An essential ingredient in the cheese making process is rennet, which is a substance used to coagulate the milk. Rennet contains enzymes that work to separate milk into solids and liquid. Many rennets were traditionally derived from animals, specifically from the stomach of a slaughtered, unweaned calf or kid.
Cheeses that are made without the use of animal rennet are considered 'vegetarian.' This can mean that the cheese is made with a rennet that is synthetically developed or that comes from vegetable sources, such as thistles or wild artichokes. The technique of using thistles in cheese production is common in Portugal and Spain and can be seen in cheeses like Serra da Estrella or Azeitao. Microbial enzymes are used in cheeses such as Lord of the Hundreds or Westfield Capri.