All About Cultured Dairy

Cultured dairy is cream or milk with friendly live bacteria (cultures) added. The bacteria convert lactose, or milk sugar, into lactic acid, making these products thick and tangy.

SOUR CREAM

Sour cream is made from cultured light cream (approximately 18 to 20 percent butterfat). When we're stirring it into stews or sauces, we always do so off the heat to keep the sour cream from separating. Sour cream makes baked goods rich and moist. You can substitute plain whole-milk yogurt, but the results will be slightly leaner. (Don't use nonfat yogurt; it lacks fat and flavor.)

CREME FRAICHE

Sometimes erroneously described as the French version of sour cream, creme fraiche is actually made from heavy (not light) cream, which means it contains a lot more fat (between 30 and 40 percent). It's rich and creamy (no kidding!) and has a subtle nutty flavor. We like a dollop in certain soups and with fruit desserts. You can make an ersatz creme fraiche by whisking together equal parts sour cream and heavy cream and letting the mixture sit out at room temperature until thickened, about 1 1/2 hours.

YOGURT

Add bacteria to whole, low-fat, or nonfat milk and you get yogurt. We add whole-milk yogurt to sauces, soups, and dressings, and use both whole-milk and low-fat yogurt to make especially moist cakes. We don't recommend cooking with nonfat yogurt. Plain yogurt thinned with milk (3 parts yogurt to 1 part milk) makes a good substitute for buttermilk in most baked goods. Greek yogurt (which also can be made with milks of varying fat levels) is thicker, drier, and tangier than ordinary yogurt. To bake with it, add a little water or other liquid to loosen it slightly.

BUTTERMILK

Buttermilk was traditionally the liquid left over from churning butter. Most modern buttermilk, however, is cultured milk. Buttermilk keeps for several weeks in the refrigerator. You can also freeze it, but the emulsion will break, so blend it after thawing to re-emulsify it for salad dressings or dip. (Thawed buttermilk is fine as is in recipes for baked goods.) If you don't have buttermilk, homemade clabbered milk is a good substitute: Add 1 tablespoon of lemon juice to 1 cup of milk and let sit for 10 minutes to thicken.