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

ABSTRACT: 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 [PLUS-MINUS SIGN] 3.1, 252 [PLUS-MINUS SIGN] 2.9 and 298 [PLUS-MINUS SIGN] 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 [MINUS SIGN]100 mV, ethanol fermentation with the high gravity (HG) media containing glucose of 201 [PLUS-MINUS SIGN] 3.1 and 252 [PLUS-MINUS SIGN] 2.9 g/L was completed at 32 and 56 h, respectively, producing 93.0 [PLUS-MINUS SIGN] 1.3 and 120.0 [PLUS-MINUS SIGN] 1.8 g/L ethanol, correspondingly. In contrast, there were 24.0 [PLUS-MINUS SIGN] 0.4 and 17.0 [PLUS-MINUS SIGN] 0.3 g/L glucose remained unfermented without ORP control. As high as 131.0 [PLUS-MINUS SIGN] 1.8 g/L ethanol was produced at 72 h when ORP was controlled at [MINUS SIGN]150 mV for the VHG fermentation with medium containing 298 [PLUS-MINUS SIGN] 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 [MINUS SIGN]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 [MINUS SIGN]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.