Greek Versus Regular Yogurt

• Milk in its natural state is a highly nutritious but highly perishable food. Curdling it and pressing it for cheese is one way to preserve it for later use, or it can be fermented in a number of ways. Yogurt is one of those fermented milk products, native to Central Asia. A variety of beneficial bacteria digest the lactose in the milk, producing mild acids that slowly solidify the milk's proteins into a soft gel. Greek-style yogurt is thicker than most because it has a high level of protein. Traditionally that was because the milk was boiled to evaporate some of its water, but commercial yogurt-makers follow the quicker technique of adding milk solids instead.

In Cooking

• Creme fraiche and whipping cream are often used in hot foods and sauces. They withstand heat readily because of their high levels of fat. Yogurt is very different, consisting largely of protein with a minimal quantity of milk fat. When yogurt is heated, the effect of acidity on the milk proteins is accentuated. Instead of a soft gel, the proteins pull together into grainy, cheese-like curds. In the process they lose their ability to trap moisture into a gel, so the yogurt's whey forms a watery pool in your casserole. This separation into curds and whey is called breaking, or curdling. Fortunately, it can be prevented.

Starch

• There are two main methods cooks can use to stabilize yogurt and other milk-based products, such as sour cream, for use in hot dishes. The first is through the use of starch thickeners. For every cup of yogurt in your recipe, whisk a tablespoon of cornstarch into three tablespoons of cold water. When it forms a smooth, thin batter, whisk in the yogurt. Bring the yogurt to a slow simmer, stirring it constantly, until it thickens. Once it does, you can safely use it in your casserole. If your recipe was originally written for sour cream, it probably already includes a similar step.

Egg Whites

• Another way of stabilizing your yogurt is with egg whites. Whip one egg white to soft peaks for every cup of yogurt in your recipe. Stir the yogurt into the egg whites until they are thoroughly combined. Don't worry about folding it in carefully, as you don't need the air in the egg whites. Whipping them just makes it easier to combine them thoroughly with your yogurt. Once the whites are thoroughly incorporated, simmer and stir the yogurt until it thickens. As soon as it thickens, you can combine it with your other sauce ingredients.