Enzymatic lipophilisation of phenolic acids through esterification with fatty alcohols in organic solvents

We investigated and optimised the synthesis of lipophilized esters between selected phenolic acids and fatty alcohols in a binary solvent system, composed of hexane and butanone. The effect of proportions of hexane and butanone was first studied by varying the volume ratio of hexane/butanone from 85:15 to 45:55. It was found that the conversion of phenolic acids strongly depended on the proportions of hexane and butanone. To examine the effect of carbon chain length of fatty alcohols on the reaction rate, the esterifications of C4–C18 straight-chain fatty alcohol with dihydrocaffeic acid (DHCA), as a model phenolic acid, were systematically evaluated. The results indicated that the conversion of DHCA was significantly affected by the number of carbon chain of fatty alcohols. Roughly 95% conversion was achieved within 3 days when hexanol was used as an acyl acceptor; while only 56% and 44% conversions were achieved when 1-butanol and octadecanol were employed, respectively. However, the conversions of ferulic and caffeic acids under the same conditions were much lower than was that of DHCA. The optimal mixture ratio of hexane to butanone was found to be 65:35. Using the reaction of octanol and DHCA as a model, the reaction parameters, such as temperature, enzyme load, reaction time and substrate molar ratio, were optimised with response surface methodology (RSM). A second polynomial model was generated and optimised reaction conditions were obtained as: temperature 60 °C, reaction time 3.9 days, enzyme load 238 mg, and substrate molar ratio 3.6 (octanol/DHCA). A validation reaction, based on the optimal conditions, was conducted, to yield 94.5% conversion of DHCA, indicating the suitability of the RSM model.