(126d) Process Development for Very-High-Gravity Ethanol Fermentation

Metabolic flux distribution may be altered by manipulating intracellular reducing equivalents. To favour ethanol synthesis by Saccharomyces cerevisiae, a reduced cytosolic environment is desired, otherwise biomass formation is favoured. Direct variation of intracellular NADH/NAD⁺ is difficult, however, indirect control through measurement of fermentation redox potential is applicable.  In this presentation, we first report a correlation between yeast growth pattern and fermentation redox potential profile under very-high-gravity (VHG) conditions [1]. When S. cerevisiae was grown under VHG conditions, it initially encounters osmotic stress due to a high glucose feed. As fermentation proceeds, ethanol built-up retards yeast propagation, leading to sudden cell death and incomplete sugar utilization. To overcome this problem, strategies for proper aeration during ethanol fermentation could be employed [2]. It is known that yeast grows 33% faster at its active state than at its inactive state. This feature was used to develop a redox potential-driven repeated-batch ethanol fermentation process. Compared to the equivalent batch and the continuous ethanol fermentation processes, the annual ethanol productivity of the repeated-batch operation is 2.4% and 13.2% greater, respectively, under 200 g feeding glucose/l conditions [3]. Incomplete glucose conversion under VHG condition is common. To overcome this problem, a fermentation process incorporating a chemostat device and an ageing vessel along with a proper operating scheme was designed and validated [4, 5].