Conversion of alpha sub(s1)-casein-(24-199)-fragment and beta -casein under cheese conditions by chymosin and starter peptidases

The sequence of proteolytic cleavages characterizing the action of chymosin on the alpha sub(s1)-casein-(24-199)-fragment ( alpha sub(s1)-I) and on beta -casein in vitro under conditions as present in Gouda cheese, and the possible intervention by the lactococcal cell-envelope proteinase (CEP) in the subsequent chain of reactions in cheese, have been established. Primary cleavage sites with approximately the same susceptibility to chymosin are Leu 149 - Phe 1501, Leu 156 - Asp 157 and Trp 164 - Tyr 165 in alpha sub(s1)-I and Leu192 - Tyr 193 in beta -casein. Two of the three main primary products of alpha sub(s1)-I degradation, viz. fragments f24-149 and f150-164, are rapidly converted by chymosin to several, mostly small fragments. These fragments, together with the primary product f165-199, are considered to be substrates for starter peptidase action in cheese. As long as cell lysis is not significant in cheese, CEP seems to be mainly responsible for further degradation of these peptides. The phosphoserine-containing alpha sub(s1)-casein fragment f24-74 and the bitter-tasting beta -casein fragment f193-209 appear to be most resistant to both chymosin and CEP action.