| Ethanol production using energy crop attracts more attentions nowadays,Sorghum bicolor(L.)Moench was one of these crops.Xylose concentration was about 30%in sweet sorghum residue hydrolysate, Pachysolen tannophilus can ferment xylose to produce ethanol,which was studied in this paperFirst,mutation was carried on P.tannophilus and DES+UV had the best results.Two strains were obtained,ethanol yield of which increased by 23.9%and 28.7%and were 0.171 g·g-1and 0.178 g·g-1.Second,properties of P.tannophilus were studied,including fermentation in glucose/xylose mixtures,xylose tolerance and ethanol tolerance as following:(1)In synthetic glucose/xylose mixtures,glucose was the preferred substrate,xylose fermentation started after glucose was exhausted.The presence of glucose could limit the xylose consumption and there was an adaptive period for P.tannophilus to utilize xylose to produce ethanol after glucose used up in fermentation on mixed sugars.The average glucose rate(1.28 g·l-1·h-1)was higher than the average xylose consumption rate(0.34 g·l-1·h-1)in fermentation on single sugar.Highest ethanol concentration(6.90 g·l-1)was obtained at the 12th hour of fermentation on 100%glucose,while the lowest ethanol concentration (2.70 g·l-1)was obtained at the 48th hour of fermentation on 100%xylose. But biomass showed an opposite result that highest biomass(6.65 g·l-1) was obtained in fermentation on pure xylose and lowest biomass(5.22 g·l-1)was obtained in fermentation on pure glucose.(2)The most favorable xylose concentration for the production of ethanol by P.tannophilus was 20 g·l-1,which gave the highest maximum specific growth rate of 0.12 h-1and the highest ethanol yield on xylose of 0.18 g·g-1.In fed-batch fermentation(total sugar 40 g·l-1),ethanol concentration increased fast after feeding,and increment(2.3 g·l-1)was bigger than ethanol concentration(1.9 g·l-1)before feeding.Xylose consumption rate also increased fast.(3)The influence of ethanol on fermentation by P.tannophilus was studied.When xylose utilization rate was 80%,ethanol concentration began to decline.Fermentation of P.tannophilus was affected by ethanol addition in the beginning of fermentation,average xylose consumption rate was 0.065 g·l-1·h-1and maximum specific growth rate was 0.07 h-1at 28 g·l-1ethanol,comparing with average xylose consumption rate of 0.38 g·l-1·h-1and maximum specific growth rate of 0.14 h-1in fermentation with no ethanol addition;P.tannophilus stopped growth at 40 g·1-1ethanol. When initial ethanol concentration was 30 g·l-1,addition of glucose in xylose media made for better growth of P.tannophilus and the most favorable glucose concentration was 15 g·l-1with the highest biomass of 1.51 g·l-1as compared with that of 0.95 g.l-1in pure xylose media.Last,fermentations on sweet sorghum residue hydrolysate and fermentation medium by P.tannophilus were performed.The hydrolysate fermentation had the lower ethanol concentration 3.5 g·l-1and ethanol yield of 0.173 g·g-1than ethanol concentration of 7.4 g·l-1and ethanol yield of 0.250 g·g-1in fermentation medium.Hydrolysate medium was studied by orthogonal experiment,and the optimum design was yeast extract 1 g·l-1,(NH4)2SO4 1 g·l-1,MgSO4 1 g·l-1,KH2PO4 2 g·l-1. Fermentation on the optimum hydrolysate medium by DU-13 had the best results:ethanol concentration was 13.5 g·l-1and ethanol yield was 0.378 g·g-1.
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