On the basis of earlier kinetic modelling for the conversion of glucose to ethanol by wild-type Z. mobilis [59], a further model has been developed for the fermentation of glucose/xylose mixtures by ZM4 (pZB5) [60]. A two — substrate model was constructed based on Monod kinetics for substrate lim­itation, as well as functions for product (ethanol) inhibition and substrate inhibition at the higher glucose and xylose concentrations. The model simu­lation data for various glucose/xylose concentrations were compared with the experimental results using a Microsoft Excel-based program and statistical analysis for error minimization. Using this approach, it was established that the model (with relevant values of the constants) provided good agreement with the experimental batch culture data for 25/25, 50/50 and 65/65 g L-1 glu — cose/xylose media. It should be noted that the model did not include any repression of xylose uptake by glucose as experimentally both glucose and xylose are taken up simultaneously even at the high initial glucose concen­trations. However, this does not preclude the possibility that some glucose repression of xylose might be occurring. The results indicate that ethanol inhibition of xylose utilization is likely to be the more dominant factor in influencing its kinetics.

2.6