What it is and where it comes from
Rennet is the set of enzymes that makes milk curdle into cheese. Traditional rennet comes from the stomach of young calves, lambs or kids. Today most cheese is made with microbial rennet from a fungus, or with chymosin made by fermentation, which is the same enzyme produced by microbes. Some cheeses use pepsin from pigs alongside it.
What scholars say
- Rennet from microbes, plants or fermentation, or from a calf slaughtered in the halal way: halal for all.
- Rennet from pigs (porcine pepsin): not halal.
- Animal rennet from calves not slaughtered in the halal way: Imam Abu Hanifa held that rennet is pure and not affected by the death of the animal, and reports say the Companions ate cheese made by Zoroastrian Persians. Many Hanafi scholars follow this.
- Most scholars of the Maliki, Shafi'i and Hanbali schools, and Abu Hanifa's two students, hold that such rennet is impure, so the cheese is not halal. Some contemporary scholars add that the rennet is a tiny amount that is used up in the cheese.
Where you find it
Cheese, and whey, whey powder and casein made from cheese making.
What to look for on the label
Halal for all: "microbial rennet", "vegetarian rennet", "non-animal rennet", "suitable for vegetarians", or a halal logo. The word "rennet" alone does not tell you the source, because it is also used for microbial rennet; "animal rennet" means it is from an animal. If the label does not say, ask the maker. Some traditional cheeses, such as Parmigiano Reggiano, must by their own rules be made with calf rennet.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.