DAIRY CHEMISTRY AND BIOCHEMISTRY other hand. are relative ely heat labile, being completely denatured by heating at 90C for 10 min Heat-induced changes in milk are discussed in Chapter 9. 4. Caseins are phosphoproteins, containing, on average, 0.85% phosphorus, while the whey proteins contain no phosphorus. The phosphate groups are responsible for many of the important characteristics of casein, especially its ability to bind relatively large amounts of calcium, making it a very nutritionally valuable protein, especially for young animals. The phosphate, which is esterified to the protein via the hydroxyl group of serine, is generally referred to as organic phosphate. Part of the inorganic phosphorus in milk is also associated with the casein in the form of colloidal calcium phosphate(c. 57% of the inorganic phosphorus)( Chap The phosphate of casein is an important contributor to its remarkably high heat stability and to the calcium-induced coagulation of rennet altered casein(although many other factors are involved in both cases) 5. Casein is low in sulphur(0.8%)while the whey proteins are relatively rich 1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentra- tions of cysteine and cystine; in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e. g. cooked flavour, increased rennet coagulation time (due to interaction between B-lactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization 6. Casein is synthesized in the mammary gland and is found nowhere else in nature. Some of the whey proteins(B-lactoglobulin and a-lactalbumin) are also synthesized in the mammary gland, while others(e.g. bovine serum albumin and the immunoglobulins) are derived from the blood 7. The whey proteins are molecularly dispersed in solution or have simple quaternary structures, whereas the caseins have a complicated quaternary structure and exist in milk as large colloidal aggregates, referred to as micelles, with particle masses of 10% D: 8. Both the casein and whey protein groups are heterogeneous, each containing several different proteins 4.2.1 Other protein fractions In addition to the caseins and whey proteins, milk contains two other groups of proteins or protein-like material, i.e. the proteose-peptone frac and the non-protein nitrogen(NPN) fraction. These fractions were gnized as early as 1938 by rowland but until recently very little was150 DAIRY CHEMISTRY AND BIOCHEMISTRY other hand, are relatively heat labile, being completely denatured by heating at 90°C for 10min. Heat-induced changes in milk are discussed in Chapter 9. 4. Caseins are phosphoproteins, containing, on average, 0.85% phosphorus, while the whey proteins contain no phosphorus. The phosphate groups are responsible for many of the important characteristics of casein, especially its ability to bind relatively large amounts of calcium, making it a very nutritionally valuable protein, especially for young animals. The phosphate, which is esterified to the protein via the hydroxyl group of serine, is generally referred to as organic phosphate. Part of the inorganic phosphorus in milk is also associated with the casein in the form of colloidal calcium phosphate (c. 57% of the inorganic phosphorus) (Chap￾ter 5). The phosphate of casein is an important contributor to its remarkably high heat stability and to the calcium-induced coagulation of rennet￾altered casein (although many other factors are involved in both cases). 5. Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentra￾tions of cysteine and cystine; in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between P-lactoglobulin and K-casein) and improved heat stability of milk pre-heated prior to sterilization. 6. Casein is synthesized in the mammary gland and is found nowhere else in nature. Some of the whey proteins (P-lactoglobulin and cr-lactalbumin) are also synthesized in the mammary gland, while others (e.g. bovine serum albumin and the immunoglobulins) are derived from the blood. 7. The whey proteins are molecularly dispersed in solution or have simple quaternary structures, whereas the caseins have a complicated quaternary structure and exist in milk as large colloidal aggregates, referred to as micelles, with particle masses of 106-109 Da. 8. Both the casein and whey protein groups are heterogeneous, each containing several different proteins. 4.2.1 Other protein fractions In addition to the caseins and whey proteins, milk contains two other groups of proteins or protein-like material, i.e. the proteose-peptone frac￾tion and the non-protein nitrogen (NPN) fraction. These fractions were recognized as early as 1938 by Rowland but until recently very little was