| Very high gravity(VHG) ethanol fermentation can not only significantly improve final ethanol concentration, but also save energy consumption in the process of ethanol fermentation. However, biomass concentration and cell viability are obviously on the low side at the end of fermentation under VHG ethanol fermentation in non-aeration condition,which led to prolong fermentation time. As a result, how to enhance biomass concentration and cell viability is critically important. In this project, micro-aeration was incorporated into glucose VHG ethanol fermentation, the fermentation characteristics of Saccharomyces cerevisiae GGSF16 during ethanol fermentation was studied. Based on the fermentation effect, the optimum initial sugar concentration and aeration strategy was determined, then applied to cassava starch hydrolysate VHG ethanol fermentation. The main results were as follows:(1) Effect of different aeration rates on VHG ethanol fermentation at 32 ?, 200 rpm, glucose concentrations 260 g/L and incoculum size 10 % in a 5.0 L fermentor was studied. The results indicated that a proper aeration is an important control factor to reduce fermentation time, increase biomass concentration, and improve the viability and ethanol inhibition of the yeast cells as compared to non-aeration condition. However, aeration led to decrease in the ethanol production and specific glucose uptake rate and specific ethanol production rate. The optimum aeration level was found to be 80 mL/min, the biomass concentration reached 14.16 g dry wt/L, the ethanol productivity achieved 3.93 g/(L·h), which increased 104 % and 70.1 % than non-aeration fermentation, the final ethanol concentration reached 117.9 g/L and the yield of ethanol reached 0.452 g/g(The stoichiometric yield was 88.4 %). Meanwhile, the time required to end fermentation under the optimal conditions was shorten from 54 h to 30 h. further increases in the rate of micro-aeration resulted in an increased biomass concentration and decreased ethanol production as more glucose was utilized for biomass synthesis.(2) In order to improve the final ethanol concentration and the efficiency of fermentation, based on the above results, effect of different initial glucose concentrations(260, 300, 340 g/L) and aeration strategy(non-aeration, full-course aeration, aeration interval) on VHG ethanol fermentation was studied. The reason of the rate of cell growth, sugar consumption and ethanol production were explored, and the relationship among cell growth, sugar consumption and ethanol production were described in ethanol fermentation. The results as follows: The optimum initial glucose concentrations and aeration strategy was found to be 300 g/L and 80 mL/min aeration interval, the final ethanol concentration reached 138.5 g/L, the ethanol productivity achieved 3.30 g/(L·h), the biomass concentration reached 12.4 g dry wt/L, the stoichiometric yield was 90.3 %, which increased 11.9 %,76.5 %,98.4 % and 11.8 % than non-aeration fermentation, the yield of ethanol reached 0.467 g/g. under the same condition of aeration strategy, fermentation time was prelonged, biomass concentration and stoichiometric yield were decreased with the increasing of initial glucose concentrations. Under the same condition of initial glucose concentrations, increasing the aeration time and aeration rates which can shorten fermentation time and enhance biomass concentration, but it was not beneficial to accumulate ethanol.(3) Effect of different aeration strategy on cassava starch hydrolysate VHG ethanol fermentation using simultaneous saccharification and fermentation(SSF) process at the condition of glucoamylase concentration 250 U/g,urea concentration 4 g/L,initial pH 5.0-5.5,inoculum size 10 % and total sugar concentration ca.320 g/L in a 5.0 L fermentor was studied. The optimum aeration strategy was found to be 80 mL/min aeration interval, the biomass concentration reached 9.6 g dry wt/L, the ethanol productivity achieved 3.17 g/(L·h), which increased 17.5 % and 38.4 % than non-aeration fermentation, the final ethanol concentration reached 133.3 g/L, and the yield of ethanol reached 0.466 g/g, the stoichiometric yield was 82.4 %. Meanwhile, the time required to end fermentation under the optimal conditions was shorten from 60 h to 42 h. |
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