Proteolysis in Cheese During Ripening

The enzymatic hydrolysis of the casein matrix is a major biochemical event that influences the development of cheese structure and flavor during ripening. Products of proteolysis are a very large number of peptides, together with free amino acids. The enzymes which mediate proteolysis originate from different sources: the milk, residual coagulant, starter lactic acid bacteria (LAB), adjunct organisms, nonstarter bacteria and, rarely, exogenous proteinases and peptidases. The principal milk proteinase, plasmin hydrolyzes β-and αs2-caseins during ripening and is of most significance in high-cook cheeses and varieties in which the pH increases significantly during ripening. Residual coagulant trapped in the curd is the major source of proteinase activity in low to medium-cooked cheeses. Residual coagulants and plasmin act directly on intact casein to form a number of large and intermediate-sized peptides. Cell envelope-associated proteinase of the starter LAB acts upon these peptides to produce shorter peptides, which are further hydrolyzed by a wide range of peptidases to release amino acids. Certain regions of the intact casein molecules are degraded extensively while some parts remain largely intact; for example, in long-ripened cheese still γ-caseins from plasmin activity on β-casein remain. Cheese proteolysis results in the formation of several hundreds or thousands of peptides, of which some contribute to umami and bitter taste. Furthermore, free amino acids contribute to cheese taste and some are catabolized to volatile flavor compounds or synthesized into flavor peptides.