Casein and Whey Proteins in Milk
Casein
Casein may be defined as the fraction of skim milk that is precipitated by acidifying to pH 4.6 – 4.7, leaving the milk serum proteins in solution. Casein is composed of three electrophoretic components, which are designated as alpha, beta and gamma in order of decreasing mobility.
Associated with the alpha-casein complex is a portion known as K-casein, which is specifically hydrolyzed from casein via the action of rennin to give para-K-caseinate. The para-K-casein is insoluble in the presence calcium ions and as consequence it precipitates as a clot or curd. This phenomenon is the basis of cheese manufacturer.
It is noteworthy that milk of ruminants (buffalo, cow, ewe, and goat), which contains at least 2.5% casein, gives a cheese type curd with rennet, whereas milk of nonruminants (human) which contains less than 2.5% does not give a cheese type curd with rennet.
Whey proteins
The soluble proteins retained in the supernatant (whey), following the removal of casein from skim milk, include lactalbumin and lactoglobulin. Unlike casein, the whey proteins are unaffected by dilute acids or rennin. Both of these proteins are coagulated by heat, but the degree of such coagulation is determined by the PH of the solution, temperature, time of holding and salt concentration.
Nonprotein Nitrogen
A variety of nonprotein nitrogen containing constituents are present in milk. These substances include amino acids, certain vitamins, urea, uric acid, ammonia, creatin, and creatinine. Little is known about this fraction, but it probably is the end product of nitrogen metabolism in the cow’s body and in the synthesis of milk in the udder. Normally this fraction amounts to about 5% of the total nitrogen in milk, but physiological disturbances such as mastitis in a higher level of non protein nitrogen.
Casein and Whey Proteins in Milk
