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Rennet (/ˈrɛnɪt/) is a complex set ofenzymes produced in the stomachs ofruminant mammals.Chymosin, its key component, is aprotease enzyme thatcurdles thecasein in milk. In addition to chymosin, rennet contains other enzymes, such aspepsin and alipase.
Rennet has traditionally been used to separate milk into solidcurds and liquidwhey, used in the production of cheeses. Rennet from calves has become less common for this use, to the point that less than 5% of cheese in the United States is made using animal rennet today.[1] Most cheese is now made usingchymosin derived from bacterial sources.
One of the main actions of rennet is itsproteasechymosin cleaving the kappacasein chain.[2]Casein is the main protein ofmilk. Cleavage removes the slightly negatively chargedglycomacropeptide (GMP) from the surface of the casein micelle. Because negative charges repel other negative charges, the GMP prevents casein micelles from adhering to each other. With the GMP removed, the casein micelles can begin to cluster and lose their polar charge, causing them to rise out of the polar water molecules and join non-polar milk fat as a portion of the cheese curd. This action is enhanced in the presence of strong ions like those formed from calcium and phosphate. As such, those chemicals are occasionally added to supplement pre-existing quantities in the cheese making process, especially in calcium phosphate-poor goat milk. The solid truncated casein protein network traps other components of milk, such as fats and minerals, to create cheese.[citation needed]
Calf rennet is extracted from the innermucosa of the fourth stomach chamber (theabomasum) of young, nursing calves as part of livestock butchering. These stomachs are abyproduct ofveal production.[3] Rennet extracted from older calves (grass-fed orgrain-fed) contains less or nochymosin, but a high level ofpepsin 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 forgoat's milk and lamb rennet forsheep's milk.[4]
Dried and cleaned stomachs of young calves are sliced into small pieces and then put into salt water orwhey, together with somevinegar orwine to lower thepH 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 2 to 4 litres of milk.[5]
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 aninactive form and are activated by thestomach acid. The acid is thenneutralized 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.[citation needed]
One kilogram of rennet extract has about 0.7 g of active enzymes – the rest is water and salt and sometimessodium benzoate (E211),0.5%–1.0% for preservation. Typically, 1 kg of cheese contains about 0.3 mg of rennet enzymes.[6][7]
Because of the limited availability of mammalian stomachs for rennet production, cheese makers have sought other ways to coagulate milk since at leastRoman times. The many sources of enzymes that can be a substitute for animal rennet range from plants and fungi to microbial sources.[8] Cheeses produced from any of these varieties of rennet are suitable forlactovegetarians, as well as those keepingKosher. Fermentation-producedchymosin is used more often in industrial cheesemaking in North America and Europe today because it is less expensive than animal rennet.[9]
Many plants have coagulating properties.Homer suggests in theIliad that the Greeks used an extract offig juice to coagulate milk.[10] Other examples include several species ofGalium,dried caper leaves,[11]nettles,thistles,mallow,Withania coagulans (also known as Paneer Booti, Ashwagandh and the Indian Cheesemaker), andground ivy. Some traditional cheese production in theMediterranean uses enzymes from thistle orCynara (artichokes and cardoons). Phytic acid, derived from unfermentedsoybeans, or fermentation-produced chymosin (FPC) may also be used.[citation needed]
Vegetable rennet might be used in the production ofkosher andhalal cheeses, but nearly all kosher cheeses are produced with either microbial rennet or FPC.[citation needed] Commercial so-called vegetable rennets usually contain an extract from themoldRhizomucor miehei described below.[citation needed]
Some molds such asRhizomucor miehei are able to produceproteolytic enzymes.[12] These molds are produced in afermenter and then specially concentrated and purified to avoid contamination with unpleasant byproducts of the mold growth.[13]
The traditional view is that these coagulants result in bitterness and low yield in cheese, especially when aged for a long time. Over the years[when?], microbial coagulants have improved greatly, largely due to thecharacterization and purification of secondary enzymes responsible for bitter peptide formation/non-specific proteolytic breakdown in cheese aged for long periods. Consequently, it has become possible to produce several high-quality cheeses with microbial rennet.[14]
It is also suitable for the elaboration ofvegetarian cheese, provided no animal-based ingredients are used in its production.[15]
Because of the above imperfections of microbial and animal rennets, many producers sought other replacements of rennet. With genetic engineering it became possible to isolate rennet genes from animals and introduce them into certainbacteria,fungi, oryeasts to make them producerecombinant chymosin during fermentation. The genetically modified microorganism is killed after fermentation and chymosin isolated from the fermentation broth, so that the fermentation-producedchymosin (FPC) used by cheese producers does not contain a GMO or any GMO DNA. FPC is identical to chymosin made by an animal, but is produced in a more efficient way. FPC products have been on the market since 1990 and, because the quantity needed per unit of milk can be standardized, are commercially viable alternatives to crude animal or plant rennets, as well as generally preferred to them in industrial production.[16]
Originally created by biotechnology companyPfizer, FPC was the first artificially-produced enzyme to be registered and allowed by theUS Food and Drug Administration.[17][18] In 1999, about 60% of UShard cheeses were made with FPC,[19] which thereafter made up to 80% of the global market share for rennet.[20] By 2017, FPC had 90% of the global market share for rennet.[21]
The most widely used FPC is produced either by the fungusAspergillus niger and commercialized under the trademark CHY-MAX[22] by the Danish companyChr. Hansen, or produced byKluyveromyces lactis and commercialized under the trademark Maxiren by the Dutch company DSM.[23]
FPC is chymosin B, so it is purer than animal rennet, which contains a multitude of proteins. FPC provides several benefits to the cheese producer compared with animal or microbial rennet: higher production yield, better curd texture, and reduced bitterness.[16]
Cheeses produced with FPC can be certified kosher[24][25] and halal,[25] and are suitable for vegetarians if no animal-based alimentation was used during the chymosin production in the fermenter.[citation needed]
Many soft cheeses are produced without use of rennet, by coagulating milk with acid, such ascitric acid orvinegar, or thelactic acid produced bysoured milk.Cream cheese,paneer,rubing, and otheracid-set cheeses are traditionally made this way.The acidification can also come from bacterial fermentation such as incultured milk.[citation needed]
Vegan alternatives to cheese are manufactured without using animal milk but instead use soy, wheat, rice or cashew. These can be coagulated with acid using sources such as vinegar or lemon juice.[26][27][28][29]
InYazidism, the Earth is believed to have coagulated and formed when rennet flowed from the White Spring of the celestialLalish in heaven into the Primeval Ocean.[30]
