Process Design for Very-high-gravity Ethanol Fermentation

AbstractMetabolic 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. To utilize fermentation redox potential into designing an ethanol fermentation process under very-high-gravity (VHG) conditions, correlations between yeast growth pattern and fermentation redox potential profile were established. Under VHG conditions, S. cerevisiae initially encounters osmotic stress resulting in a lengthy lag phase. As fermentation proceeds, the built-up of ethanol inhibits yeast propagation, resulting in sudden cell death and incomplete sugar conversion. Additionally, an operational scheduling for a continuous VHG ethanol fermentation, consisting of a chemostat device and an ageing vessel, was proposed and compared to the equivalent batch operation. Results show that the proposed operational scheduling is superior to the batch counterpart. Process design criteria for a chemostat device connecting to several equal-size ageing vessels were developed in an attempt to increase annual ethanol productivity