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. 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.