MILK PROTEINS 149 Appendix 4A. We have retained the term cystine to indicate two disulphide linked cysteines 4.2 Heterogeneity of milk proteins Initially, it was believed that milk contained only one type of protein bi about 100 years ago it was shown that the proteins in milk could be fractionated into two well-defined groups On acidification to pH 4.6(the oelectric pH)at around 30C, about 80% of the total protein in bovine milk precipitates out of solution; this fraction is now called casein. The protein which remains soluble under these conditions is referred to as whey or serum protein or non-casein nitrogen. The pioneering work in this area was done by the German scientist, Hammarsten, and consequently isoelec- tric(acid) casein is sometimes referred to as casein nach Hammarsten The ratio of casein; whey proteins shows large interspecies differences; in human milk, the ratio is c. 40: 60, in equine(mare's)milk it is 50: 50 while in the milks of the cow, goat, sheep and buffalo it is c. 80: 20. Presumably, hese differences reflect the nutritional and physiological requirements of the young of these species There are several major differences between the caseins and whey proteins, of which the following are probably the most significant, especially from an industrial or technological viewpoint 1. In contrast to the caseins, the whey proteins do not precipitate from solution when the pH of milk is adjusted to 4.6. This characteristic is used as the usual operational definition of casein. This difference in the properties of the two milk protein groups is exploited in the preparation of industrial casein and certain varieties of cheese(e. g. cottage, quarg and 2. Chymosin and some other proteinases(known as rennets) produce a very ight, specific change in casein, resulting in its coagulation in the presence of Ca-t. Whey proteins undergo no such alteration. The coagulability of casein through the action of rennets is exploited in the manufacture of most cheese varieties and rennet casein; the whey proteins are lost in the whey. The rennet coagulation of milk is discussed in Chapter 10 3. Casein is very stable to high temperatures; milk may be heated at its natural pH (c. 6.7)at 100C for 24 h without coagulation and it withstands heating at 140C for up to 20 min. Such severe heat treat ments cause many changes in milk, e.g. production of acids from lactose resulting in a decrease in pH and changes in the salt balance, which eventually cause the precipitation of casein. The whey proteins, on theMILK PROTEINS 149 Appendix 4A. We have retained the term cystine to indicate two disulphide￾linked cysteines. 4.2 Heterogeneity of milk proteins Initially, it was believed that milk contained only one type of protein but about 100 years ago it was shown that the proteins in milk could be fractionated into two well-defined groups. On acidification to pH 4.6 (the isoelectric pH) at around 30°C, about 80% of the total protein in bovine milk precipitates out of solution; this fraction is now called casein. The protein which remains soluble under these conditions is referred to as whey or serum protein or non-casein nitrogen. The pioneering work in this area was done by the German scientist, Hammarsten, and consequently isoelec￾tric (acid) casein is sometimes referred to as casein nach Hammarsten. The ratio of casein : whey proteins shows large interspecies differences; in human milk, the ratio is c. 40 : 60, in equine (mare's) milk it is 50: 50 while in the milks of the cow, goat, sheep and buffalo it is c. 80 : 20. Presumably, these differences reflect the nutritional and physiological requirements of the young of these species. There are several major differences between the caseins and whey proteins, of which the following are probably the most significant, especially from an industrial or technological viewpoint: 1. In contrast to the caseins, the whey proteins do not precipitate from solution when the pH of milk is adjusted to 4.6. This characteristic is used as the usual operational definition of casein. This difference in the properties of the two milk protein groups is exploited in the preparation of industrial casein and certain varieties of cheese (e.g. cottage, quarg and cream cheese). Only the casein fraction of milk protein is normally incorporated into these products, the whey proteins being lost in the whey. 2. Chymosin and some other proteinases (known as rennets) produce a very slight, specific change in casein, resulting in its coagulation in the presence of Ca2+. Whey proteins undergo no such alteration. The coagulability of casein through the action of rennets is exploited in the manufacture of most cheese varieties and rennet casein; the whey proteins are lost in the whey. The rennet coagulation of milk is discussed in Chapter 10. 3. Casein is very stable to high temperatures; milk may be heated at its natural pH (c. 6.7) at 100°C for 24h without coagulation and it withstands heating at 140°C for up to 20min. Such severe heat treat￾ments cause many changes in milk, e.g. production of acids from lactose resulting in a decrease in pH and changes in the salt balance, which eventually cause the precipitation of casein. The whey proteins, on the