Effect of zinc ion on ethanol tolerance and floc size of self-flocculating yeast
The effects of metal ions on ethanol tolerance of self-flocculating yeast was studied by using a defined medium in shake flask culture.The observed improvement of zinc ion supplementation on ethanol tolerance promoted the further examination of zinc ion effect on continuous ethanol fermentation.Zinc sulfate was supplemented into the fermentation medium at the concentrations of 0.01,0.05 and 0.1 g·L-1,respectively.The average floc size of the self-flocculating yeast decreased in zinc-addition cultures,but both its ethanol tolerance and thermal tolerance were significantly improved,which correlated well with the increased ergosterol and trehalose contents in the yeast flocs.Intracellular zinc content was found to be about six folds higher than that of control,and no significant difference was observed in the three addition levels.Correlation analysis indicated close correlation between zinc addition and ethanol/thermal tolerance.Ethanol production increased in all the zinc-supplemented cultures,with the most significant effect observed when 0.05 g·L-1 zinc sulfate was supplied.Thus,zinc ion supplementation was proved to be an efficient strategy for enhanced ethanol tolerance,thermal tolerance and ethanol production in continuous ethanol fermentation of the self-flocculating yeast.