Deep eutectic solvents for enzymatic esterification of racemic menthol

Enzymatic esterification is an effective route that can contribute for the separation of racemic (rac) mixtures and is thus of great importance for the pharmaceutical, food, and cosmetic industries. All the processes involved in this methodology should be environmentally friendly, biocompatible, fast, and cost-effective. The use of deep eutectic solvents (DESs) can comply with all these metrics. In this work, we report on the use of DESs in the enzymatic esterification of menthol, one of the largest traded flavors and aromas worldwide. Solid rac-menthol was mixed with solid lauric acid at different molar ratios, yielding a DES liquid at room temperature or slightly above. Rac-menthol and lauric acid were the DES components and also the esterification reaction substrates. The DES reacted promptly when Candida rugosa lipase was added, and the conversions of rac-menthol reached up to 44% after 3 h of reaction and an enantiomeric excess of the product of 62%. Enzyme activity was dependent on the molar ratio of the substrates as well as on the water activity in the DES. The use of DES combined with enzymes paves the way for efficient and more sustainable biocatalytic processes.