Animal rennets

The traditional milk-coagulating extracts from the abomasum — the fourth stomach — of young ruminants, principally calves, containing the enzymes chymosin and pepsin. For millennia these were the only cheese coagulants, prized for producing clean-breaking curds and the complex flavour development of long ripening. They remain mandatory for protected-designation cheeses such as Parmigiano-Reggiano, where tradition is law. Limited supply from veal slaughter and vegetarian objections have driven the shift to fermentation-produced chymosin — identical enzyme made by microbes — and to microbial coagulants, which now curdle most of the world’s cheese.