A commonly used homemade substitute for buttermilk is made by mixing an acid with milk (usually a tablespoon of vinegar for every cup of milk). When you've done this at home, you've probably noticed that the milk curdles and becomes lumpy over time. If you've ever wondered why acids curdle milk, I've got the answer.
Curdling Milk Is a Matter of pH
Acidifying milk — essentially lowering its pH — causes the milk proteins, like casein, to unwind and unfold in a process known as protein denaturing. The unfolded proteins are then free to interact with each other and clump together in a way they could not do when they were properly folded. The milk takes on a curdled appearance from the lumps of proteins that are binding one another.
Ways to Lower the pH of Milk
To lower the pH, an acid must be introduced or formed by some means. The most obvious sources of acid include vinegar or lemon juice (like when you make "sour milk," a homemade substitute for buttermilk).
Another way of introducing an acid is through lactose-digesting bacterial cultures, which transform the lactose sugar found in milk into lactic acid, thereby lowering the pH of the milk. These bacterial cultures are used in the preparation of yogurt, certain cheeses, sour cream, and many "cultured" dairy products.
What About Rennet?
Introducing rennet is another way of curdling milk, and requires adding much less acid than if you were to use vinegar, for example. Rennet is actually a complex of enzymes, originally extracted from the stomach of calves, one of which specifically cleaves (cuts) the casein protein found in milk: chymosin.
Today, chymosin is produced using techniques in biotechnology, expressing the proteins in bacteria. Chymosin breaks off a piece of the casein protein, allowing what's left to coagulate and clump together, curdling the mixture and forming curds.
Rennet is often used in the cheese-making process to make batches of cheese with less acid, leading to the growth of more flavor-producing bacteria in the cheese, which have a big impact on taste (as well as texture). Using rennet in the cheese-making process yields cheese curds that are stronger and more rubbery.