A number of dairy products are cultured by using bacteria to convert the lactose in milk or cream to lactic acid. Depending on the type of bacteria used and on the fat and acid content, the milk or cream converts to sour cream, yogurt or buttermilk. These cultured dairy products tend to separate when heated, so they should never be boiled and are usually added toward the end of cooking. In commercial products, not all the cultures remain active. When using them to make crème fraîche, yogurt or cheese, be sure to use “active” or “live-culture” types.