Chymosin /ˈkaɪməsɪn/ or rennin /ˈrɛnɪn/ is a protease found in rennet. It is an aspartic endopeptidase belonging to MEROPS A1 family. It is produced by newborn ruminant animals in the lining of the abomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. It is widely used in the production of cheese. Bovine chymosin is now produced recombinantly in E. coli, Aspergillus niger var awamori, and K. lactis as alternative resource.
ALTHOUGH proteolytic activity has been shown by Berridge1 in a partially crystalline preparation of rennin, and by De Baun, Connors and Sullivan2 in a preparation 75 per cent pure, there has hitherto been no published proof that the activity is not due to contamination. Moreover, it has been implied that rennin has only weak proteolytic activity3.
The action of rennin on kappa-casein was studied as a function of time using several methods to measure activity. The first indication of rennin cleavage of kappa-casein is precipitability in .1 M acetate buffer at pH 5.2 and 5 C. A longer exposure to rennin is required to alter kappa-casein so that it forms a precipitate with calcium ions and loses its ability to stabilize alpha s-casein. The least sensitive indication of rennin activity is measurement of nitrogen soluble in 2% trichloroacetic acid. Electrophoresis experiments showed that these methods detect various stages in the conversion of kappa-casein para-kappa-casein.
Chymosin, known also as rennin, is a proteolytic enzyme related to pepsin that synthesized by chief cells in the stomach of some animals. Its role in digestion is to curdle or coagulate milk in the stomach, a process of considerable importance in the very young animal. If milk were not coagulated, it would rapidly flow through the stomach and miss the opportunity for initial digestion of its proteins.
Rennet is a liquid, paste, or powder preparation containing the enzyme, rennin. It is usually prepared by extracting the fourth stomach (abomasum) of milk-fed calves with a sodium chloride solution, and is used in the manufacture of cheese, rennet casein, and various specialty products such as junket or rennet custard. Rennet is probably one of our oldest commercially-used enzyme preparations. Several hundred-thousand gallons of extract are produced each year in addition to large amounts of paste and powder. In spite of its long and extensive use, many properties of rennet, as well as rennin itself, are not fully understood. The milk-clotting action of rennet has been known for centuries, but the nature of this action has never been satisfactorily explained. Many theories of rennin action have been advanced, but none have been soundly established by experimental evidence. Much of the material on rennin in the literature is contradictory.
Because of the complexity of milk and the complexity of the clotting reaction, no completely satisfactory method has yet been devised for measuring rennin activity and studying the influence of pH, temperature, and possible activators and inhibitors on the activity of the enzyme itself. While it is known that calcium ions and other di- or trivalent cations are essential for the clotting of milk in the presence of rennin, the function of the ions in clot formation has never been demonstrated. Rennin has some proteolytic activity, but its importance in the proteolytic breakdown of cheese during ripening has been questioned from time to time. These are but a few of the unsolved problems which stand in our way of a better understanding of the nature of rennin and the role it plays in the cheese-making operation.
The effect of temperature and time of heating whole milk on the renninclotting time, the primary phase of rennin action and the protein (mainly β-lactoglobulin) soluble in 2% trichloroacetic acid (TCA), have been studied. Considerable changes in these parameters occurred above 60C. The primary phase was inhibited (the degree of inhibition being both temperature and time-dependent), the clotting time was increased, and the protein soluble in 2% TCA decreased considerably.
n. an enzyme produced in the stomach that coagulates milk. It is secreted by the gastric glands in an inactive form, prorennin, which is activated by hydrochloric acid. Rennin converts caseinogen (milk protein) into insoluble casein in the presence of calcium ions. This ensures that the milk remains in the stomach, exposed to protein-digesting enzymes, for as long as possible. The largest amounts of rennin are present in the stomachs of young mammals.
Rennin, or chymosin, is the active protease in rennet. Rennin splits the milk protein at a specific point, affecting the clotting. After splitting, the casein clumps together and separates from the whey, forming curds. Rennin is naturally found in the calf stomachs, where they use the enzyme to prolong the digestion of milk. In fact, it is believed that cheese was first discovered by accident when milk was kept in a bag made of the stomach of a calf or goat. However, now the more common practice is to either extract the rennin from the frozen stomach of a calf and incubate it before adding it to a vat of milk or by extracting rennin from thistles or cynara can also be used (Frankhauser, 2009).
However, there are environmental issues surrounding the production of rennet. For thousands of years, cheese makers have relied on traditional rennet sources. Over the last few decades the demand for cheese has increased at such an unprecedented rate, that supplies of rennin simply cannot keep up. Since the 1990s, cheese makers have begun using genetically modified rennin as an alternative to natural renin. There are many controversies that come with the use of GMOs and cheese makers are divided in this regard. France is one of only two countries that outlaw the use of genetically modified rennin, and due to not technically being an ingredient, is often not required to be listed as a GMO on the labels. Unlike the cheese makers in most countries, the French believe that natural renin is both better for the environment, but also provides better taste and more complex flavors (Chymosin, 2010).