Rennet

Rennet /ˈrɛnɪt/ is a complex set of enzymes produced in the stomachs of ruminant mammals. Chymosin, its key component, is a protease enzyme that curdles the casein in milk. In addition to chymosin, rennet contains other important enzymes such as pepsin and a lipase. Rennet /ˈrɛnɪt/ is a complex set of enzymes produced in the stomachs of ruminant mammals. Chymosin, its key component, is a protease enzyme that curdles the casein in milk. In addition to chymosin, rennet contains other important enzymes such as pepsin and a lipase. Rennet is used to separate milk into solid curds (for cheesemaking) and liquid whey, and so is used in the production of most cheeses. One of the main actions of rennet is its protease chymosin cleaving the kappa casein chain. Casein is the main protein of milk. Cleavage causes casein to stick to other cleaved casein molecules and form a network. It can cluster better in the presence of calcium and phosphate, which is why it is occasionally added in cheese making especially from calcium phosphate-poor goat milk. The solid truncated casein protein network traps other components of milk like fats and minerals to create cheese. In digestion it is followed by other proteases cutting casein further to ultimately release and absorb the component amino acids and minerals. In a nutshell, curdling is chymosin cleaving casein, a process that is part of natural digestion and cheese making. Calf rennet is extracted from the inner mucosa of the fourth stomach chamber (the abomasum) of young, unweaned calves as part of livestock butchering. These stomachs are a byproduct 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 species, milk-specific rennets are available, such as kid goat rennet for goat's milk and lamb rennet for sheep's milk. Dried and cleaned stomachs of young calves are sliced into small pieces and then put into salt water 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 g of this solution can normally coagulate 2 to 4 L of milk. Deep-frozen stomachs are milled and put into an enzyme-extracting solution. The crude rennet extract is then activated by adding acid; the enzymes in the stomach are produced in an inactive form and are activated by the stomach acid. The acid is then neutralized and the rennet extract is filtered in several stages and concentrated until reaching a typical potency of about 1:15,000; meaning 1 g of extract can coagulate 15 kg of milk. One kg of rennet extract has about 0.7 g of active enzymes – the rest is water and salt and sometimes sodium benzoate (E211),0.5% - 1.0% for preservation. Typically, 1 kg of cheese contains about 0.0003 g of rennet enzymes. Because of the limited availability of mammalian stomachs for rennet production, cheese makers have sought other ways to coagulate milk since at least Roman times. The many sources of enzymes that can be a substitute for animal rennet range from plants and fungi to microbial sources. Cheeses produced from any of these varieties of rennet are suitable for lactovegetarians. Fermentation-produced chymosin is used more often in industrial cheesemaking in North America and Europe today because it is less expensive than animal rennet. Many plants have coagulating properties. Homer suggests in the Iliad that the Greeks used an extract of fig juice to coagulate milk. Other examples include several species of Galium, dried caper leaves, nettles, thistles, mallow, and ground ivy. Enzymes from thistle or Cynara are used in some traditional cheese production in the Mediterranean. Phytic acid, derived from unfermented soybeans, or fermentation-produced chymosin (FPC) may also be used.