Optimization of enzymatic synthesis of ethyl hexanoate in a solvent free system using response surface methodology (RSM)

Abstract The present paper demonstrates application of biocatalysis to the synthesis of ethyl hexanoate, i.e. pineapple flavour ester, in a solvent free system. In order to evaluate the effect of various process parameters on reaction conversion, response surface methodology (RSM) complemented by central composite design (CCD) was employed. A maximum conversion of 88.57% was obtained while changing one factor at a time, at optimum conditions of temperature (50 °C), enzyme dose (2%), molar ratio acid to alcohol (1:3), speed of agitation 250 rpm and reaction time of 120 min. Based on this RSM study, the optimum predicted conditions were: 1:3.39 alcohol to acid ratio, 2.35% enzyme loading and 48.83 oC, for a predicted conversion of 90.99%. The activation energy for the enzymatic esterification was determined and calculated to be 25.76 kJ/mol. The positive values of Gibbs-free energy (ΔG), enthalpy (ΔH) and negative value of entropy (ΔS) revealed that the esterification reaction was non-spontaneous and an endothermic reaction. The reaction seems to follow bi-substrate Ping Pong Bi Bi mechanism with inhibition by both substrates.