Effect of somatic cell count and polymorphonuclear leucocyte content of milk on composition and proteolysis during ripening of Swiss-type cheese

During the manufacture of Swiss-type cheese varieties, such as Emmental, Gruyère and Comté, a high scald temperature is used, which inactivates the added coagulant, decreasing the primary proteolysis of αs1-casein during ripening and resulting in the development of the rubbery texture characteristic of this type of cheese (Kosikowski & Mistry, 1997). Swiss-type cheese may be made from raw, thermized or pasteurized milk, depending on the type of manufacture and the country (Beuvier et al. 1997). The suitability of milk for cheesemaking is often assessed from the somatic cell count (SCC) of the raw milk, with increased milk SCC being linked to impaired cheesemaking properties and cheese yield potential (Barbano et al. 1991; Auldist et al. 1996). Late lactation milk, which often has elevated SCC, also yields poor quality cheese (Lucey, 1996). High SCC milk has elevated levels of the milk alkaline proteinase plasmin (EC 3.4.21.7), but its activity is not generally considered to be detrimental during cheese ripening (Farkye & Fox, 1992; Bastian & Brown, 1996). Milk somatic cells are themselves also associated with a number of proteolytic enzymes of differing properties, the activities of which are ill-defined in dairy products (Verdi & Barbano, 1991), with the exception of the acid proteinase cathepsin D, which has a chymosin-like activity towards αs1-casein (McSweeney et al. 1995). Preliminary evidence of cathepsin D activity in Swiss cheese during ripening has been reported (Igoshi & Arima, 1993; Beuvier et al. 1997). In the present study the effect of milk SCC on the ripening of Swiss-type cheese was examined. Of particular interest was the role of polymorphonuclear leucocytes (PMN), the predominant cell type in high SCC mastitic milk (Heegaard et al. 1994) and late lactation milk, when the frequency of milking varies (Kelly et al. 1998). PMN possess a range of proteolytic enzymes, including cathepsin D (Verdi & Barbano, 1991) and plasminogen activators (PA; Heegaard et al. 1994). The experimental strategy chosen was to examine the effects on the quality and proteolysis during ripening of Swiss-type cheeses of adding high SCC milk, whose cell population is mostly PMN, to creamery milk.