Continuous Ethanol Fermentation Coupled with Recycling of Yeast Flocs

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 220 g/L residual reducing sugar, supplemented with 1.5 g/L (NH4)2HPO4 and 2.5 g/L KH2PO4, was used as the ethanol fermentation substrate and fed into the first fermentor at the dilution rate of 0.057 h−1. The yeast flocs separated by sedimentation were recycled into the first fermentor as two 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 high dilution rates. The broth containing more than 101 g/L ethanol, 3.2 g/L residual reducing sugar, and 7.7 g/L residual total sugar was produced. The ethanol productivity was calculated to be 5.77 g L−1 h−1, which increased by more than 70 % compared with that achieved in the same tank in series system without recycling of yeast cells.