Recent Developments in the Zymomonas Process for Ethanol Production

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Abstract
The bacterium Zymomonas mobilis, which is used in the tropics to make pulque and alcoholic palm wines, appears to have considerable potential for industrial alcohol fermentations. Some of the advantages of the Zymomonas process reported in studies from our laboratory1-24 are 1. There are significantly higher specific rates of sugar uptake and ethanol production compared to those found for yeasts. 2. Considerably higher volumetric ethanol productivities found in continuous cell recycle systems (up to 120 to 200 g/hr). 3. There are higher ethanol yields and lower biomass production than for yeasts. The lower biomass concentrations would seem to be a consequence of the lower metabolic energy available for growth. Zymomonas metabolize glucose via the Entner-Doudoroff pathway while yeasts convert glucose to ethanol via glycolysis. 4. Zymomonas cultures grow anaerobically and, unlike yeasts, do not require the controlled addition of oxygen to maintain viability at high cell concentrations. 5. The ethanol tolerance of some selected strains of Zymomonas is comparable if not higher than strains of Saccharomyces cerevisiae. Ethanol concentrations of 85 g/(up to 11% v/v) have been achieved in continuous culture and up to 130 g/(16% v/v) in batch culture.