Production of bioethanol by Zymomonas mobilis in high-gravity extractive fermentations

Continuous removal of ethanol during production by the anaerobic bacterium Zymomonas mobilis mitigated ethanol inhibition. Ethanol inhibited its own production at a concentration of >20 g L-1. Ethanol productivity was enhanced by continuous in situ extraction with water immiscible organic solvents. Several solvents were evaluated for biocompatibility with Z. mobilis and the ability to extract ethanol from a batch fermentation. In situ batch extractive fermentations were carried out in a 2-L stirred bioreactor (1:1 volume ratio of solvent and fermentation broth) with an optimal glucose concentration of 150 g L-1, 35 degrees C, and an agitation speed of 150 rpm. Oleyl alcohol, iso-octadecanol and 2-octyl-1-dodecanol were the three solvents that were most compatible with Z. mobilis. With these solvents, the cell viability relative to control (no solvent) was 1.48 +/- 0.40, 1.03 +/- 0.18 and 1.05 +/- 0.07, respectively, after two days of exposure. All solvents tested improved the final biomass concentration, the glucose consumption and ethanol production relative to control, but iso-octadecanol was clearly the most effective solvent. Using iso-octadecanol, the maximum ethanol concentration of 75 3 g L-1 was attained. This was nearly 1.25-fold that of the control fermentation. For this solvent, the ethanol yield on glucose was 0.485 +/- 0.005 gg(-1) compared to a yield of 0.468 +/- 0.005 gg(-1) for the control culture. (C) 2016 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.