Continuous Ethanol Fermentation Coupled With Recycling Of Yeast Flocs And From Non-grain Raw Materials

A continuous ethanol fermentation system composed of three-stage tank fermentors in series coupled with two sediment tanks was established. A self-flocculating yeast strain developed by protoplast fusion of Saccharomyces cerevisiae and Schizosaccharomyces pombe was applied. Two-stage corn powder enzymatic hydrolyzate containing reducing sugar 220 g/L, together with 1.5 g/L (NH₄)₂HPO₄ and 2.5 g/L KH₂PO₄, was used as 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 in two ways, respectively, activation-recycle and direct recycle. The relatively chemostat-states were obtained for both operations. Activation process was found necessary for maintaining a good ethanol fermentation performance at the high dilution rate. The broth containing 101 g/L ethanol and 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 more than 70% compared with that for continuous ethanol fermentation in tank fermentors without recycling of yeast cells using the same yeast strain.Based on the achievement of corn powder material, the new extension study on cassava and cane molasses, which are abundant in south of China, was developed respectively: Two-stage cassava powder enzymatic hydrolyzate containing reducing sugar 220 g/L, together with 5.0g/L (NH₄)_<sub>2HPO₄ and 1.55 g/L KH₂PO₄, was used as ethanol fermentation substrate and fed into the first fermentor of three-stage fermentors system at the dilution rate of 0.036 h^-1, the relatively chemostat-state was obtained and the broth containing more than 106.4 g/L ethanol and 6.1 g/L residual reducing sugar and 17 g/L residual total sugar was produced. The ethanol productivity was calculated to be 3.83 g L^-1h^-1;multi-stage treated cane molasses diluent containing reducing sugar 190 g/L, together with 2 g/L (NH₄)₂HPO₄ was used as ethanol fermentation substrate and the broth containing more than 81 g/L ethanol and 35 g/L residual reducing sugar, the ethanol productivity was calculated to be 4.05 g L^-1 h^-1.