Milk-clotting enzymes are used during the production of cheese to coagulate the casein, allowing the formation of a three-dimensional network that entraps the milk fat. Commercially available milk-clotting enzymes differ with respect to source, specificity, optimum pH and thermostability. All are acid proteinases that can cleave κ-casein resulting in the coagulation of milk. Chymosin (EC 3.4.23.4) is specific for the Phe–Met bond in κ-casein at the natural pH of milk (6·7). Recombinant chymosin is available commercially from a variety of sources and has a maximum activity at 40 °C. Recombinant chymosins are purified from the fermentation of recombinant strains of Aspergillus niger, Asp. oryzae or Kluyveromyces marxianus. These enzyme preparations are chemically and functionally identical to calf chymosin. Rennets are purified from the abomasum of bovines and can contain from 60 to 100% chymosin with the remainder being primarily bovine pepsin (Wigley, 1996). Microbial proteinases (EC 3.4.23.6) are generally more proteolytic than chymosin, with varying heat stability. These enzymes liberate more non-protein N from casein and can cleave α- and β-casein as well as κ-casein at the natural pH of milk. Acid proteinases from Cryphonectria parasitica are more heat labile than those from Rhizomucor miehei, which are characterized as thermostable (Ernstrom & Wong, 1974).

The objective of this research was to characterize milk-clotting enzymes with respect to thermal inactivation in skim milk. This information has applications in milk and whey processing.