By Harold McGee
There are dozens of different proteins floating around in milk. When it comes to cooking behavior, fortunately, we can reduce the protein population to two basic groups: Little Miss Muffet’s curds and whey. The two groups are distinguished by their reaction to acids. The handful of curd proteins, the caseins, clump together in acid conditions and form a solid mass, or coagulate, while all the rest, the whey proteins, remain suspended in the liquid. It’s the clumping nature of the caseins that makes possible most thickened milk products, from yogurt to cheese. The whey proteins play a more minor role; they influence the texture of casein curds, and stabilize the milk foams on specialty coffees. The caseins usually outweigh the whey proteins, as they do in cow’s milk by 4 to 1.