Rennet is an enzyme traditionally used in the cheese-making process to coagulate or set milk, leading to the formation of curds. Its primary active component is chymosin.

There are four main sources of rennet:

  1. Animal Rennet: Traditionally, rennet is derived from the stomach lining of young ruminant animals, usually calves. Specifically, it’s taken from the fourth stomach or the abomasum. This is because the enzyme aids the animal in digesting its mother’s milk.
  2. Vegetable Rennet: Some plants produce enzymes that can curdle milk. Examples include the thistle plant and fig tree sap. These are used in some traditional cheese-making contexts and are considered vegetarian alternatives to animal rennet.
  3. Microbial Rennet: Produced by fermenting specific fungi or bacteria. Microbial rennets are suitable for vegetarians but not always for those who adhere strictly to some religious dietary guidelines, since the source fungi or bacteria might be derived from non-Halal sources or undergo transformations in non-Halal mediums.
  4. Fermentation-Produced Chymosin (FPC): This is the result of a more modern process where genes that produce chymosin are inserted into certain microorganisms, such as yeast or fungi. These organisms then produce chymosin during fermentation. FPC is the primary rennet source in many countries due to its consistent performance in cheese-making.

From an Islamic perspective, if animal rennet is used, it is essential that the animal is slaughtered according to Halal standards for the enzyme to be Halal. Otherwise, the cheese or product containing the rennet might not be considered Halal. When in doubt, it’s advisable for Muslims to opt for products that are clearly labeled Halal or to choose cheese made with non-animal rennet.