Very high gravity ethanol fermentation by flocculating yeast under redox potential-controlled conditions

Background: Very high gravity (VHG) fermentation using medium in excess of 250 g/L sugars for more than 15% (v) ethanol can save energy consumption, not only for ethanol distillation, but also for distillage treatment; however, stuck fermentation with prolonged fermentation time and more sugars unfermented is the biggest challenge. Controlling redox potential (ORP) during VHG fermentation benefits biomass accumulation and improvement of yeast cell viability that is affected by osmotic pressure and ethanol inhibition, enhancing ethanol productivity and yield, the most important techno-economic aspect of fuel ethanol production. Results: Batch fermentation was performed under different ORP conditions using the flocculating yeast and media containing glucose of 201 +/- 3.1, 252 +/- 2.9 and 298 +/- 3.8 g/L. Compared with ethanol fermentation by non-flocculating yeast, different ORP profiles were observed with the flocculating yeast due to the morphological change associated with the flocculation of yeast cells. When ORP was controlled at -100 mV, ethanol fermentation with the high gravity (HG) media containing glucose of 201 +/- 3.1 and 252 +/- 2.9 g/L was completed at 32 and 56 h, respectively, producing 93.0 +/- 1.3 and 120.0 +/- 1.8 g/L ethanol, correspondingly. In contrast, there were 24.0 +/- 0.4 and 17.0 +/- 0.3 g/L glucose remained unfermented without ORP control. As high as 131.0 +/- 1.8 g/L ethanol was produced at 72 h when ORP was controlled at -150 mV for the VHG fermentation with medium containing 298 +/- 3.8 g/L glucose, since yeast cell viability was improved more significantly. Conclusions: No lag phase was observed during ethanol fermentation with the flocculating yeast, and the implementation of ORP control improved ethanol productivity and yield. When ORP was controlled at -150 mV, more reducing power was available for yeast cells to survive, which in turn improved their viability and VHG ethanol fermentation performance. On the other hand, controlling ORP at -100 mV stimulated yeast growth and enhanced ethanol production under the HG conditions. Moreover, the ORP profile detected during ethanol fermentation with the flocculating yeast was less fluctuated, indicating that yeast flocculation could attenuate the ORP fluctuation observed during ethanol fermentation with non-flocculating yeast.