protein in nature. The NPN content of milk is about 5% of the total nitrogen content. This group of substances includes amino acids, urea, uric acid, creatine, creatinine, orotic acid, hippuric acid etc., and is present in the aqueous solution.

Gases

As milk is drawn from the udder, it contains gases which get incorporated in to it shortly after milking. The carbon dioxide content of milk is about 5-10 vol.%. It also contains 2-3 vol. % nitrogen and 0.5-1.0 vol.% oxygen. During hand milking, foaming is produced which results in the loss of 50% of the total gases present in milk. Further storage results in gradual loss of gases. When the milk is exposed to the atmosphere, there is a shift in the pattern of carbon dioxide and oxygen. Carbon dioxide escapes out and oxygen and nitrogen gain entry in to milk.

Organic acids

Naturally occurring citric acid is present in milk at the level of 2.45 g per kg of milk. Other acids such as lactic acid, butyric acid etc. are present in milk as metabolic end products of fermentation processes by microorganisms. The lactic acid, which is produced during the fermentation of lactose, plays a major role and helps in reducing the pH in a controlled manner due to its production and dissociation, e.g. during the manufacture of acidified / fermented dairy products.

The lactic acid produced during fermentation is of two types, L(+) lactic acid and D(-) lactic acid. D and L are designated for the spatial position of the OH group of the C2 atom (L = left position, D = right position); the polarization of the light is characterized by (+) and (-). An L(+)lactic acid turns it right while a D(-) lactic acid turns it left. Apart from this, there is a small amount of the optically inactive raceme DL-lactic acid.

The lactic acid bacteria determine the percentages of L(+) and D(-) lactic acid produced during the fermentation. For example, L(+) lactic acid is produced mainly by Lactococcus lactis subsp. lactis and Streptococcus salivarius subsp. thermophilus. Lactobacillus delbrueckii subsp. bulgaricus and Lactobacillus delbrueckii subsp. lactis produce mainly D(-) lactic acid.

In the human body, L(+) lactic acid is also formed and has some important metabolic functions although D(-) lactic acid is not formed. Hence, those combinations of microorganisms which are capable of producing more than 50% of L(+) lactic acid are used during the manufacture of fermented products.

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