Enzymatic synthesis of l‐menthyl esters in organic solvent‐free system

Abstract

Abstract l‐Menthol has been widely used as a food additive and an ingredient of cosmetics, and it is esterified to moderate the strong flavor. We attempted esterification of l‐menthol with long‐chain unsaturated fatty acid in an organic solvent‐free enzymatic system. Commercially available lipases were screened, and Candida rugosa lipase was selected as a catalyst. Several factors affecting the esterification were investigated, and the reaction conditions were determined as follows: A reaction mixture of l‐menthol/fatty acid (1:3, mol/mol), 30% water, and 700 units of the lipase per gram of reaction mixture was incubated at 30°C with stirring. After 24 h under these conditions, the esterification extents of l‐menthol with oleic, linoleic, and α‐linolenic acids reached 96, 88, and 95%, respectively. The structure of the esterified product was confirmed by mass, infrared, and nuclear magnetic resonance spectroscopies. Bacause Candida lipase acted strongly on l‐menthol and very weakly on d‐menthol, dl‐menthol was esterified with oleic acid under the same conditions. The reaction showed high enantioselectivity; the enantiomeric ratio ( E ) was 31, and enantiomeric excess (ee) of l‐menthyl oleate reached 88% after 32 h.