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( Rennet)
Rennet (pronounced /'r?n?t/) is an enzyme containing substance produced in any mammalian stomach to digest the mother's milk, and is often used in the production of cheese. Rennet contains many enzymes, including a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). The active enzyme in rennet is called chymosin or rennin (EC 3.4.23.4) but there are also other important enzymes in it, e.g., pepsin or lipase. There are non-animal sources for rennet substitutes. The chief use of rennet is in the making of cheese, curd, and junket. Chymosin reacts specifically with ?-casein, cleaving the protein between the amino acids phenylalanine(105) and methionine (106), producing two fragments. The soluble fragment (residues 106-169), which becomes part of the whey, is known as glyco macro peptide and contains the glycosylation sites for ?-casein. The other component (residues 1-105) is insoluble, and in the presence of calcium ions causes the coagulation of the casein micelles to form a curd. Natural calf rennet is extracted from the inner mucosa of the fourth stomach chamber (the abomasum) of young calves. These stomachs are a by-product of veal production. If rennet is extracted from older calves (grass-fed or grain-fed) the rennet contains less or no chymosin but a high level of pepsin and can only be used for special types of milk and cheeses. As each ruminant produces a special kind of rennet to digest the milk of its own mother, there are milk-specific rennets available, such as kid goat rennet especially for goat's milk and lamb rennet for sheep milk. Rennet or digestion enzymes from other animals, like swine-pepsin, are not used in cheese production. Dried and cleaned stomachs of young calves are sliced into small pieces and then put into saltwater or whey, together with some vinegar or wine to lower the pH of the solution. After some time (overnight or several days), the solution is filtered. The crude rennet that remains in the filtered solution can then be used to coagulate milk. About 1 gram of this solution can normally coagulate 2000 to 4000 grams of milk.
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