Continuous ethanol production using self-flocculating yeast in a cascade of fermentors

Continuous ethanol fermentation using a self-flocculating yeast strain SPSC01 was examined in a four-stage tanks in series fermentation system with a total working volume of 4000 ml. The first tank was designated for seed cultivation and the others for ethanol fermentation. Two-stage enzymatic hydrolysate of corn powder containing total sugar of 120 g l−1, supplemented with 2.0 g l−1 (NH4)2HPO4 and K2HPO4, respectively, was used as seed culture substrate and fed into the seed fermentor at the dilution rate of 0.017 h−1. Meanwhile, the hydrolysate containing total sugar of 220 g l−1, supplemented with 0.5 g l−1 (NH4)2HPO4 and 0.15 g l−1 K2HPO4 was used as ethanol fermentation substrate and fed into the second tank at the dilution rates of 0.017, 0.025, 0.033, 0.040 and 0.050 h−1, respectively. Steady states were observed when the fermentation system was operated at the dilution rates of 0.017, 0.025, 0.033 and 0.050 h−1. However, when the fermentation system was operated at the dilution rate of 0.040 h−1, unsteady states and oscillations that characterized by big fluctuations of residual sugar, ethanol and biomass were detected. Compared with freely suspended yeast cell ethanol fermentation system, this self-flocculating yeast strain could partly self-immobilize within the tanks. The effluent containing 95.6 g l−1 ethanol, 1.5 g l−1 reducing sugar and 2.5 g l−1 total sugar was steadily produced when the fermentation system was operated at the dilution rate of 0.033 h−1. The ethanol productivity of 3.44 g l−1 h−1 was achieved, which almost doubled that of freely suspended yeast cell system when the same levels of ethanol and residual sugars were reached. The conversion yield of ethanol to total sugar in the media was calculated to be 0.465, equivalent to 91.1% of its theoretical value of 0.511.