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

Area of Science:

• Biotechnology
• Fermentation Science
• Biofuel Production

Background:

• Very High Gravity (VHG) fermentation (>250 g/L sugars) offers energy savings in fuel ethanol production.
• Stuck fermentation, characterized by prolonged times and unfermented sugars, is a major challenge in VHG fermentation.
• Controlling Oxidation-Reduction Potential (ORP) can improve yeast biomass, viability, and ethanol yield under osmotic stress and ethanol inhibition.

Purpose of the Study:

• To investigate the effect of controlled ORP on yeast performance during VHG ethanol fermentation.
• To optimize ORP conditions for enhanced ethanol productivity and yield using flocculating yeast.
• To compare fermentation dynamics between flocculating and non-flocculating yeast under varying ORP.

Main Methods:

• Batch fermentation experiments were conducted using flocculating yeast with glucose concentrations of 201±3.1, 252±2.9, and 298±3.8 g/L.
• Oxidation-Reduction Potential (ORP) was controlled at -100 mV and -150 mV.
• Ethanol concentration, residual glucose, and fermentation time were monitored.

Main Results:

• Controlled ORP at -100 mV completed fermentation in 32 h (201 g/L glucose) and 56 h (252 g/L glucose), yielding 93.0±1.3 g/L and 120.0±1.8 g/L ethanol, respectively.
• Without ORP control, significant glucose remained (24.0±0.4 g/L and 17.0±0.3 g/L).
• At -150 mV ORP, VHG fermentation (298 g/L glucose) produced 131.0±1.8 g/L ethanol in 72 h, with improved yeast viability.

Conclusions:

• ORP control significantly improved ethanol productivity and yield, eliminating the lag phase with flocculating yeast.
• An ORP of -150 mV enhanced yeast viability and VHG fermentation performance, while -100 mV stimulated yeast growth for High Gravity (HG) conditions.
• Flocculating yeast exhibited less ORP fluctuation, suggesting yeast flocculation can stabilize fermentation.