What is the name of the enzyme usually added to milk to coagulate it, to make "curds and whey"?
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Curd.txt Whey.txt Rennet.txt
Curd Whey Rennet
Curds are a dairy product obtained by coagulating milk in a process called curdling. The coagulation can be caused by adding rennet or any edible acidic substance such as lemon juice or vinegar, and then allowing it to sit. The increased acidity causes the milk proteins (casein) to tangle into solid masses, or curds.  Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria) will also naturally produce curds, and sour milk cheeses are produced this way. Producing cheese curds is one of the first steps in cheesemaking; the curds are pressed and drained to varying amounts for different styles of cheese and different secondary agents (molds for blue cheeses, etc.) are introduced before the desired aging finishes the cheese. The remaining liquid, which contains only whey proteins, is the whey. In cow's milk, 80% of the proteins are caseins.

Uses

Curd products vary by region and include cottage cheese, curd cheese (both curdled by bacteria and sometimes also rennet), farmer cheese, pot cheese, queso blanco, and Indian paneer (milk curdled with lime juice). The word can also refer to a non-dairy substance of similar appearance or consistency, though in these cases a modifier or the word curdled is generally used.

In England, curds produced from the use of rennet are referred to as junket, with true curds and whey only occurring from the natural separation of milk due to its environment (temperature, acidity).

Cheese curds, drained of the whey and served without further processing or aging, are popular in some French-speaking regions of Canada, such as Quebec, parts of Ontario, and Atlantic Canada. In Quebec, Eastern Ontario and the Eastern provinces such as New Brunswick, cheese curds are popularly served with french fries and gravy as poutine. In some parts of the Midwestern U.S., especially in Wisconsin, they are breaded and fried, or are eaten straight.

In Turkey, curds are called keş and are very commonly used as an aphrodisiac and for breakfast served on fried bread and are also eaten with macaroni in the provinces of Bolu and Zonguldak.

In Mexico, the chongos zamoranos is a dessert prepared with milk curdled with sugar and cinnamon.

Albanian gjiza is made by boiling whey for about 15 minutes and adding vinegar or lemon. The derivative is drained 3 to 4 times with a napkin or piece of cloth and salted to taste. Gjiza can be served immediately or refrigerated for a couple of days.

Formation

Lactobacillus is a genus of bacteria which can convert sugars into lactic acid by means of fermentation. Milk contains a sugar called lactose, a disaccharide (compound sugar) made by the glycosidic bonding between glucose and galactose (monosaccharides). When pasteurized milk is heated to a temperature of 30-40 °C, or even at room temperature or refrigerator temperature, and a small amount of old curd or whey added to it, the lactobacillus in that curd or whey sample starts to grow. These convert the lactose into lactic acid, which imparts the sour taste to curd. Raw milk naturally contains lactobacillus.

Curds in song and poetry

Curds are mentioned in the Middle Irish parodic tale Aislinge Meic Con Glinne (The Vision of Mac Conglinne), the relevant portion of which reads:Stately, pleasantly it sat,A compact house and strong.Then I went in:The door of it was dry meat,The threshold was bare bread,cheese-curds the sides.Smooth pillars of old cheese,And sappy bacon propsAlternate ranged;Fine beams of mellow cream,White rafters - real curds,Kept up the house. 

Curds also appear in the nursery rhyme Little Miss Muffet. Whey is the liquid remaining after milk has been curdled and strained. It is a co-product of the manufacture of cheese or casein and has several commercial uses. Sweet whey is manufactured during the making of rennet types of hard cheese like cheddar or Swiss cheese. Acid whey (also known as "sour whey") is a co-product produced during the making of acid types of dairy products such as cottage cheese or strained yogurt.

Whey proteins consist of α-lactalbumin, β-lactoglobulin, serum albumin, immunoglobulins, and a proteose-peptones. 

Production 

To produce cheese, rennet or an edible acid is added to heated milk. This makes the milk coagulate or curdle, separating the milk solids (curds) from the liquid whey. Sweet whey is the co-product of rennet-coagulated cheese and acid whey (also called sour whey) is the co-product of acid-coagulated cheese. Sweet whey has a pH greater than or equal to 5.6, acid whey has a pH less than or equal to 5.1.

Uses 

Whey is used to produce whey cheeses such as ricotta, whey butter, so-called brown cheeses such as Brunost (technically not cheeses at all), and many other products for human consumption. The fat content of whey is low; for example 1,000 kg of whey are required to make typically 1 kg of whey butter.[https://archive.org/stream/CAT31283652/circbai161_djvu.txt C.F. Doane, "Whey Butter", Bureau of Animal Industry, U.S. Department of Agriculture, Circular 161 [full text], 23 June 1910.] It is also an additive in many processed foods, including breads, crackers, and commercial pastry, and in animal feed. Whey proteins consist primarily of α-lactalbumin and β-lactoglobulin. Depending on the method of manufacture, whey may also contain glycomacropeptides (GMP).

Dairy whey remaining from home-made cheesemaking has many uses.  It is a flour conditioner and can be substituted for skim milk in most baked good recipes that require milk (bread, pancakes, muffins, etc.).

Whey protein (derived from whey) is often sold as a nutritional supplement. Such supplements are especially popular in the sport of bodybuilding.  In Switzerland, where cheese production is an important industry, whey is used as the basis for carbonated soft drinks such as Rivella and Montino.
In Iceland, liquid whey is sold as Mysa.

Throughout history, whey was a popular drink in inns and coffee houses.  When Joseph Priestley was at college at Daventry Academy 1752–1755, he records that, during the morning of Wednesday 22 May 1754, he “went with a large company to drink whey.”   This was probably ‘sack whey’ or ‘wine whey.'

Another use of whey is to make ‘Cream of Tartar Whey’:  "Put a pint of blue milk [blue milk is characterized by the appearance on its surface, eighteen or twenty-four hours after it is drawn, of small, indigo-blue fungal spots that rapidly enlarge until the whole surface is covered with a blue film.] over the fire, when it begins to boil, put in two tea spoonfuls of cream of tartar, then take it off the fire, and let it stand till the curd settles to the bottom of the pan, then put it into a basin to cool, and drink it milk warm.”

Whey was also used in central Spain to enrich bakery products. In some traditions, it was used instead of water to produce bread dough.

In areas where cheese is made, excess whey byproduct is sometimes sprayed over hay fields as a fertilizer.

Whey cream and butter

Cream can be skimmed from whey. Whey cream is saltier, tangier, and “cheesier” than ("sweet") cream skimmed from milk, and can be used to make whey butter. Due to the low fat content of whey the yield is not high, with typically 2 to 5 parts of butter manufactured from 1,000 parts of whey. Whey cream and butter are suitable for making butter-flavoured food, as they have a stronger flavour of their own. They are also cheaper to manufacture than sweet cream and butter.

Health

Because whey contains lactose, it should be avoided by those who are lactose intolerant. Dried whey, a very common food additive, contains 6% lactose. When used as a food additive, whey can contribute to quantities of lactose far above the level of tolerance of most lactose-intolerant individuals.

Liquid whey contains lactose, vitamins, protein, and minerals, along with traces of fat.

In 2005 researchers at Lund University in Sweden found that whey can help regulate and reduce spikes in blood sugar levels in people with type 2 diabetes by increasing insulin secretion.

People can be allergic to whey or other milk proteins (this is an allergy, not lactose intolerance). As whey proteins are altered by high temperatures, whey-sensitive people may be able to tolerate evaporated, boiled or sterilised milk. Hard cheeses are high in casein but low in whey proteins, and are the least allergenic for those allergic to whey proteins. However, casein proteins (which are heat-stable) are the most important allergens in cheese, and an individual may be allergic to either or both types of protein.

Protein

Whey protein is the name of globular proteins that can be isolated from whey. It is typically a mixture of globinstagers beta-lactoglobulin (~65%), alpha-lactalbumin (~25%), and serum albumin (~8%), which are soluble in their native culture forms, independent of pH. Rennet is a complex of enzymes produced in the stomachs of ruminant mammals.  Chymosin, its key component, is a protease enzyme that curdles the casein in milk.  This helps young mammals digest their mothers' milk. Rennet can also be used to separate milk into solid curds for cheesemaking and liquid whey.  In addition to chymosin, rennet contains other important enzymes such as pepsin and a lipase.

Rennet is used in the production of most cheeses. The mammal's digestive system must be accessed to obtain its rennet. Non-animal alternatives for rennet are suitable for consumption by vegetarians.

Extraction of calf rennet

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.

Traditional method 

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.

Modern method 

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.

Alternative sources

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.

Vegetable 

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 dried caper leaves,  nettles, thistles, mallow, and Ground Ivy (Creeping Charlie).  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.

Vegetable rennets are also suitable for vegetarians. Vegetable rennet might be used in the production of kosher and halal cheeses, but nearly all kosher cheeses are produced with either microbial rennet or FPC.   Commercial so-called vegetable rennets usually contain rennet from the mold Mucor miehei.

Microbial 

Some molds such as Rhizomucor miehei are able to produce proteolytic enzymes. These molds are produced in a fermenter and then specially concentrated and purified to avoid contamination with unpleasant byproducts of the mold growth.

The flavor and taste of cheeses produced with microbial rennets tend towards some bitterness, especially after longer maturation periods.  Cheeses produced this way are suitable for vegetarians, provided no animal-based alimentation was used during the production.

Fermentation-produced chymosin

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 certain bacteria, fungi, or yeasts to make them produce chymosin during fermentation. The genetically modified microorganism is killed after fermentation and chymosin isolated from the fermentation broth, so that the fermentation-produced chymosin (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 have been considered in the last 20 years the ideal milk-clotting enzyme. 

FPC was the first artificially produced enzyme to be registered and allowed by the US Food and Drug Administration. Staff, National Centre for Biotechnology Education, 2006. [http://www.ncbe.reading.ac.uk/ncbe/gmfood/chymosin.html Case Study: Chymosin]  In 1999, about 60% of US hard cheeses were made with FPC,  and it has up to 80% of the global market share for rennet. 

By 2008, about 80% to 90% of commercially made cheeses in the US and Britain were made using FPC.  Today, the most widely used FPC is produced either by the fungus Aspergillus niger and commercialized under the trademark CHY-MAX®  by the Danish company Chr. Hansen, or produced by Kluyveromyces lactis and commercialized under the trademark MAXIREN®  by the Dutch company DSM.

FPC is chymosin B, so is more pure than animal rennet, which contains a multitude of proteins. FPC can deliver several benefits to the cheese producer compared with animal or microbial rennet, such as higher production yield, better curd texture, and reduced bitterness.

Cheeses produced with FPC can be certified kosher [https://www.thecheesemaker.com/pdf/Chymosin_Rennet_Specs.pdf Chymax spec sheet] and halal, and are suitable for vegetarians if no animal-based alimentation was used during the chymosin production in the fermenter.

Nonrennet coagulation 

Many soft cheeses are produced without use of rennet, by coagulating milk with acid, such as citric acid or vinegar, or the lactic acid produced by soured milk. Cream cheese, paneer, and rubing are traditionally made this way (see :Category:Acid-set cheeses for others).
The acidification can also come from bacterial fermentation such as in cultured milk.

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.
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