Rennet, an enzyme found in the stomachs of young ruminants, is a key ingredient in the process of making cheese. Rennet is an animal product, and thus is inappropriate for people leading a vegetarian or kosher lifestyle. In these cases, alternatives to rennet are available for both home and commercial cheese makers.
Rennet is an enzyme found in the fourth stomach of young ruminants (cows, sheep, and goats, among others) that allows these young animals to digest mother's milk. This enzyme curdles the milk, separating curds from whey and thus completing the first step in the process of making cheese. Today, rennet is available at the store in either tablet form or as a liquid. These products are created through a process that first creates a slurry of the stomach lining, then subjects the slurry to a chemical compound that precipitates the enzymes (mostly the enzyme rennin).
Traditional Rennet and Cheese Making
Traditionally, rennet was created by washing and salting the animal's stomach and allowing it to dry. The cheese maker would then snap off a small piece of the stomach and use this in the same manner that you use modern rennet tablets or liquid.
Rennet, or some other source of enzyme that will curdle milk and separate curds and whey, is an absolutely essential step in the process of making cheese.
For successful cheese making, the first step is to inoculate milk with buttermilk or yoghurt by adding the inoculant to warmed milk in a sterilised pot, covering with a sterilised lid, and leaving overnight to sit at room temperature (preferably about 21.1 degrees C). This lowers the pH of the milk and assists the rennet in breaking down a component of milk known as casein. The following morning, you again warm the milk and add rennet using a very careful measurement, as the wrong amount of rennet can ruin a batch of cheese. You allow the mixture to sit for an hour, or until curds and whey are separated completely. At this point, cut the curd (the solid part which rises to the top) into cubes without removing it from the pot and then heat it. The temperature to which you heat the curd determines whether a finished cheese will be soft or hard. The curd itself, with slight processing into smaller pieces, is often known as "cottage cheese."
Alternatives to Rennet
Rennet alternatives may be necessary for ideological reasons -- such as vegetarianism, veganism or kosher lifestyles -- or even for those who simply don't like the idea of "baby animal stomach" in their cheese. It is important to note that commercial cheese makers don't have to reveal whether the source of their rennet enzyme is artificially synthesised, is real rennet gleaned from a stomach, or is a non-rennet alternative. Thus, when shopping for cheeses, look for cheeses marked "vegetarian" or "kosher" to ensure that you are avoiding natural rennet.
Alternatives that can achieve the same result as rennet include vinegar and lemon juice. Other options are to acquire "vegetable rennet" made from one of several plants (thistle, nettle and mallow, to name a few), or "microbial rennet" acquired from mould. Genetically engineered rennet, which was never actually in the stomach of an animal but rather produced in a lab, is another possible (yet slightly creepy) option.