Rennet is used in cheesemaking to coagulate cow, goat or sheep’s milk and to separate the curds from the whey: Curds are the lumps that become the cheese, and whey is the liquid by-product discarded after separation. Taken from the stomachs of young calves, rennet contains an active enzyme called chymosin that accelerates the curdling process.
Traditionally, animal rennet is used when making cheese. The curd is cut into pieces, heated and manipulated until the whey is expelled, while bacteria grow to form lactic acid. After the process is complete, the curds are pressed into molds and aged until ready for sale.
Three alternatives to animal rennet are used for vegetarians, microbial rennet, vegetable rennet or fermentation-produced chymosin. The latter is used in the production of nearly 90 percent of cheese made in the United States commercially, according to Formaggio Kitchen. Microbial rennet is taken from mold, and can cause bitterness in the cheese it yields. Vegetable rennet is harvested from plants that have similar coagulation chemicals to its animal counterpart; historically, cheese from Spain and Portugal use this method for production.
Cheeses imported from Europe have strict restrictions concerning their specific ingredients and the process used to make them. Many such cheeses, including Parmigiano-Reggiano, Gruyere, Gorgonzola and Camembert, use animal rennet and are not vegetarian friendly, as outlined by The Huffington Post.