Rennet is used to get milk to curdle in cheese making.
Traditionally, Rennet came from the stomachs of calves, usually newly-born male ones that aren't wanted and so are slaughtered at birth. Specifically, it comes from their fourth stomach. The lining of that stomach has an enzyme called "rennin", that adult cows don't have. The enzyme helps calves digest milk, which of course they great in great quantities, getting it straight from the source.
In the past, the stomach lining would be washed and dried, and then a small piece put into the batch of cheese being made. More recently, a liquid extract was made by soaking the small pieces in a brine for 4 to 5 days at 86 F (30 C).
Vegetarian Rennet alternatives made from vegetable-based enzymes are also available.
In the past, vegetarian sources of a curdler came from plants such as thistles, nettles, fig leaves, safflower, etc.
Now it is being more industrially made in one of two ways. The first is fermenting either a fungus called "Mucor miehei", or a bacteria: either "Bacillus subtilis" or "Bacillus prodigiosum". This kind of Rennet, though, can't be used for cheddar or hard cheeses, giving it limited applications.
The other way is by growing genetically modified bacteria in tubs, which secrete rennin as they grow. This rennin was approved for use in America by the American Food and Drug Administration (FDA) in 1989. Half the rennin sold is now made in this way (2003), as it is cheaper than animal-based Rennet.
So there are two things that vegetarians have to come to grips with: they may be avoiding some commercial cheeses now for no reason, given that the non-animal based Rennet is on its way to becoming the standard. The second is that cheese marked vegetarian may contain a genetically modified food stuff.
Marketers of rennin are now preferring the word "chymosin", to move away entirely from the old animal-based connection. You can buy Rennet in tablet or liquid form for home cheesemaking.
Not all cheeses require Rennet. Fresh cheeses such as Cottage Cheese, Cream Cheese and Ricotta don't use it (though Sweet Curd Cottage Cheese does).
History
It is the Greeks who are credited with discovering Rennet as a cheese-making tool.
Language Notes
Rennet comes from the German word "rennen", meaning to run together.
See Also
Junket
Other entries for Cheese
Affinage, American Cheeses, Casu Marzu, Cheese Rinds, Creamery, Double/Triple-Cream Cheese, Extra-Hard Cheeses, Firm Cheeses, Goat's Milk Cheeses, Mexican Cheeses, Pate (of a Cheese), Processed Cheese, Queso Fundido, Rennet, Semi-Firm Cheeses, Sheep's Milk Cheeses, Skim-Milk Cheeses, Smear-Ripened Cheeses, Soft Cheeses, Surface-Ripened Cheeses, Sweet Curd Cheeses, The Crumblies, Truckle, Washed-Curd Cheeses, Washed-Rind Cheeses, Yak Cheese, Yeel Cheese
Other entries for Dairy
Butterfat, Butter, Milk, Nondairy Topping
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Also called:
 Rennin
Presure (French)
Cuajo (Spanish)
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