Kinetics and performance of a co-immobilised system of amyloglucosidase and Zymomonas mobilis

High operational stability and productivity of co-immobilised systems are i mportant aspects for their successful application in industrial processes. A dynamic model is required to describe artificially co-immobilised systems because the time needed to reach steady state normally exceeds the operati onal life span of these systems. Time dependent intraparticle concentration profiles and macroscopic conversion were modelled to study the operational stability and productivity of these systems theoretically. The model was u sed to describe experimental results of ethanol production from maltose by a co-immobilised system of amyloglucosidase and Zymomonas mobilis. Furtherm ore, the influence of the immobilisation procedure with glutaraldehyde and polyethyleneimine could also be studied with and incorporated in the model. From the model it could be derived that co-immobilised systems performing a consecutive reaction evolve towards a steady state, characterised by a co nstant concentration of the intermediate in the particle if product inhibit ion is neglected. Such a situation develops independently of the biomass co ncentration and the radial position, and has important consequences for co- immobilised systems. When the concentration of the intermediate in the bulk liquid is lower than this constant value in the biocatalyst particle, two regions may be distinguished in the particle: an inactive peripheral region without biomass and an active core with a biomass concentration depending on the substrate and immobilised enzyme concentration. Unlike immobilised s ingle cell systems, it is possible to obtain a real steady state and theref ore a stable situation for co-immobilised systems. However, a high operatio nal life time could only be achieved at the expense of the productivity of the biocatalyst particle. A stability criterion is derived which agrees ver y well with the simulation results. (C) 1999 John Wiley & Sons, Inc.