A light, airy mixture that usually begins with a thick egg yolk-based sauce or puree that is lightened by stiffly beaten egg whites. Soufflés may be savory or sweet, hot or cold. Baked soufflés are much more fragile than those that are chilled or frozen because the hot air entrapped in the soufflé begins to escape (causing the mixture to deflate) as soon as the dish is removed from the oven. Savory soufflés are usually served as a main dish, are almost always hot and can be made with a variety of ingredients including cheese, meat, fish or vegetables. Dessert soufflés may be baked, chilled or frozen and are most often flavored with fruit purees, chocolate, lemon or liqueurs. Both sweet and savory soufflés are often accompanied by a complementary sauce. Soufflés are customarily baked in a classic soufflé dish, which is round and has straight sides to facilitate the soufflé's rising. These special dishes are ovenproof and come in a variety of sizes ranging from 3 1/2-ounce (individual) to 2-quart. They're available in kitchenware shops and the housewares section of most department stores. Foil or parchment "collars" are sometimes wrapped around the outside of a soufflé dish so that the top of the foil or paper rises about 2 inches above the rim of the dish. Such collars are used for cold dessert soufflés so that the sides of the frozen or molded mixture are supported until they set. Once the collar is removed, the soufflé stands tall and appears to "rise" out of the dish.