Hydroxy fatty acid (HFA) esters of long-chain alcohols, such as hydroxy stearates, have potential applications from lubricants to cosmetics. These esters were synthesized enzymatically to overcome the problems associated with chemical processes. An immobilized lipase, Rhizomucor miehei, was employed as catalyst in the esterification reaction between hydroxy-stearic acid as a source of HFA and monohydric fatty alcohols (C 8 -C 18 ). The yields of esters were in the range of 82-90% by conducting the reactions at 65 ± 2°C, 2-5 mm Hg pressure, and 10% lipase concentration. The products were analyzed by infrared spectroscopy, and some of their analytical characteristics were determined. JAOCS 75, 1057-1059 (1998).KEY WORDS: Enzymatic esterification, hydroxy fatty acid, hydroxy stearates, lipase, Rhizomucor miehei, wax esters.Hydroxy fatty acids (HFA), derived from several natural and synthetic sources, have many applications. HFA and their derivatives are used in cosmetics, in paints and coatings, in lubricants, and in the food industry. They are useful intermediates in the synthesis of fine chemicals and pharmaceuticals (1).HFA are found in many natural sources. For example, castor oil contains 12-hydroxy oleic acid, or ricinoleic acid, which incorporates a hydroxyl group at the C 12 -atom. When subjected to hydrogenation, this castor oil fatty acid gives 12-hydroxy stearic acid. Hydroxy stearates are usually known in the form of wax esters. These wax esters form gelatinous crystalline matrixes; by contrast, with the nonhydroxy derivatives, either no crystals at all are formed, or the crystals are not held in suspension. The hydroxy derivatives have wide application in leather coatings, for oil resistance and water imperviousness, and in roll leaf foils, because of their alcohol solubility and excellent wetting and adhesion to metallic particles (2). These wax esters can be produced either by hydrogenation of castor oil fatty acid esters or directly by reacting the fatty acid and an alcohol at high temperature in the presence of tin, titanium, or sulfuric acid catalyst for up to 20 h. This high-temperature process can lead to degradation of the ester and undesired side reactions; additionally, the resulting energy costs are high. Recently, the possibility of carrying out reactions with microbial lipases has been explored because of regio-and stereospecificity of the enzymes and because of the greater degree of purity that can be obtained for the desired reaction products.Application of enzymes in various biochemical modifications of fats and oils is well established, and the catalytic activity of lipases toward hydroxy fatty acids is also well studied by various workers (3-11). According to this information, 1,2-specific lipases cannot act upon the hydroxyl group of HFA; this property has been taken advantage of in the esterification of HFA and alcohol so that estolide or lactone formation is avoided. For example, Rhizomucor miehei lipase has been employed in the esterification of several different HFA (3,5,11) and ...