The earliest cheeses were produced using the rennet from an animal part, specifically the stomach of young goats, calves, and lambs. A chemical, chymosin, found in the stomachs of baby animals will cause milk to coagulate, or form curds. Only animals that are still nursing, and not eating any foods, contain this chemical in their stomachs; therefore, it can only be obtained from very young animals. The traditional method for extracting rennet involves slaughtering the animal, removing the stomach lining, and then using salt water, wine, whey, or vinegar to extract the chymosin. Although many cheese makers still follow traditional methods, more modern methods for producing animal rennet used in cheese making use enzyme extracting chemicals to get the rennet from the stomach lining.
For vegetarians that consume dairy products, cheese made from vegetable rennet is an option. There are several plants that have natural coagulation qualities and can be used in place of animal rennet to make cheese. Thistle is the most commonly used plant for making rennet. Another source of vegetable rennet is safflower. Dried caper leaves and the bark of a fig tree can also be used to make vegetable rennet. In addition, some soybeans that have been genetically modified are also capable of producing rennet that can be used in cheese making. The cheese making process using vegetable rennet is the same as the one used for making a cheese with animal rennet; however, the two are not necessarily interchangeable. Cheeses made using vegetable rennet will have different qualities than those made using animal rennet.
Another source of rennet used in cheese making is microbial rennet. Because microbial rennet uses no animals as its source, it is also appropriate for vegetarian cheeses. Certain types of molds, fungus, and bacteria have coagulating properties that will form curds in milk. The enzymes of the microbes can be used in the cheese making process.
In addition, cow DNA can now be added to some specific yeasts, fungi, and bacteria to produce genetically engineered animal rennet. This rennet more closely resembles the rennet made from animals; therefore, the cheeses produces using this type of microbial rennet are more similar in taste and quality as animal rennet cheeses. Depending on how strict your vegetarian practices are, this may be a suitable substitution, since no animals are harmed to make the rennet.