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% 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 1½ ounces (50 mL) buttermilk, and letting the mixture stand in a warm place until slightly thickened, about 6 to 24 hours.
