Correlations between reduction–oxidation potential profiles and growth patterns of Saccharomyces cerevisiae during very-high-gravity fermentation

When Saccharomyces cerevisiae was grown under three glucose concentrations (ca. 200, 250, and 300 g/l), controlled at three reduction–oxidation (redox) potentials (no control, −150 and −100 mV) by manipulating two aerations (0.82 and 1.3 vvm), we observed that the recorded redox potential profiles resembled bathtub curves, and the profiles correlated well to the growth patterns measured under the same conditions. According to the shape of bathtub curve, we subdivided the curve into four regions. Region I features an abrupt decline in redox potential (corresponding to the growth phase from lag and logarithmic to the onset of stationary phase) that correlates to rapid yeast propagation, resulting from fast glucose uptake. Region II (corresponding to the stationary phase in yeast growth, characterizes a constant level of redox potential) is maintained by proper sparging and constant agitation. The continual buildup of ethanol causes growth arrest of yeast, resulting in the reduction of net NADH production. As a result, an uprising of redox potential is the feature of Region III, which signifies the end of stationary phase followed by the commencement of death phase. The severity of growth environment due to ethanol toxicity results in a rapid decrease in yeast population. Region IV (corresponding to the death phase during yeast growth) characterizes a drastic reduction in yeast viability and a gradual leveling of redox potential. A low glucose feed correlates to a fast decline of redox potential, a small basin in the bathtub curve, short fermentation duration, and complete glucose utilization. Imposing the current redox potential settings to low glucose feeds exerts no appreciable effect on ethanol production. In contrast, a high glucose feed connects to a sluggish bathtub curve for all four regions and incomplete glucose utilization. Proximate analysis on carbon balance indicates that controlling redox potential at −150 mV and under ca. 250 and 300 g glucose/l conditions, gave the highest fermentation efficiency as compared to other conditions; but there were no beneficiary effect to control redox potential under ca. 200 g glucose/l conditions.