Fatty Acids, a large group of organic compounds that are weakly acidic. They are called fatty because the first of them to be isolated came from natural fats and oils (liquid fats). These acids are compounds of carbon, hydrogen, and oxygen. The lower members of the group (those having a small number of carbon atoms in each molecule, such as acetic acid and butyric acid) are volatile liquids. The higher members are solids. Combined with glycerol, forming compounds called glycerides, the higher fatty acids are found in natural fats and oils. All fatty acids are soluble in alcohol, benzene, chloroform, and ether.

A fatty acid is classified as saturated or unsaturated, depending on whether or not it can contain additional hydrogen atoms. A mono unsaturated fatty acid has one pair of carbon atoms to which additional hydrogen atoms can become attached; a poly unsaturated fatty acid has two or more such pairs of carbon atoms. (For a more detailed explanation, see the subtitle Chemical Structure.) Vegetable oils are sometimes processed to solidify them by adding hydrogen to their unsaturated fatty acids. This process, a form of hydrogenation, creates fatty acids called trans fatty acids.

The most important saturated fatty acids in edible fats and oils are palmitic, stearic, lauric, and butyric acid. The most abundant unsaturated fatty acid is a monounsaturated acid called oleic acid. Polyunsaturated fatty acids include linoleic acid and linolenic acid. Biochemists refer to the polyunsaturated acids as essential fatty acids because they cannot be produced by the body and they are needed by the body for proper growth and regeneration of tissues.

Fatty acids are used commercially in making soaps and detergents, lubricants, paints, and rubber products. The fatty acids are usually produced from natural fats with glycerol as a by-product. In the most commonly used process, fat molecules are split into fatty acids and glycerol by high-pressure steam. Fatty acids are produced synthetically by oxidation of hydrocarbons obtained from petroleum.

Fatty acids are composed of molecules having open chains of carbon atoms and a single carboxyl group (COOH). Most of the fatty acids found in natural fats and oils have molecules with an even number (6 or 8 to 22) of carbon atoms arranged in a straight chain. Each carbon atom can have four chemical bonds. In a saturated fatty acid, each carbon atom within the carbon chain is connected by single bonds to its adjoining carbon atoms and to two hydrogen atoms. In an unsaturated fatty acid, two adjoining carbon atoms each lack a hydrogen atom and they share the extra bond, forming a double chemical bond. A polyunsaturated fatty acid has two or more such double bonds in the carbon chain.