Cheese is a fermented food derived from the milk of various mammals. Since humans began to domesticate milk-producing animals around 10,000 B.C., they have known about the propensity of milk to separate into curds and whey. As milk sours, it breaks down into curds, lumps of phosphoprotein, and whey, a watery, grey fluid that contains lactose, minerals, vitamins, and traces of fat. It is the curds that are used to make cheese, and practically every culture on Earth has developed its own methods, the only major exceptions being China and the ancient Americas.

The first cheeses were "fresh," that is, not fermented. They consisted solely of salted white curds drained of whey, similar to today's cottage cheese. The next step was to develop ways of accelerating the natural separation process. This was achieved by adding rennet to the milk. Rennet is an enzyme from the stomachs of young...