ABSTRACT
A continuous ethanol fermentation system composed of three-stage tanks in series coupled with two sedimentation tanks was established. A self-flocculating yeast strain developed by protoplast fusion from Saccharomyces cerevisiae and Schizosaccharomyces pombe was applied. Two-stage enzymatic hydrolysate of corn powder containing 220g/L of reducing sugar, supplemented with 1.5g/L (NH4)2HPO4 and 2.5g/L KH2PO4, was used as the ethanol fermentation substrate and fed into the first fermentor at the dilution rate of 0.057h(-1). The yeast flocs separated by sedimentation were recycled into the first fermentor as two different models: activation-recycle and direct recycle. The quasi-steady states were obtained for both operation models after the fermentation systems experienced short periods of transitions. Activation process helped enhance the performance of ethanol fermentation at the high dilution rates. The broth containing more than 101g/L ethanol, 3.2g/L residual reducing sugar and 7.7g/L residual total sugar was produced. The ethanol productivity was calculated to be 5.77g/(L x h), which increased by more than 70% compared with that achieved in the same tank in series system without recycling of yeast cells.
Subject(s)
Biomass , Ethanol , Metabolism , Fermentation , Flocculation , Saccharomyces cerevisiae , MetabolismABSTRACT
During the dilute acid pretreatment of lignocellulosic materials such as corn stover, hemicellulose is hydrolyzed into monosaccharides, and meanwhile, toxic by-products are simultaneously generated, which may influence ethanol fermentation thereafter. Studies on the inhibitory effects of the by-products on ethanol fermentation are of practical use for further improvement of ethanol yield from lignocellulosic materials. Five by-products, including acetic acid, formic acid, vanillin, furfural and 5-hydroxymethylfurfural, were identified to be the main components in the hydrolysate of dilute acid pretreatment of local corn stover, which were added into the medium at different concentrations to study their impacts on the growth and ethanol fermentation of a recombinant xylose-utilizing yeast strain, S. cerevisiae 6508-127. The ethanol production was inhibited by formic acid and acetic acid to a lesser extent than that to the growth, and formic acid was shown to be much more toxic than acetic acid, showing severe inhibitory effects at the concentration of 1g/L, half of the concentration for acetic acid which showed remarkably negative effects on ethanol fermentation. Vanillin caused a much longer lag-phase in growth when the concentration was 2g/L, and the lag-phase was not obvious at lower concentrations. At the concentration of 6g/L, vanillin completely inhibited the fermentation as well as the cell growth. 5-Hydroxymethylfurfural was showed to remarkably inhibit ethanol production, but the biomass yield was higher by exogenous addition of 5-Hydroxymethylfurfural than control. Furfural at 0.5~1.5g/L inhibited the cell growth, but the ethanol yield was higher than that of the control experiment. It was also found that vanillin, furfural and 5-hydroxymethylfurfural could be assimilated and metabolized by S. cerevisiae 6508-127 under the experimental conditions.