Study On Ethanol Production By Enzymatic Hydrolysis And Fermentation Of Corncob

Corncob can be used to produce fuel ethanol as a cheap and abundantly available lignocellulosic resource instead of limited grain feedstocks. Bioconversion of corncob to fuel ethanol is very meaningful in the aspects of new energy development and environmental protection. In this thesis, technologies in enzymatic hydrolysis of corncob, ethanol production from cellulosic hydrolysate, simultaneous saccharification and fermentation using a thermotolerant yeast strain and a recombinant yeast respectively were investigated.Enzymatic hydrolysis of dilute acid-pretreated corncob was investigated. The optimal dosage of cellulase was 20 FPIU/g substrate. Supplementing cellobiase greatly reduced the inhibitory effect caused by cellobiose accumulation and helped to increase the hydrolysis yield. Addition of 1% Tween-80 and 0.5% trace element solution effectively improved enzymatic hydrolysis, it was found that the reducing sugar concentration reached 68.2 g/L with the hydrolysis yield of 93.1% under optimized conditions. Fed-batch hydrolysis was performed to increase the reducing sugar concentration, which could help to obtain higher ethanol concentration in subsequent ethanol fermentation, while maintaining a relatively acceptable hydrolysis yield. Further ethanol fermentation from glucose in the cellulosic hydrolysate was performed using a thermotolerant strain Saccharomyces cerevisiae HTR-11, and the ethanol concentration reached 25.1 g/L within 18 h and the ethanol yield reached 93.7% of the theoretical yield.Simultaneous saccharification and fermentation (SSF) of dilute acid-pretreated corncob by S. cerevisiae HTR-11 was carried out for ethanol production. Preadjustment of pH value to 4.8-5.0 in SSF improved ethanol yield. Compared with direct SSF, SSF with prehydrolysis was found to have negative effect on the overall ethanol yield. Under the optimized conditions of 38 °C and 10 g/L peptone, 29.7 g/L ethanol was obtained within 92 h with ethanol yield of 0.500 g ethanol/g cellulose. Due to the elimination of feedback inhibition on cellulase caused by glucose in SSF, the ethanol yield increased by 18.2% compared with separate saccharification andfermentation.SSF of alkaline pretreated-corncob by S. cerevisiae ZU-10 was carried out forethanol production. Compared with direct SSF, SSF with prehydrolysis was found to have negative effect on the overall ethanol yield. Further addition of xylanase increased the ethanol yield. The optimized conditions were as follows: 30°C, xylanase dosage of 400 IU/g substrate, 7.5 g/L peptone, inoculum density of 10% and agitation speed of 120 rpm. Ethanol concentration finally reached 37.8 g/L at 116 h with ethanol yield of 0.408 g ethanol/g carbohydrate (cellulose and hemicellulose). As the substrate concentration increased from 100 g/L to 200 g/L, ethanol concentration increased from 37.8 g/L to 66.6 g/L, thus decreasing the energy cost in ethanol distillation and recovery.The above three techniques of ethanol production (Separate hydrolysis and fermentation of dilute acid-pretreated corncob by S. cerevisiae HTR-11, SSF of dilute acid-pretreated corncob by S. cerevisiae HTR-11 and SSF of alkaline pretreated-corncob by S. cerevisiae ZU-10) were compared in terms of ethanol yield. The highest ethanol yield (0.278 g ethanol/g corncob) was obtained in SSF of alkaline pretreated corncob by S. cerevisiae ZU-10. By this technique, the simultaneous conversion of cellulose and hemicellulose to ethanol was realized, which is very meaningful in decreasing the ethanol production cost and increasing the productivity of ethanol fermentation.