How does liquid milk turn into solid cheese?

Milk turns into cheese when its proteins are made to clump together and the watery part is drained away. Cheesemakers do this with two helpers: bacteria that turn the milk sugar, lactose, into lactic acid, and an enzyme called chymosin, found in rennet, that snips a single protein in the milk.

Milk's main protein is casein, carried in tiny clusters called micelles. Each cluster is protected by a slightly negatively charged tail, a fragment called glycomacropeptide, and because negative charges repel one another, the clusters float apart and the milk stays liquid. Chymosin cuts that tail off. Without it the clusters can stick to each other, and they build a network that traps the milk fat. That network is the curd. The leftover liquid is the whey.

Then comes draining. The more whey the cheesemaker lets out, the drier and firmer the cheese. After that, bacteria and sometimes moulds ripen it, which is why one starting liquid can become fresh curd, a hard cheddar or a blue cheese.

Nobody knows who first did it. One theory pictures nomadic herdsmen carrying milk in vessels made from the stomachs of sheep and goats. Those linings contain rennet and lactic acid bacteria, so the milk would ferment, curdle, and could be strained into something like cheese.