What it is
Rennet is a set of enzymes, chiefly one called chymosin, that makes liquid milk clot into the curds every cheese starts from. The traditional source is bracing if you have not heard it before: the enzyme is drawn from the lining of the fourth stomach, the abomasum, of young calves, and the FDA's ingredient definition names it plainly as an extract from the fourth stomachs of calves, kids or lambs. Those stomachs are a byproduct of slaughter, mostly of veal calves, which is why traditional rennet is off the table for vegetarians as much as for vegans. What complicates the verdict is that most cheese has quietly stopped using it: fermentation-produced chymosin, made by putting the calf gene into bacteria, fungi or yeast and letting them brew the identical enzyme, has been sold since 1990 and by 2017 held around 90 per cent of the world rennet market. Alongside it sit microbial rennets from moulds and much older plant coagulants from thistle, nettle and fig sap, none of which involve an animal at all.
How to tell what your cheese used
The maddening part is that the label seldom names the coagulant, and where it does the words are slippery. 'Animal rennet' or a plain 'rennet' points to the calf-stomach original, while 'microbial rennet', 'vegetable rennet' and 'non-animal rennet' all mean an animal-free coagulant, and a bare 'enzymes' could be either. Because fermentation-produced chymosin counts as much the same substance as the calf version, cheeses made with it need carry no special note, and in North America roughly nine cheeses in ten are now made this way without saying so. A 'suitable for vegetarians' mark is a genuine clue, but read it precisely: it tells you the rennet is animal-free, not that the cheese is vegan, because the cheese is still made of milk. Which is the honest heart of it for a vegan: rennet is the wrong thing to worry about, since the dairy already settles the question, and the rennet only matters when you are choosing between vegetarian cheeses.
The three kinds
Rennet is the enzyme complex, chiefly chymosin, that curdles milk by cleaving a specific bond in casein. Traditional animal rennet is extracted from the fourth stomach chamber of unweaned calves, kids or lambs, then salted and filtered into liquid or powder. Microbial rennet comes instead from protease-producing fungi, with no animal genetics involved at all. Fermentation-produced chymosin is the third route: the calf gene for chymosin is inserted into a host microorganism, which ferments and excretes pure chymosin, and the microbe is filtered out afterwards.
That third route is where the genuine disagreement sits. The enzyme contains no animal tissue and no animal is slaughtered for it, which is why most vegetarians and most manufacturers treat it as acceptable; the FDA approved it in 1990 as the first bioengineered food product. But the gene sequence originated in a calf, so some authorities decline to call it animal-free, and USDA organic standards exclude it from certified organic cheese while permitting true microbial rennet. The question is not whether animal tissue is present, because it is not. It is whether animal-originated genetic material disqualifies it.
How much cheese still uses animal rennet
Less than most people assume. Industry figures cited by the Vegetarian Resource Group put fermentation-produced chymosin at around 90 percent of US commercial cheese production as of 2012, up from roughly 60 percent of US hard cheeses in 1999. No regulator publishes a current national figure, so that remains the best-sourced number available rather than a live one. European industrial cheese has followed a similar path, with the notable exception of the protected traditional names.
The cheeses that legally cannot switch
A cluster of Italian protected-designation cheeses still require animal rennet by their own registered production rules. Parmigiano Reggiano and Grana Padano both specify calf rennet in their consortium specifications, and Grana Padano’s also permits lysozyme from egg white as an antimicrobial, which is a second hidden animal ingredient in the same cheese. Pecorino Romano and Gorgonzola are traditionally made with animal rennet as well. Because EU law ties the protected name to the production method, a cheese made with microbial rennet cannot legally be sold as Parmigiano Reggiano however good it is.
How to tell
US labels typically say only enzymes or rennet, because cheesemaking enzymes are often treated as processing aids that need not name their source. In practice that vagueness leans animal, since a producer using microbial rennet almost always advertises it to capture vegetarian buyers. Look for explicit microbial rennet, vegetable rennet, vegetarian or vegan wording. Absent that, assume any protected-name Italian hard cheese uses animal rennet, because its specification requires it.