Milk protein

Of the major organic constituents in milk, milk protein is arguably the most important, nutritionally and economically.Milk protein is of high biological value, it has none of the anti-nutritive factors often associated with high-protein foods of plant origin, it has good processing characteristics and is bland, making it useful for incorporation into a wide variety of food products for human consumption.The proportion of milk produced in Britain that is used for manufacturing is increasing year by year and a greater commercial emphasis is being placed on the milk protein.This paper is concerned specifically with that constituent and deals particularly with the relationship between the cow's nutrition and the content and composition of the protein in her milk. The protein constituents of milkConventionally, milk 'total protein' is estimated as 6.38 x total nitrogen content (this assumes that milk 'protein' contains 156.8 gN/kg, although modem analysis suggests that 158.9 gN/kg might be a more appropriate figure).Of this 'protein' a small proportion is non-protein N but typically more than 94% is true protein.The largest component in the non-protein N fraction is urea which diffuses freely throughout body water so that the concentration of urea in milk is in equilibrium with that in blood plasma (see Thomas, 1980).The milk true protein is a mixture of caseins and milk serum or whey proteins.Most of the proteins present are synthesized in the mammary gland though there is a transfer of immunoglobulins and albumin from the blood.In early studies of the chemistry of milk, the proteins were routinely fractionated using precipitation techniques (Rowland, 1938; Aschaffenburg & Drewry, 1959) to give, for example, casein, p-lactoglobulin, residual albumin (which is mainly a-lactalbumin), globulin and a proteose-peptone component.Application of gel electrophoresis and gel chromatography techniques has more recently allowed fuller analyses to be made and coupled with this there have been determined efforts to rationalize and standardize nomenclature for both the major and minor protein components (see Brunner, 1981).The main components of the casein fraction are now regarded as asI-, asz-, p-, y-and K-caseins, whilst the milk serum proteins consist of P-lactoglobulin, a-lactalbumin, bovine serum albumin, immunoglobulins and a proteose-peptone fraction.There is still disagreement in the literature about the exact proportions of the total protein that are accounted for by these various protein components, but this partly reflects methodological differences in analysis.Some recent analyses of the average content and composition of the protein in creamery milk samples are given in Table I , and these are probably typical of milks produced under British conditions.