Rennet Coagulation of Milk

The initial step in most manufacture of most cheese varieties involves the enzymatic hydrolysis of one of the milk proteins, κ-casein. The enzyme involved is called rennet. During the primary stage, κ-casein is cleaved by rennet at the Phe105–Met106 bond, resulting in a reduction in both the net negative charge and steric repulsion, such that rennet-altered micelles become susceptible to aggregation and after a lag phase, a three-dimensional gel network is formed. Historically, rennet was extracted from the fourth stomach of young calves but today various other forms are available including recombinant chymosin, which is produced by several genetically modified organisms. The coagulation reaction involves two phases: the primary enzymatic phase involving hydrolysis of κ-casein and the secondary aggregation reaction which involves the association of micelles that have most of the κ-casein peptides hydrolyzed by rennet. The rennet coagulation of milk is greatly impacted by environmental conditions, such as, temperature, pH and calcium content.