Rennet

Rennet is a natural complex of enzymes produced in any mammalian stomach to digest the mother's milk. Rennet contains a enzyme  that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). The active in rennet is called rennin or chymosin but there are also other important enzymes in it, e. g.,  or. There are non-animal sources for rennet substitutes.

Uses
The chief use of rennet is in the making of cheese and Junket. Chymosin reacts specifically with &kappa;-casein, cleaving the protein between the amino acids (105) and (106), producing two fragments. The soluble fragment (residues 106-169) which becomes part of the whey is known as and contains the  sites for &kappa;-casein. The other component (residues 1-105) is insoluble. This, in the presence of ions, causes the coagulation of the casein micelles to form a curd.

Production of natural calf rennet
Natural rennet is produced in the inner of the fourth stomach of young ruminants. These stomachs are a of  production.

Traditional method
Today this method is only used by traditional cheese-makers in central, e.g. in , Jura in France, or Alp-Sennereien in.

Dried and cleaned stomachs of young calves are sliced into small pieces and then put into saltwater, together with some or  to get a lower pH. After some time – overnight or several days - this solution has to be filtered. This crude rennet can then be used for coagulation of the milk. 1 gram of this solution should then coagulate 2000 to 4000 grams of milk.

Modern method
Deep-frozen stomachs are milled and put into an extracting solution – in this solution the enzymes are extracted. The crude rennet extract is then activated by adding acid – the enzymes in the stomach are produced in an inactive pre-form and are activated by the stomach acid. After of the acid, the rennet extract is filtered in several stages and concentrated until reaching the required potency - e.g. 1:15.000 (1 kg of rennet would have the ability to coagulate 15,000 litres of milk). This is depending on the success of the extraction.

1 kg of rennet extract contains about 0.7 grams of active enzymes and no other organic material – the rest is water and salt and sometimes some, , 0.5% - 1% for preservation. 1 kg of cheese contains about 0.0003 grams of rennet enzymes.

Alternative coagulants
Due to the limited availability of proper stomachs for rennet production, cheesemakers have always looked for other ways to coagulate the milk. Only about 35% of worldwide cheese production uses animal rennet. Artificial coagulants are a useful alternative, especially for cheap or lower-quality cheeses.

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

Vegetable rennet
Many plants have coagulating properties. Some examples include bark,, thistles, , and. Commercial so-called vegetable rennets usually contain rennet from the Mucor miehei - see microbial rennet below. Rennet from thistle or  is used in some traditional cheese production in the.

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 GM rennet. Worldwide there is also no industrial production for vegetable rennet.

Microbial rennet
There are some like   that have the capability to produce proteolytic enzymes. These moulds are produced in a and then especially concentrated and purified to avoid contamination with unpleasant side products of the mould growth.

The flavour and taste of cheeses produced with microbial rennets tend towards some bitterness, especially if longer maturation is wanted. These so-called "microbial rennets" are suitable for vegetarians if there was no animal-based alimentation used during the production in the fermenter.

Genetically engineered rennet
Because of the above imperfections of microbial rennets, some big companies sought further replacements of natural rennet. With the development of genetic engineering, it suddenly became possible to use calf-genes to modify some, or  to make them produce. produced by genetically modified organisms was the first artificially produced enzyme to be registered and allowed by the in the USA. In, about 60% of U.S. hard cheese was made with genetically engineered Chymosin.

Today the most widely-used genetic rennet is produced by the fungus. The problems of destroying the or the  marker genes seem to be solved.

Cheese production with genetic rennet is similar to production with natural calf rennet. Genetic rennet only contains one of the known main chymosin types – either type A or type B. Some of the other chymosin types found in genetic rennet do not exist in natural rennet. This is also the reason why special analysis can determine what kind of coagulant has been used.

Often a mixture of genetic 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 -- but only for vegetarians who are not opposed to GM-derived foods.

Acid coagulation
Milk can also be coagulated by adding some, e.g. . This form of coagulation is sometimes used in cheap mozzarella production without maturation of the cheese. Paneer is also made this way. The acidification can also come from bacteria as happens in.