Chymosin-induced hydrolysis of caseins: Influence of degree of phosphorylation of alpha-s1-casein and genetic variants of beta-casein

The objective of this study was to investigate the impact of natural variations in α_S1-casein and β-casein composition of milk on chymosin-induced hydrolysis of these caseins in milk gels and in sodium caseinate solutions. At 50% casein degradation, 15% more of α_S1-casein with eight phosphate groups was hydrolysed compared with α_S1-casein with nine phosphate groups in chymosin-induced milk gels. Furthermore, in sodium caseinate solutions, >10% more β-casein A2 was degraded compared with β-casein A1 and B at 50% casein degradation. Proteolysis by chymosin was not impacted by natural variation in the α_S1-casein/β-casein ratio. Natural variation in α_S1-casein/β-casein ratio did not impact upon firmness and gel strength in milk gels. Overall, comparison of sodium caseinate solutions to milk gels showed that differences in either phosphorylation of α_S1-casein or amino acid composition of β-casein caused significant differences in degradation by chymosin, possibly due to changes in physical conformation of caseins.