Vivek K. Upadhyay

Proteolysis in Cheese during Ripening

This chapter discusses proteolysis process in cheese during ripening. Proteolysis contributes to: (1) The development of cheese texture: via hydrolysis of the protein matrix of cheese; via a decrease in aw through changes to water binding by the new carboxylic acid and amino groups liberated on hydrolysis of peptide bonds. These groups are ionized at the pH of cheese and thus bind water; indirectly via an increase in pH caused by the liberation of ammonia from amino acids produced by proteolysis. (2) Flavor and perhaps the off-flavor of cheese, directly by the production of short peptides and amino acids, some of which have flavors; indirectly by the liberation of amino acids, which act as substrates for a range of catabolic reactions, which generate important volatile flavor compounds; by facilitating the release of sapid compounds from the cheese matrix during mastication. Proteolysis in cheese during ripening is catalyzed by proteinases and peptidases from six sources: (1) the coagulant—the enzymes involved depend on the type of coagulant used. (2) the milk—a number of indigenous proteinases are present in milk, the most important of which is plasmin, which is produced from an inactive precursor, plasminogen. (3) starter lactic acid bacteria (LAB) contain a cell envelope-associated proteinase, which contributes to ripening principally by hydrolyzing intermediate-sized and short peptides produced from the caseins by the action of chymosin or plasmin. The other three sources are nonstarter lactic acid bacteria (NSEAB), secondary starter ( Propionibacterium freudenreichii subsp, shermanii in Swiss-type cheese), and exogenous proteinases and peptidases.

Acceleration of proteolysis during ripening of Cheddar-type cheese using of a streptokinase-producing strain of Lactococcus

Bovine milk contains a number of indigenous proteolytic enzymes, of which plasmin is the most important (Grufferty & Fox, 1988; Bastian & Brown, 1996; Kelly & McSweeney, 2003). Plasmin (EC 3.4.21.7) is a serine proteinase with pH and temperature optima of 7·5 and 37 °C, respectively. In milk, most of the plasmin is present as its inactive precursor, plasminogen, which is converted to active plasmin by plasminogen activators (PA) present in milk, e.g., urokinase-type (u-PA) and tissue-type PA (t-PA) (Bastian & Brown, 1996). Since plasmin, plasminogen and PA are associated with casein micelles, they are incorporated into cheese curd, while plasmin inhibitors and inhibitors of PA are lost with the whey. Plasmin incorporated in cheese curd acts on its substrate, the caseins, contributing significantly to primary proteolysis during ripening (Upadhyay et al. 2004b).

Biochemistry of Cheese Ripening: Proteolysis

Abstract 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 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 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.