The aerobic fungus Aspergillus awamori and the anaerobic bacterium Zymomonas mobilis were coimmobilized in alginate beads and cultivated in a synthetic medium in stirred tank and airlift tower loop reactors. In the coimmobilized mixed culture, the fungus grows in the outer aerobic shell and on the surface of the beads. The fungus converts starch and maltose to glucose. Glucose is consumed by both of the microorganisms. Z. mobilis, growing in the anaerobic core of the beads, partially converts glucose to ethanol, which is consumed by the fungus after the glucose has been exhausted. The coimmobilized mixed culture forms a stable steady state. Biological parameters of the free and immobilized monocultures on soluble starch, maltose and glucose as substrates, and the effective diffusion coefficients of starch, maltose, glucose, and ethanol as well as oxygen within the beads were experimentally determined.

The coimmobilized aerobic/anaerobic mixed culture was mathematically modelled. The model describes substrate uptake, growth, and product formation, both, microscopically and macroscopically. Four typical cultivations in stirred tank and airlift tower loop reactors were investigated. The agreement between simulation and measurements was satisfactory. The boundary between the anaerobic and the aerobic section moves outward during the batch phase. A sensitivity analysis illustrates the significance of single parameters.