Certain natural fats, such as butter fat and lard, are used as food with little or no preparation. Certain unsaturated oils, such as cottonseed and peanut oils, are often partly hydrogenated to increase their melting points and make them suitable for use as shortening and cooking fats. Tallow, the animal fats and oils from sheep and cattle, is used for candlemaking and in margarine. Natural oils that contain esters of unsaturated acids are known as drying oils and possess the property of forming a permanent dry film when exposed to air. Linseed oil and other oils of this type are extensively used in the production of paints.
Fats also serve as a raw material in the manufacture of soap .
The latter, which are more complex substances, include lipids and sterols. The glycolipids do not contain phosphorus but are derived from carbohydrate s, fatty acids, and nitrogen compounds. Sterols are composed of complicated molecules, each containing 20 or more carbon atoms in an interlocking or fused ring structure. The fats seem to provide a concentrated, efficient source of energy for the cells. The oxidation of a gram of typical fat releases 9300 calorie s of energy.