Yogurt's tang and deceptively rich flavor make it a useful and versatile ingredient in many recipes, from simple fruit salads to intricate and elegant Middle Eastern dishes. Most recipes that call for yogurt are cold dishes, because it tends not to heat well. This is true for the thicker Greek-style yogurt and the relatively loose mainstream American variety, so it must be used carefully in hot or warm dishes.
Yogurt is a Turkish word, with a root meaning "thick." It's milk that has been heated, and then thickened by the action of beneficial bacteria from the Lactobacillus and Streptococcus families. These bacteria consume the natural sugars in the milk and produce acids, which cause the proteins in the milk to coagulate -- like an egg white in your frying pan -- and gently thicken the milk into a gel. Mainstream commercial yogurt often uses gelatin to provide additional thickening, but Greek-style yogurt is made thicker through added milk solids. Traditionally that was done by boiling the milk to concentrate it, but commercially powdered milk is added instead.
The strands of thickened casein proteins in the yogurt form a sort of three-dimensional web, like a sponge, holding the milk's whey in place as a soft gel. As the yogurt is warmed, the whey becomes runnier and the casein strands become firmer and less elastic. At a warm room temperature, the yogurt tends to slowly leak whey. In a sauce used on hot foods such as a hamburger or falafel sandwich, the loss of whey is slow enough not to matter.
That changes if you attempt to cook with the yogurt, or use it in a hot sauce. The proteins in the yogurt contract in the heat, and cause it to separate into tight, grainy curds of protein and a puddle of liquid whey. Greek yogurt is especially prone to "breaking," because of its additional milk proteins. The cuisines of the Middle East, Central Asia and India have two main techniques for using yogurt in their hot dishes. Indian stir-fried meals add the yogurt to the pan in small amounts, stirring vigorously. The whey separates and then evaporates, leaving the meats and vegetables coated with a thin layer of tangy yogurt solids. The other technique uses a thickener to stabilize the yogurt.
Stabilizing the Yogurt
One method for stabilizing yogurt relies on egg whites. For each quart of Greek yogurt, beat one egg white to soft peaks. Whisk the yogurt until it's loosened and smooth, then whisk in the egg white. Alternatively whisk one tablespoon of cornstarch into three tablespoons of cold water, and whisk that into the yogurt. With either thickener, heat the yogurt gently until it comes barely to a boil. Turn it down slightly and let it simmer, stirring occasionally, for 10 minutes. Use the stabilized yogurt immediately in your recipe, or refrigerate it for later use.
- On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee
- A New Book of Middle Eastern Food; Claudia Roden
- Photo Credit Jupiterimages/Comstock/Getty Images