Rennet, also called 
chymosin (bovine chymosin: 
PDB 1CMS (
http://www.rcsb.org/pdb/cgi/explore.cgi?pid=203441034360021&page=0&pdbId=1CMS), EC 3.4.23.4), 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
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