Proteolysis cheese curd

Cathepsin D (EC3.4.23.5). It has been known for more than 20 years that milk also contains an acid proteinase, (optimum pH ss 4.0) which is now known to be cathepsin D, a lysozomal enzyme. It is relatively heat labile (inactivated by 70°C x 10 min). Its activity in milk has not been studied extensively and its significance is unknown. At least some of the indigenous acid proteinase is incorporated into cheese curd its specificity on asl- and / -caseins is quite similar to that of chymosin but it has very poor milk-clotting activity (McSweeney, Fox and Olson, 1995). It may contribute to proteolysis in cheese but its activity is probably normally overshadowed by chymosin, which is present at a much higher level. 

The other major casein in cheese is /3-casein, but it is generally not hydrolyzed by rennet in low-pH cheeses. Alkaline milk protease (plas-min) plays the major role in the hydrolysis of /3-casein (Richardson and Pearce 1981). The plasmin level in cheese is related to the pH of the curd at whey drainage, since plasmin dissociates from casein micelles as the pH is decreased. Richardson and Pearce (1981) found two or three times more plasmin activity in Swiss cheese than in Cheddar cheese. Swiss cheese curds are drained at pH 6.4 or higher, while Cheddar cheese curds are drained at pH 6.3 or lower. Proteolysis of /3-casein is significantly inhibited by 5% sodium chloride. The inhibitory influence of sodium chloride is most likely due to alteration of /3-casein or a reduction in the attractive forces between enzyme and substrate (Fox and Walley 1971). 

Shakeel-Ur-Rehman, Waldron D., Fox P.F. 2004. Effect of modifying lactose concentration in cheese curd on proteolysis and in quality of Cheddar cheese. Int. Dairy J. 14, 591-597. 

On the assumption that proteolysis is the rate-limiting event in cheese ripening, there has been interest for several years in adding exogenous proteinases to cheese curd. The first problem encountered is the method of enzyme addition. Direct addition of the proteinase to the cheesemilk ensures its uniform distribution throughout the curd but since most proteinases are water-soluble, most of the added enzyme is lost in the whey, which is economically undesirable, and significant proteolysis may occur prior to coagulation with consequent loss of peptides in the whey and a reduction in cheese yield. 

The coagulant is major contributor to proteolysis in most cheese varieties, notable exceptions being high-cooked varieties, e.g. Emmental and Parmesan, in which the coagulant is extensively or totally denatured during curd manufacture. 

The primary (enzymatic) phase of renneting overlaps somewhat with the secondary phase of aggregation. The gel subsequently undergoes syneresis to produce curds and whey while a slow but more general proteolysis of the caseins begins, which eventually contributes substantially to the distinctive flavor and texture of cheese. The enzymatic coagulation of milk and formation of the curd has been reviewed by Dalgleish (1987). Here, attention will be confined to parts of the subject that most clearly relate to the structure and stability of bovine casein micelles. 

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