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substance containing rennin, an enzyme having the property of clotting, or curdling, milk. It is used in the making of cheese and junket. Rennet is obtained from the stomachs of young mammals living on milk, especially from the inner lining of the fourth, or true,
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(also chymosin), a proteolytic enzyme produced by the mucous membrane of the abomasum, or fourth stomach, of young ruminants. Rennin, in the form of abomasal extract, has long been used for coagulating milk in cheese-making. It is synthesized as an inactive precursor, renninogen, which is converted into rennin in an acid medium at pH < 5.0. The starting substrate for rennin is the milk protein κ-casein, which under the action of rennin loses the ability to stabilize the casein micelles. Rennin is obtained in crystalline form; its molecule is composed of a polypeptide chain (molecular weight ~ 34,000) in which dicarboxylic amino acids predominate. Rennin resembles pepsin in many of its properties.