(716f) Impact of Floc Size On Ethanol Tolerance of the Self-Flocculating Yeast SPSC01 and Underlying Mechanism

High cost is the biggest challenge in fuel ethanol production, in which energy consumption comprises about 30%, the second largest only after feedstock consumption, and thus saving energy consumption is one of the most effective strategies to make fuel ethanol more economically competitive. Very high gravity (VHG) fermentation can significantly increase ethanol titer in the fermentation broth, which not only saves energy consumption for the downstream distillation but also for the waste distillage treatment due to its significant reduction. However, high ethanol concentration severely inhibits yeast cells, and stuck fermentation frequently occurs, with more sugars unfermented and ethanol yield compromised correspondingly. Therefore, exploration of the mechanism underlying the ethanol tolerance of yeast cells is one of the prerequisites for developing effective VHG ethanol fermentation process.

The impact of the size of the yeast flocs on the ethanol fermentation was studied by controlling their size at 100, 200 and 300 mm, which was characterized by their average chord monitored by the focused beam reflectance measurement. The experimental results illustrated that the yeast flocs with the size of 300 mm exhibited maximum ethanol production, ethanol tolerance and glucose consumption, with 110.0 g l^-1 ethanol produced, 58.0% viability remained for the yeast cells after 20% (v/v) ethanol shock for 5 h and glucose uptake rate of 286.69 C-mmol l^-1 h^-1. Moreover, the retention of the self-flocculating yeast was improved for the yeast flocs with relatively large size and higher biomass concentration was achieved within the bioreactor, contributing to more glucose consumed and carbon flux to ethanol production. The decrease in the fluxes to the growth of the yeast cells such as DNA, RNA, protein and lipid biosynthesis as well as the increase of ATP consumption for the maintenance indicated that improved ethanol production resulted from the increased yeast floc size raised more energy burden on the yeast cells, and glucose uptake of the yeast flcos was compromised under VHG conditions, making the improvement of glucose uptake rate the key to improve VHG ethanol fermentation.