Rennet, also called chymosin (bovine chymosin: PDB 1CMS, EC 18.104.22.168), is an enzyme that is added to milk as the first step in making cheese, or for making junket. The enzyme converts calcium caseinate particles in the milk to relatively insoluble calcium paracaseinate, which in the presence of calcium ions coagulates to form a curd.
The usual source of rennet is the abomasum (fourth stomach) of slaughtered new-born calves or other young ruminants. It is also possible to produce rennet from fungi or genetically modified yeast or bacteria, allowing the production of cheese that is considered vegetarian.
See also: pepsin