The activity of lipase and peroxidase in Turkish hazelnuts was measured experimentally as a function of temperature at a constant pH of 4.5, and as a function of pH at a constant temperature of 25 degreesC. Lipase revealed an iso-enzyme system with two clearly distinguishable peaks in activity with pH, whereas peroxidase only showed one large peak of activity. The activity data obtained were analysed by non-linear regression using an existing model for enzyme activity as a function of pH and temperature simultaneously. The existing model was adapted to account for the iso-enzyme system found to be present in hazelnut lipase activity. The percentage variances accounted for (R-adj(2)) obtained for peroxidase were 85.6% and 96.2% for temperature and pH dependence, respectively. For lipase the percentages Were found to be 72.1% and 89.6%, respectively. These results corroborate significantly the previously developed fundamental model. (C) 2002 Elsevier Science Ltd. All rights reserved.