rennet

Rennet (pronounced /ˈrɛnɪt/) is a natural 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). The active enzyme in rennet is called chymosin or rennin (EC 3.4.23.4) but there are also other important enzymes in it, e.g., pepsin or lipase. There are non-animal sources for rennet that are suitable for vegetarian consumption.

Production of natural calf rennet

Natural calf rennet is extracted from the inner mucosa of the fourth stomach chamber (the abomasum) of young 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 mother, there are milk-specific rennets available, such as kid goat rennet especially 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 2000 to 4000 grams of milk.

Today this method is used only by traditional cheese-makers in central Europe: Switzerland, Jura, France, Romania, 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 preform and are activated by the stomach acid. After neutralization of the acid, the rennet extract is filtered in several stages and concentrated until reaching the required potency: about 1:15000 (1 kg of rennet would have the ability to coagulate 15000 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 will 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.^[1] Other examples include nettles, thistles, mallow, and Ground Ivy (Creeping Charlie). Enzymes from thistle or cynara are used in some traditional cheese production in the Mediterranean. People with soy-based allergies should take care. Phytic acid, derived from unfermented soybeans, or 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 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 EFSA 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.^[2] 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 use calf genes to modify some 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 U.S. Food and Drug Administration. In 1999, about 60% of U.S. hard cheese was made with genetically engineered chymosin^[3] and it has up to 80% of the global market share for rennet^[4]. By 2008, approximately 80 - 90% of commercially made cheeses in the United States and Great Britain were made utilizing GMO-based rennet.^{[citation needed]} 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. Genetic 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 genetic rennet. This is also the reason why special analysis can determine what kind of coagulant has been used by analyzing what bonds have and haven't been cleaved.

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, those with soy-based allergies should beware as GM soy rennet or phytic acid, derived from unfermented soybeans may be used.

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^{[citation needed]}.

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

See also

• Pepsin

References

External links

• Fankhauser's Page on Rennet history and use
• Appendix D - Assessment of filamentous fungi - Qualified Presumption of Safety
• FDA-registration of recombinant chymosin
• Recombinant Chymosin
• Cheese Yield Experiments and Proteolysis by Milk-Clotting Enzymes
• Validation of recombinant and bovine chymosin by mass spectrometry