Continuous Ethanol Fermentation Using Self-flocculating Yeast Cells Coupled With Distillage Recycling

High equipment capital investment and energy consumption hindered the technology, distillers dried grains with solubles (DDGS), to be applied for large scale fuel ethanol plant constructions in China where energy cost is much higher than most developed countries in the world. Potentially, distillage could be fully recycled during continuous ethanol fermentation using self-flocculating yeast cells as yeast cells were self-immobilized and less inhibitory by-products were expected in the waste distillage. A single suspended-bed fermentor and a bioreactor system composed of four-stage suspended-bed fermentors in series were established respectively. Results were obtained by analyzing the experimental data for further industrial application of this clean technique.A single suspended-bed fermentor with a working volume of 1200 ml was established. Continuous ethanol fermentations using self-flocculating yeast cells with and without distillage recycling were carried out, respectively, when two-stage enzymatic hydrolyte of corn powder was fed at the designated dilution rate of 0.05h?1. The experimental results showed that fully recycling use of distillage in continuous ethanol fermentation using self-flocculating yeast cells was feasible, both technically and theoretically. Although residual reducing sugar, inorganic phosphor, and total solids in the broth were detected to accumulated and reached relatively higher levels, 26.0, 11.0 and 85g/L, respectively, compared with the levels of 11.4, 4.3 and 62g/L for the fermentation without distillage recycling, the ethanol concentration did not decreased significantly, which guaranteed the ethanol productivity unaffected. Inorganic nitrogen in the broth was detected to be lower when distillage was recycled because the system's buffering was improved and ammonia water consumption for pH adjustment was decreased.A bioreactor system composed of four-stage suspended-bed fermentors in series and with a total working volume of 4800 ml was further established to simulate industrial production case. Two-stage enzymatic hydrolyte of corn powder with initial reducing sugar of 220 g/L was fed at the designated dilution rate of 0.05 h?1, and the broth with levels of 11.7% (v/v) for ethanol and 7.2 g/L for residual reducing sugar was continuously produced for 120 days. Then, we collected the waste broth for yeast batch culture and fermentation. Inhibition of yeast growth and fermentation were observed when the medium contain a certain level of soluble solids which derived from continuous ethanol fermentation coupling with distillage recycling.In the operation mentioned above, four intermediate metabolites that were expected to be the main by-products of yeast cells and remained in the distillage, including glycerol, lactic acid, pyruvic acid andα?ketoglutaric acid, were analyzed by HPLC, and no significant accumulations of these intermediate metabolites were observed, which indicated that these intermediate metabolites could not be the potential inhibitors for yeast cell growth and ethanol fermentation under the condition of distillage recycling.