When the grass was tall and thick in the woods and the cows were giving plenty of milk, that was the time to make cheese.
Somebody must kill a calf, for cheese could not be made without rennet, and rennet is the lining of a young calf’s stomach. The calf must be very young, so that it had never eaten anything but milk.
-Laura Ingalls Wilder, Little House in the Big Woods (p. 186)
When my mother first read those words to me, I must have been five or six years old. And I was horrified. Making cheese involved killing a baby cow? It’s not that I didn’t eat meat. But cheese had, up to this point, seemed like one of the most innocent of foods. It’s taken me a long time to understand that there are no innocent foods. And even the best dairy operations kill animals, whether they make their own rennet or not.
Rennet is the magic ingredient in cheesemaking. It coagulates milk: divides it into the smooth, solid curd (the future cheese) and the liquid, translucent whey. Left on its own, milk will curdle when it gets sour enough. If you’ve seen strong kefir or clabber, you know what that looks like. And while simple cheeses can be made by straining these curds, they are too soft and too acidic to make most of the cheeses we’re familiar with.
Milk is mostly water based, but it also contains fats and proteins. Of course, fat doesn’t normally dissolve in water and some proteins don’t either. Complex interactions between various protein molecules keep the fat-soluble portions of milk suspended in liquid. Coagulating milk involves disrupting these interaction so that the milk separates into the fat-soluble portion (the curd) and the water-soluble portion (the whey.) The most active (and best understood) enzyme in animal rennet is called chymosin, or rennin. It cuts molecules of the protein kappa casein in a certain spot, which causes the various protein molecules to disassociate and the milk to coagulate.
Today, commercial cheeses are often made from microbial rennet. Some molds produce enzymes similar to chymosin that, when processed and purified, can be used to coagulate cheese. But microbial rennet is also made from genetically engineered bacteria, fungi, and yeasts. Industrial chymosin producers insert genes from a cow into these microorganism to stimulate them to make the enzyme. Chymosin from transgenic organisms was used to make most US hard cheeses as early as 1999. Although true vegetable rennets exist, and I’ll talk about them in my next post, they are rarely used in commercial production. Commercial cheeses that say they contain “vegetable enzymes” or “vegetable rennet” are made with microbial rennet. And unless the cheese is organic or specifically states that the rennet is non-GE, it was probably made with GE chymosin.
Other concerns about genetic engineering aside, GE chymosin is indisputably an industrial product. I have no recipe for it, and I doubt that farmers or home cheesemakers will be trying to make it themselves anytime soon.
Animal rennet is made from the fourth stomach – the abomasum – of a very young dairy animal. Traditionally, calf rennet is used to coagulate cow’s milk, kid rennet for goat’s, and lamb rennet for ewe’s. The abomasum is the stomach that ruminants use to digest milk, and the active ingredients in animal rennet are the animal’s digestive enzymes.
This is a kid goat’s dried abomasum. It’s still filled with the milk the kid was digesting when it died – that chalky white stuff smells just like aged parmesan cheese. To make rennet the traditional way, clean the stomach after slaughter and soak it in saltwater with a bit of vinegar. The liquid is liquid rennet, and you can store it in the refrigerator for at least a year. Because the rennet’s strength isn’t standardized like it is in commercial rennet, you’ll have to determine the strength by trial and error. I have never actually made rennet (yet), but this guy has, and he has some detailed information.
You could also simply salt and dry the stomach. You’ll end up with something like the picture above. Then you can crush it into a powder and add the powder to your milk. But it will be hard to measure this kind of rennet accurately, because different parts of the stomach will have different concentrations of enzymes.
Stay tuned for Rennet Part II: True Vegetable Rennets.