The selectivity of chymosin action on alpha(s1)- and beta-caseins in solution is modulated in cheese

The primary proteolysis of caseins by chymosin in Gouda cheese was assessed by identifying the peptides present in the water-soluble fraction of a starter-free cheese. The results show that the selectivity of chymosin, as observed in solution, is affected by the cheese environment. Of all known preferred cleavage sites in alpha(sl)- and beta-caseins in solution, the enzyme failed to cleave susceptible bonds in the C-terminal part of alpha(sl)-casein in cheese. While alpha(sl)-casein is a poorly structured molecule in buffered solution, a specific structural arrangement is believed to occur in cheese and to exclusively affect the accessibility of the segment which includes the bonds 164-165, 156-157 and 149-150 that are highly susceptible in solution. Initially unevenly distributed salt in Gouda cheese is responsible for a limited extent of aggregation of beta-casein; it allows a relatively rapid degradation of the monomeric form during the first weeks of ripening. A broader specificity of chymosin on the accessible part of alpha(sl)-casein as well as on beta-casein is observed in cheese than in solution; this most probably results from an effect of the cheese environment on the substrate and/or the enzyme which affects the interactions between forces regulating binding. (C) 1997 Elsevier Science Limited.