By Harold McGee
One of the remarkable qualities of milk is that it invites its own preservation. It can spontaneously foster a particular group of microbes that convert its sugar into acid, and thereby preserve it for some time from spoiling or harboring disease. At the same time, the microbes also change the milk’s texture and flavor in desirable ways. This benign transformation, or fermentation, doesn’t happen all the time, but it happened often enough that milks fermented by bacteria became important among all dairying peoples. Yogurt and soured creams remain widely popular to this day.