By Harold McGee
Solid fat tissue is seldom prepared as such: instead we usually extract the fat from its storage cells, and then use it as both a cooking medium and an ingredient. There are two major exceptions to this rule. The first is caul fat, a thin membrane of connective tissue with a lacework of small fat deposits embedded in it. This membrane is the omentum or peritoneum, usually from the pig or sheep, which covers the organs of the abdominal cavity. Caul fat has been used at least since Roman times as a wrap to hold foods together and protect and moisten their surface while they are cooked. During the cooking, much of the fat is rendered from the membrane and the membrane itself is softened, so that it all but disappears into the food.