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Rennet is the substance used to curd milk for cheese.
Rennet can be of various origins:
- animal : an enzyme called "chymosin" obtained by recovering gastric juice from the fourth stomach (abomasum, or curd) of unweaned calves, goats or lambs;
- synthesis : enzymatic microbial coagulant, of chemical nature (for example from Rhizomucor miehei, Rhizomucor pusillus, Mucor miehei...).
- genetic engineering: GMO coagulant called "recombinant bovine chymosin" subsisting on "bovine chymosin" and using the term "rennet." This coagulant has been developed to meet the demand of the cheese agribusiness industry and these large production volumes.
The active substance in rennet is a proteolytic digestive enzyme called Chymosin. It serves the young animal to curd the milk in his stomach; the cheese maker will therefore use it for these purposes. Chymosin attacks a specific fraction of milk proteins (casein Kappa) and destroys them. This destabilizes the structure of the milk, thus causing coagulation.
Depending on the cheese technology and the cheese made, the cheese maker will use a rennet more or less concentrated in chymosin. It thus benefits from a greater or lesser contribution of enzymatic environment rich in amino acids, small peptides and other enzymes. The richness of this enzymatic environment depends on the maceration time of the stones. This maceration is carried out in extractors where the pebbles are placed and an extraction solution composed mainly of water, salt and various preservatives. At the end of maceration, the juice is removed and the stones are pressed to recover as much as possible, and therefore as many enzymes as possible. This juice will undergo flocculation, then coarse filtration, and finally bacteriological filtration in order to avoid any contamination before packaging.
Because of its potential animal origin, some vegetarians refrain from eating cheeses that may contain it. Similarly, cheeses in some countries (United Kingdom) are sometimes labelled "vegetarian" when the rennet used in their manufacture is clearly of plant or synthetic origin, a display which is rarely found in France.
Rennet is sometimes added in some homemade yogurt recipes.