A new approach to improving the performance ofZymomonas in continuous ethanol fermentations

Abstract

Although most fermentation ethanol is currently produced in traditional batch processes with yeast, the ethanologenic bacteriumZymomonas mobilis is recognized as an alternative process organism for fuel alcohol production. Different strategies for improving the productivity of ethanol fermentations are reviewed. In batch and open-type continuous fermentations the advantage of replacing yeast byZymomonas relates principally to the 10% higher fermentation efficiency (product yield), whereas in high cell density, closed-type continuous systems (operating with cell recycle or retention) the superior kinetic properties ofZymomonas can be exploited to affect about a five-fold improvement in volumetric productivity. Unlike yeast, the rate of energy supply (conversion of glucose to ethanol) inZymomonas is not strictly regulated by the energy demand and a nongrowing culture exhibits a maintenance energy coefficient that is at least 25 times higher than yeast. As an alternative to process improvement through genetic engineering of the process organism this investigation has taken a biochemical and physiological approach to increasing the kinetic performance ofZ. mobilis through manipulation and control of the chemical environment. Energetically “uncoupled” phenotypes with markedly increased specific rates of ethanol production were generated under conditions of nutritional limitation (nitrogen, phosphate, or potassium) in steady-state continuous culture. The pH was shown to influence energy coupling inZymomonas affecting the maintenance coefficient (me) rather than the max growth yield coefficient (Yxsάx). Whereas the pH for optimal growth ofZ. mobilis (ATCC 29191) in a complex medium was 6.0–6.5, the specific rate of ethanol production in continuous fermentations was maximal in the range 4.0–4.5. Fermentation conditions are specified for maximizing the specific productivity of aZymomonas-based continuous ethanol fermentation where the potential exists for improving the volumetric productivity in dense culture fermentations with an associated 35–40% reduction in capital costs of fermentation equipment and an estimated savings of 10–15% on cost of product recovery (distillation), and 3–7% on overall production costs based on the projected use of inexpensive feedstocks.