fatty acid, any of the organic carboxylic acids present in fats and oils as esters of glycerol. Molecular weights of fatty acids vary over a wide range. The carbon skeleton of any fatty acid is unbranched. Some fatty acids are saturated, i.e., each carbon atom is connected to its carbon atom neighbors by single bonds; and some fatty acids are unsaturated, i.e., contain at least one carbon-carbon double bond (see chemical bond). When fats and oils are hydrolyzed with an alkali, the fatty acids are liberated as their metal salts; these salts are soaps. Butyric acid is a fatty acid found in butter.