CONVERSION OF ALPHA(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 chy mosin on the alpha(s1)-casein-(24-199)-fragment (alpha(21)-I) and on b eta-casein in vitro under conditions as present in Gouda cheese, and t he possible intervention by the lactococcal cell-envelope proteinase ( CEP) in the subsequent chain of reactions in cheese, have been establi shed. primary cleavage sites with approximately the same susceptibilit y to chymosin are Leu 149 - Phe 150, Leu 156 - Asp 157 and Trp 164 - T yr 165 in alpha(s1)-I and Leu192 - Tyr193 in beta-casein. Two of the t hree main primary products of alpha(s1)-I degradation, viz. fragments f24-149 and f150-164, are rapidly converted by chymosin to several, mo stly small fragments. These fragments, together with the primary produ ct f165-199, are considered to be substraces for starter peptidase act ion in cheese. As long as cell lysis is not significant in cheese, CEP seems to be mainly responsible for further degradation of these pepti des. The phosphoserine-containing alpha(s1)-casein fragment f24-74 and the bitter-tasting beta-casein fragment f193-209 appear to be most re sistant to both chymosin and CEP action.