Correctly called lipids, fats are organic substances insoluble in water. For cooking purposes, a fat is solid at room temperature; oils are liquid. All fats contain the same amount of calories per gram. Fats are the richest source of energy in food and essential for health. They are used as a cooking medium; they provide an insulating layer for meat and poultry, which keeps the meat tender and moist when cooked; they tenderize baked goods; and add richness, character and flavour to foods. Fats may be saturated, unsaturated or trans–fatty. They are of either animal or vegetable origin, or they may be a form of shortening, a fatty substance made from vegetable oils to which animal fats are sometimes added. Fats may also be invisible, such as the fats used as texture enhancers in processed foods, or the fats in milk, cheese, yoghurt, avocado, nuts and seeds. Internationally, the type of fat chosen for use in cooking varies enormously. In some areas of France, goose and duck fat, lard and butter are used; in India, cooks vary in their use of dalda (hydrogenated fat), coconut oil or ghee; in both Italian and Chinese cuisine, vegetable oils and a flavoursome rendered pork fat (lard) are used; in Jewish cooking, chicken fat is used in place of dairy fats such as butter because dairy and meat products cannot be mixed. Store fats well wrapped as they pick up other flavours and smells. Exposure to air causes them to go rancid so ingredients such as nuts (high in fat) need to be kept sealed.