Interestingly, significant differences in the peak development of-casein A1 and A2 as well as for S1-casein with 8P and 9P were observed. Therefore, the mass spectra of peaks 4e9 were analysed after 8 weeks storage to investigate the components present in the peaks. The results are summarised in Table 2 and show that pro- teose peptones 5 and 8 (slow) from -casein A2 are co-eluting with S2-casein along with a polypeptide from S1-casein f(1e124). The
corresponding polypeptide S1-casein f(125e199) was co-eluting with S1-casein 8P. While the peaks of S1-casein 9P and -casein A1, either were not or only slightly contaminated with polypeptides, 1- and 2-caseins were co-eluting with -casein A2 and 3-casein with -la. This explains the difference in the peak development of S1-casein 8 and 9P as well as -casein A1
and A2 .A comparison of the mass spectra (data not shown) showed that the rate of hydrolysis was similar for the two genetic variants A1 and A2 of -casein and for the differently phosphorylated S1-caseins. For this reason, S1-casein 9P and -casein A1 were chosen for a relative quantification of the proteolysis based on the UV chromatograms.