High Gravity And High Productivity Ethanol Fermentation With Self-flocculating Yeast Cells

Very high gravity(VHG) ethanol production technologies have garnered great attention. However,as a toxic metabolite,ethanol exerts a strong inhibition on yeast cell growth and ethanol production,which limits ethanol concentrations in the broth to no more than 13%(v) for most ethanol production plants at the present time.In this study,the effect of ethanol inhibition process on the viabilities of yeast cells was investigated by using a self-flocculating yeast strain SPSC01.Compared with those suddenly subjected to a high ethanol environment, the survival time of cells was prolonged by a factor of 2.5 when the ethanol concentration was increased gradually.Rapid loss of cell viability was also observed after a long-time inhibition of high ethanol.These experimental results indicated that about 24.6%,79.1%and 100% cell viability were lost after being subjected to 120 g·L^-1 ethanol for 10 h,45 h and 90 h, respectively,even the ethanol tolerance of yeast cells had already been induced.A repeated batch operation strategy was then proposed for high gravity ethanol fermentation.Yeast cells were separated by self-sedimentation after each turn of fermentation to hold a high level of biomass concentration,while ethanol was removed selectively before the next turn of fermentation to maintain a high level of cell viability.Ten batches have been run for 142.73 h during the whole fermentation process.Totally,13.44 L fermentation broth with high ethanol was obtained.The final ethanol concentration,residual glucose and ethanol yield were 122 g·L^-1,0.46 g·L^-1 and 0.46,respectively.The ethanol productivity was calculated to be 6.62 g·L^-1·h^-1,which increased by factors of 1.4 and 1.7 compared with those achieved by batch and continuous,fermentations using free yeast cells,and the ethanol productivity was increased 12.8%compared with continuous fermentation using the same self-flocculating yeast strain.Furthermore,about 88%cell viability was maintained at the end of the fermentation.The inoculation volume and amount of nutrients in the medium were further optimized to reduce the whole production cost.Considering the ethanol yield,ethanol productivity and cost,the optimal volume of broth for seed cultivation was determined to be 2 L,and the concentrations of yeast extract and peptone in the medium were further optimized to be 12 g·L^-1and 8 g·L^-1,respectively.