Oscillation In Continuous High-gravity Ethanol Fermentation And Corresponding Attenuation Mechanism(s)

The impact of added ethanol on the performance of continuous ethanol fermentation wasstudied by feeding the low gravity medium supplemented with ethanol into the fermentationsystem. The quasi-steady state observed for the original medium was interrupted after themedium supplemented with ethanol was fed into the fermentation system, and oscillationsimilar to those previously observed for the high gravity medium was developed. After theoriginal low gravity medium was restored and the ethanol concentration of the fermentationsystem decreased, the steady state was also restored. These experimental results indicatedethanol, whether added or produced by yeast during fermentation, is the main factortriggering the oscillation. The lag response of yeast cells to ethanol inhibition was alsoverified through comparing the oscillation profiles of ethanol and glucose with yeast cellbiomass.A bioreactor system composed of a CSTR and three tubular bioreactors in series wasestablished. The mixing performance of the packed tubular bioreactors was investigated bymeans of the step-input technique using xylose as a tracer. The residence time distributionsillustrated that the mixing performance of these packed tubular bioreactors was close to aCSTR rather than a PFR, and the hypothesis for the mechanism of the oscillation attenuationfrom the packing, i.e, the alleviation of the backmixing, cannot be supported.When the working volumes of the tubular bioreactors were maintained at the same levelsby packing them with non-adsorptive glass beads, the attenuation effect observed in thewood-chips packing system could not be achieved, indicating that the shift of dilution rate isnot the reason for the oscillation attenuation. Compared with the bioreactor systems packedwith the polyurethane particles but without attenuation effect, it was found not only higherbiomass concentration but also higher yeast cell viability was achieved for the system packedwith the wood chips, making the oscillations effectively attenuated. Investigation furtherrevealed that the ethanol tolerance of yeast cells in the wood chip packed system wasimproved, which might further contribute to the oscillation attenuation.