DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 9.2 Time-temperature for the destruction of berculosis (.), inactivation of alkaline phosphatase ()and creaming ability of milk (--)(from Webb and Johnson, 1965) This may cause the release of H, S (which can result in the development of an off-flavour) and disulphide interchange reactions with whey proteins, leading to the formation of a layer of denatured whey proteins on the fat globules at high temperatures(>100.C). The membrane and or whey proteins may participate in Maillard browning with lactose and the cysteine may undergo B-elimination to dehydroalanine, which may then react with lysine to form lysinoalanine or with cysteine residues to form lanthionine, leading to covalent cross linking of protein molecules(section 9.6.3). Mem brane constituents, both proteins and pl lost from the membrane to the aqueous phase at high temperatures. Much of the indigenous copper in milk is associated with the MFGM and some of it is transferred to the serum on heat processing. Thus, severe heat treatment of cream improves the oxidative stability of butter made from it as a result of the reduced concentration of pro-oxidant Cu in the fat phase and the antioxidant effect of exposed sulphydryl groups The consequences of these changes in the MFGM have been the subject of little study, possibly because severely heated milk products are usually homogenized and an artificial membrane, consisting mainly of casein and some whey proteins, is formed; consequently, changes in the natural mem brane are not important. Damage to the membrane of unhomogenized products leads to the formation of free(non-globular) fat and consequently to 'oiling-off and the formation of a cream plug'( Chapter 3)3 50 DAIRY CHEMISTRY AND BIOCHEMISTRY 50 60 70 80 Temperature ("C) Figure 9.2 Time-temperature curves for the destruction of M. tuberculosis (. . .), inactivation of alkaline phosphatase (-) and creaming ability of milk (---) (from Webb and Johnson, 1965). This may cause the release of H,S (which can result in the development of an off-flavour) and disulphide interchange reactions with whey proteins, leading to the formation of a layer of denatured whey proteins on the fat globules at high temperatures (> l0OT). The membrane and/or whey proteins may participate in Maillard browning with lactose and the cysteine may undergo p-elimination to dehydroalanine, which may then react with lysine to form lysinoalanine or with cysteine residues to form lanthionine, leading to covalent cross-linking of protein molecules (section 9.6.3). Mem￾brane constituents, both proteins and phospholipids, are lost from the membrane to the aqueous phase at high temperatures. Much of the indigenous copper in milk is associated with the MFGM and some of it is transferred to the serum on heat processing. Thus, severe heat treatment of cream improves the oxidative stability of butter made from it as a result of the reduced concentration of pro-oxidant Cu in the fat phase and the antioxidant effect of exposed sulphydryl groups. The consequences of these changes in the MFGM have been the subject of little study, possibly because severely heated milk products are usually homogenized and an artificial membrane, consisting mainly of casein and some whey proteins, is formed; consequently, changes in the natural mem￾brane are not important. Damage to the membrane of unhomogenized products leads to the formation of free (non-globular) fat and consequently to 'oiling-off and the formation of a 'cream plug' (Chapter 3)