Modelling the production of ethyl butyrate catalysed by Candida rugosa lipase immobilised in polyurethane foams

Response surface methodology was used to model and optimise the esterification of ethanol with butyric acid in n-hexane, catalysed by Candida rugosa   lipase immobilised in two hydrophilic polyurethane foams (“FHP 2002™” and “FHP 5000™”). Experiments were carried out following central composite rotatable designs (CCRD), as a function of the initial water activity of the biocatalyst (awaw), initial butyric acid concentration (A  ) and ethanol:acid molar ratio (MR) in the organic medium. Ester production increased with increasing awaw of the biocatalysts, probably due to the hydrophilicity of both substrates in contrast with the hydrophobicity of the product, which is released to the bulk medium. Thus, for each biocatalyst (aw=0.98aw=0.98) another CCRD was performed as a function of A and MR. With both preparations, higher conversions (>95%) were observed for low A values. For the “FHP 2002™” system, a maximum ester production of 0.23 M is expected, after 18-h reaction, at initial 0.35 M A   and 1.51 MR, corresponding to awaw of 0.95 and 0.84 M A and 1.65 M ethanol in lipase microenvironment. With “FHP 5000™” system, predicted initial conditions of 0.54 M A and 0.75 MR (0.32 M A; 0.75 M ethanol in microenvironment; awaw of 0.95), will lead to the maximum ester production of 0.27 M. These maxima were experimentally confirmed.