Rennet

Rennet is a complex of enzymes 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). They are also very important in the stomach of young mammals as they digest their mothers milk. The active enzyme in rennet is called chymosin or rennin but there are also other important enzymes in it, e.g., pepsin and lipase. There are non-animal sources for rennet that are suitable for consumption by vegetarians.

Production of natural calf rennet
Natural calf rennet is extracted from the inner mucosa of the fourth stomach chamber (the abomasum) of young, unweaned 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 species, there are milk-specific rennets 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 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 2 to 4 liters of milk.

This method is still used by some traditional cheese-makers, e.g. in Switzerland, Greece, France, Romania, Italy, Sweden, United Kingdom and Alp-Sennereien in Austria.

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 gram of extract can coagulate 15 kg (15 litres) of milk.

In 1 kg of rennet extract, there are about 0.7 grams of active enzymes – the rest is water and salt and sometimes sodium benzoate, E211, 0.5% - 1% for preservation. Typically, 1 kg of cheese contains about 0.0003 grams of rennet enzymes.

Alternative sources of rennet
Because of the limited availability of proper stomachs for rennet production, cheese makers have looked for other ways to coagulate the milk since at least Roman times. There are many sources of enzymes, ranging from plants, fungi, and microbial sources, that can substitute for animal rennet. Cheeses produced from any of these varieties of rennet are suitable for lacto-vegetarians to consume. GMO-Microbial rennet (see below) is used more often in industrial cheesemaking in North America today because it is less expensive than animal rennet, whereas cheese from Europe is more likely to be made from animal rennet due to tradition.

As the proper coagulation is done by enzymatic activity, the task was to find enzymes for cleaving the casein that would result in a taste and texture similar to those yielded by animal-based rennet.

Vegetable 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 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 genetically modified (GM) soy rennet may also be used.

These real 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 genetically modified rennet. Worldwide, there is no industrial production for vegetable rennet. Commercial so-called vegetable rennets usually contain rennet from the mold Mucor miehei - see microbial rennet below.

Microbial rennet
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. At the present state of scientific research, governmental food safety organizations such as the European Food Safety Authority deny QPS (Qualified Presumption of Safety) status to enzymes produced especially by these molds.

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

Genetically engineered rennet
Because of the above imperfections of microbial rennets, some producers sought further replacements of natural rennet. With the development of genetic engineering, it became possible to insert cow genes into certain bacteria, fungi or yeasts to make them produce chymosin. Chymosin produced by genetically modified organisms was the first artificially produced enzyme to be registered and allowed by the US Food and Drug Administration. In 1999, about 60% of US hard cheese was made with genetically engineered chymosin and it has up to 80% of the global market share for rennet. By 2008, approximately 80% to 90% of commercially made cheeses in the US and Britain were made using GM-based rennet. One example of a commercially available genetically engineered rennet is Chymax, created by Pfizer. Today, the most widely used genetically engineered rennet is produced by the fungus Aspergillus niger.

Cheese production with genetically engineered rennet is similar to production with natural calf rennet. GMO-produced rennet contains only one of the known main chymosin types, either type A or type B. Other chymosin types found in natural rennet do not exist in GMO-produced rennet.

Often, a mixture of genetically engineered chymosin and natural pepsin is used to imitate the complexity of natural rennet and to get the same results in coagulation and in development of flavour and taste.

The so-called "GM rennets" are suitable for vegetarians if there was no animal-based alimentation used during the production in the fermenter. However, genetically engineered rennet is often produced from soy or phytic acid, which is unsuitable for people who have soy-based allergies.

Acid coagulation
Milk can also be coagulated by adding an acid, such as citric acid.

Cream cheese, paneer, and rubing are traditionally made this way (see Category:Acid-set cheeses for others), and this form of coagulation is sometimes used in cheap mozzarella production without maturation of the cheese.

The acidification can also come from bacterial fermentation such as in cultured milk.