Sour cream has been cultured or fermented by added lactic acid bacteria, which makes it thick and slightly tangy in flavor. It has about 18 percent fat.
Crème fraîche (kremm fresh) is a slightly aged, cultured heavy cream. It is widely used for sauce-making in Europe because of its pleasant, slightly tangy flavor and its ability to blend easily into sauces. Unlike regular heavy cream, it usually doesn't require tempering and can be added directly to hot sauces. It is available commercially but is expensive. A close approximation can be made by warming 1 quart (1 L) heavy cream to about 100°F (38°C), adding 1V2 ounces (50 mL) buttermilk, and letting the mixture stand in a warm place until slightly thickened, about 6 to 24 hours.
Buttermilk is fresh, liquid milk, usually skim milk, which has been cultured or soured by bacteria. It is usually called cultured buttermilk to distinguish it from the original buttermilk,which was the liquid left after butter-making. Buttermilk is used in recipes calling for sour milk.
Yogurt is milk (whole or low-fat) cultured by special bacteria. It has a custardlike consistency. Most yogurt has additional milk solids added, and some of it is flavored and sweetened.
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