View rennet
Rennet is produced in stomachs of young mammals in order to help digest milk. One of the steps in digestion is to coagulate the milk, causing milk solids (curds) and liquid (whey) to separate. So, it is also used for that purpose in cheese making.
While there are many enzymes in rennet, the key one is chymosin, also called rennin. But, there are additional enzymes that influence cheese such as lipase.
Rennet is most often produced by extracting it from the fourth stomach of a young calf, and is a by-product of veal production.
Many plants also have coagulating ingredients and these plants are used to produce vegetarian rennet. Nettles and thistles are popular plant-based rennets and are often found in artisan cheeses produced for vegetarians or kosher diets. However, vegetable rennet is not produced on a large scale and some non-meat-byproduct rennets are derived from molds or are genetically engineered.