On Food and Cooking

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.
Both caseins and whey proteins are
unusual among food proteins in being largely
tolerant of heat. Where cooking coagulates the
proteins in eggs and meat into solid masses, it
does not coagulate the proteins in milk and
cream — unless the milk or cream has
become acidic. Fresh milk and cream can be
boiled down to a fraction of their volume