With the addition of live cultures, cream and milk become sour cream and yogurt, respectively. Both are versatile ingredients, used from appetizer to dessert courses, but cooks should be aware of subtle differences in their behavior during cooking and flavor.
Making Sour Cream
Adding a starter culture of lactic acid to cream turns it into sour cream, which, as the name suggests, has a deliciously tart and tangy flavor to complement the smooth texture of the cream. In the U.S., cream has to be pasteurized, so fermenting agents are necessary, but in France the use of unpasteurized cream allows the bacteria already present in the cream to ferment by themselves, which produces creme fraiche. Sour cream, by contrast, has to be re-pasteurized once fermentation is complete. Sour cream has a high fat content at approximately 20 percent, along with added stabilizers and thickeners such as gelatin and rennin. Light and nonfat sour cream versions are available, but the natural version contains no trans fats. A saving grace for weight-watchers is that sour cream is most commonly used in small portions.
Sour Cream Recipes
Sour cream adds a luxuriant mouth-feel to soups and stews, as well as a distinct sharpness, but isn't as heavy as straightforward cream. In Eastern Europe, it is the go-to ingredient for rich, complex stews such as stroganoff and goulash, a topping for baked potatoes, or a condiment for blinis and smoked fish. As a key ingredient in cheesecakes and other sponge cakes, sour cream also adds depth for baked goods. Boiling or simmering sour cream ultimately causes it to curdle as the proteins disintegrate, so either it should be stirred in to the sauce once the pan is off the stove, or combined with a little flour to prevent curdling. Creme fraiche, sour cream's French counterpart, doesn't curdle, however, but using it sacrifices some of the tartness of sour cream.
Yogurt is produced by introducing fermenting agents to milk. Specifically, the Food and Drug Administration requires two bacteria to be present, Lactobacillus bulgaricus and Streptococcus thermophilus. Unlike sour cream, yogurt does not need re-pasteurization. The easiest way to make yogurt at home is to add a small amount of yogurt with live active cultures to milk, whole or 2 percent for the creamiest yogurt. The milk needs to be first heated to a rolling boil, just under 200 degrees Fahrenheit, then simmered in a covered pan at around 110 degrees Fahrenheit. As the yogurt cools, fermentation will thicken the product. A clear liquid called whey may rise to the surface, but should be stirred back into the cream. Because the live cultures break down lactose, yogurt is a viable dairy option for the mildly lactose intolerant.
Cooking With Yogurt
Yogurt doesn't have the silky smoothness of sour cream, but has a light, neutral flavor. Consequently, yogurt makes a pleasant base for fruit smoothies or savory refreshers such as Indian lassi. Greek yogurt is a thicker type of yogurt made by straining the yogurt to remove liquid, whey and lactose. It is much higher in protein and saturated fat and tends to have a tangier flavor. As a result, it makes a good substitute for sour cream. However, Greek yogurt is not as effective as sour cream for baking as the milk proteins tend to separate, and the yogurt has a tendency to thin the longer it is out of the refrigerator. If cooking with yogurt, avoid using aluminum bakeware, which will give an unpleasant metallic taste. Yogurt is ideal for clean, smooth dips such as Greek tzatziki or Indian cucumber raita, as well as a formidable marinade base, most notably for tandoori spices.