Casein proteins are the primary constituents of animal milk protein (accounting roughly around 80% of the total milk protein). The amount of casein in cow?s milk varies according to the breed of cow andstage of lactation, but is generally in the range of 24-29 g L-1. Casein contains 0.7-0.9 % of phosphorus, covalently bound to the protein by a serine ester linkage. Casein is consequently known as phopho-protein. Casein protein contains all the amino acids essential to man in high proportion, with the possible exception of cysteine. Thus, casein may be considered as a highly nutritious protein.
Casein exists in milk in complex groups of molecules (referred to as calcium phopho-caseinate)that are called ?micelles?. The micelles consist of casein molecules, calcium, inorganic phosphate and citrate ions, and have a typical molecular weight of several hundred millions. Casein micelles exist in milk as a very stable colloidal dispersion. The caseins, as proteins, are made up ofmany hundreds of individual amino acids, each of which may have a positive or a negativecharge, depending on the pH of the [milk] system. At some pH value, all the positive chargesand all the negative charges on the [casein] protein will be in balance, so that the net chargeon the protein will be zero. That pH value is known as the isoelectric point (IEP) of theprotein and is generally the pH at which the protein is least soluble. For casein, the IEP isapproximately 4.6 and it is the pH value at which acid casein is precipitated. In milk, whichhas a pH of about 6.6, the casein micelles have a net negative charge and are quite stable. Casein has been shown to consist of several individual casein components, referredto as ?s1-, ?s2-, ?- and ?-casein.