Dairy Chemistry And Biochemistry

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS^385

Figure 10.2 Schematic representation of the tertiary structure of an aspartyl proteinase,

showing the cleft which contains the active site; arrows indicate p structures and cylinders the

%-helices (from Foltmann, 1987).

Due to increasing world production of cheese and the declining supply
of young calf stomachs (referred to as vells), the supply of calf rennet has
been inadequate for many years. This has led to a search for suitable
substitutes. Many proteinases are capable of coagulating milk but most are
too proteolytic relative to their milk-clotting activity, leading to a decrease
in cheese yield (due to excessive non-specific proteolysis in the cheese vat
and loss of peptides in the whey) and defects in the flavour and texture of
the ripened cheese, due to excessive or incorrect proteolysis. Only six
proteinases are used commercially as rennet substitutes: porcine, bovine and
chicken pepsins and the acid proteinases from Rhizomucor miehei, R. pusillus
and Cryphonectria parasitica. Chicken pepsin is quite proteolytic and is used
widely only in Israel (for religious reasons). Porcine pepsin enjoyed limited
success about 30years ago, usually in admixtures with calf rennet, but it is
very sensitive to denaturation at pH values above 6 and may be denatured
extensively during cheesemaking, leading to impaired proteolysis during
ripening; it is now rarely used as a rennet substitute. Bovine pepsin is quite
effective and many commercial calf rennets contain up to 50% bovine
pepsin. Rhizomucor miehei proteinase, the most widely used microbial
rennet, gives generally satisfactory results. Cryphonectria parasitica pro-
teinase is, in general, the least suitable of the commercial microbial rennet
substitutes and is used only in high-cooked cheeses in which extensive
denaturation of the coagulant occurs, e.g. Swiss-type cheeses.
The gene for calf chymosin has been cloned in Kluyveromyces marxianus
var. lactis, Aspergillus niger and E. coli. Microbial (cloned) chymosins have