Proteolysis of casein is the principal cause of textural changes and flavour development in ripened cheese (Fox & McSweeney, 1996). Caseins are degraded into small peptides and free amino acids during a complex process, described by Grappin et al . (1985) as a two-step scheme. Caseins are initially broken down into large, well characterised fragments. This initial step, called primary proteolysis, is catalysed principally by the residual coagulant (chymosin and pepsin), and to a variable extent by endogenous milk proteases, such as plasmin, cathepsin D, and possibly somatic cell proteinases. Enzymes originating from either rennet or milk are active in most ripened cheese varieties. However, their relative contributions vary substantially depending on manufacturing practices. For instance, in Swiss-type cheeses, cooking the curd extensively inactivates the coagulant, and simultaneously enhances plasmin activity, which therefore becomes predominant (Ollikainen & Kivela, 1989).