The changes in the concentration of available lysine, available methionine, tryptophan and arginine in the samples tested are shown in Table 1. Heating Na caseinate reduced the levels of available lysine and methionine by about 5 and 10%, respectively, compared to the controls. Destruction of lysine probably occurred due to protein–protein interactions involving ε-NH2 groups with the amide groups of asparagine and glutamine, resulting in the formation of cross links. Intermolecular protein cross links in the system can be demonstrated by SDS-PAGE which showed that the major bands were smeared and indistinct, indicating that considerable protein–protein interaction or protein crosslinking occurred during heating ( Fig. 1: slots 2 and 3). The loss of methionine may be due to damage to the methyl–thiol group by alkylation or other interactions during heating. However, the apparent values for arginine and tryptophan were increased after heating. The reasons for the increases are unknown; it is probable that some specific compounds were formed during heating which interfered with the determination of arginine and tryptophan.