Response surface methodology was used to model and optimise the production of ethyl butyrate, catalysed by Rhizopus oryzae lipase immobilised in a hydrophilic polyurethane foam. Experiments were carried out following a central composite rotatable design, as a function of reaction temperature (T: 22-38 degrees C) initial butyric acid concentration (A: 0.031-0.619 M) and initial molar ratio ethanol/acid (MR; 0.257-2.443). After 48 h reaction time, the production of ethyl butyrate could be fitted to a surface described by a second-order polynomial model. A maximum ethyl butyrate concentration of 0.106 M, corresponding to 47% conversion into ester and a productivity of 2.21 mu mole/mL h, is expected at initial reaction conditions of T, A and MR of 33 degrees C, 0.225 M and 1.637, respectively. This maximum was experimentally confirmed. (c) 2012 Elsevier Ltd. All rights reserved.