Effect of the size of yeast flocs and zinc supplementation on continuous ethanol fermentation performance and metabolic flux distribution under very high concentration conditions.

Taking continuous ethanol fermentation with the self-flocculating yeast SPSC01 under very high concentration conditions as an example, the fermentation performance of the yeast flocs and their metabolic flux distribution were investigated by controlling their average sizes at 100, 200, and 300 microm using the focused beam reflectance online measurement system. In addition, the impact of zinc supplementation was evaluated for the yeast flocs at the size of 300 microm grown in presence or absence of 0.05 g L(-1) zinc sulfate. Among the yeast flocs with different sizes, the group with the average size of 300 microm exhibited highest ethanol production (110.0 g L(-1)) and glucose uptake rate (286.69 C mmol L(-1) h(-1)), which are in accordance with the increased flux from pyruvate to ethanol and decreased flux to glycerol. And in the meantime, zinc supplementation further increased ethanol production and cell viability comparing with the control. Zinc addition enhanced the carbon fluxes to the biosynthesis of ergosterol (28.6%) and trehalose (43.3%), whereas the fluxes towards glycerol, protein biosynthesis, and tricarboxylic acid cycle significantly decreased by 37.7%, 19.5%, and 27.8%, respectively. This work presents the first report on the regulation of metabolic flux by the size of yeast flocs and zinc supplementation, which provides the potential for developing engineering strategy to optimize the fermentation system.