Biochemistry of Cheese Ripening: Proteolysis

The enzymatic hydrolysis of the casein matrix is a major biochemical event that occurs during cheese 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, coagulant, starter lactic acid bacteria (LAB), and adjunct organisms, nonstarter bacteria and, rarely, exogenous proteinases and peptidases. The principal indigenous proteinase in milk, plasmin, hydrolyzes β- and α_s2-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 coagulant and plasmin act directly on the caseins to form a number of large and intermediate-sized peptides. The peptides are then acted upon by the cell envelope-associated proteinase of the starter LAB to produce shorter peptides, which are then degraded by a wide range of peptidases to form free amino acids. Only certain regions of the caseins are degraded extensively, and perhaps 75% of the caseins in mature Cheddar cheese remain intact or are present as large polypeptides. Proteolysis results, however, in the formation of perhaps hundreds of peptides and a complement of free amino acids which can be converted to volatile flavor compounds via amino acid catabolism.