By Harold McGee
Milk is rich in nutrients, but its most readily tapped energy source, lactose, is a sugar found almost nowhere else in nature. This means that not many microbes have the necessary digestive enzymes at the ready. The elegantly simple key to the success of the milk bacteria is that they specialize in digesting lactose, and they extract energy from lactose by breaking it down to lactic acid. Then they release the lactic acid into the milk, where it accumulates and retards the growth of most other microbes, including those that cause human disease. They also make some antibacterial substances, but their main defense is a pleasantly puckery tartness, one that also causes the casein proteins to gather together in semisolid curds and thicken the milk.