Study On Ethanol Production From Xylose By A Recombinant Yeast

Bioconversion of xylose to ethanol is an important process in ethanol production from plant lignocellulosic materials. Sacchromyces cerevisiae has traditionally been used for ethanol fermentation, but it can't utilize xylose. Several yeasts, such as Pichia stipitis and Candida shehatae, can be used for the ethanol fermentation from xylose. However, their ethanol tolerance is low and semi-oxygen is required for optimal ethanol productivity. In this thesis, a recombinant yeast, Sacchromyces cerevisiae ZU-10, is used for the ethanol fermentation from xylose by free and immobilized cells, and then the ethanol production from lignocellulosic materialswere investigated.The suitble xylose concentration, initial pH value and tempetature of xylosefermentation were 80 g/L, 5.5 and 30 °C respectively, and 29.3 g/L ethanol was obtained within 96 h. However, ethanol fermentation by S. cerevisiae ZU-10 would be impacted obviously when there was higher than 0.25 g/L acetic acid or higher than 0.08 g/L furfural in xylose substrate.Addition of glucose favored xylose fermentation, 85.7% xylose was utilized within 36 h when adding 40 g/L glucose to culture containing 30 g/L xylose. This research was valuable for industrial application of S. cerevisiae ZU-10.The recombinant yeast of S. cerevisiae ZU-10 was immobilized by entrapping the cells into calcium alginate gels for ethanol fermentation. The optimized conditions of xylose fermentation by immobilized cells were investigated. Comparing with free cells , period of the fermentation by immobilized cells was shorter, and the tolerance to acetic acid and furfural were enhanced. The xylose consumption rate reached 100%, the yield of ethanol increased from 29.3 g/L to 31.7 g/L, anti-acid and anti-furfural concentration increased from 0.08 g/L, 0.25 g/L to 0.12 g/L, 1.2 g/L respectively. During repeated ethanol fermentation by immobilized cells of 8 batches, the xylose consumption rate exceeded 99% and the average ethanol yield reached 0.39.S. cerevisiae ZU-10 was sensitive to inhibitors, different detoxification methods were used on corn stover hemicellulose hydrolyzate for ethanol fermentation. Most of the acetic acid and furfural can be removed by evapotation, and SO₄^2- can be removed by lime neutralization, and the major inhibitors could be effectively removed when these two methods combined together. Hemicellulose hydrolyzate after detoxification was fermented by S. cerevisiae ZU-10, it was found that the immobilized cells could utilize xylose more quickly and shorten fermentation period, and the xylose concentration decreased from 71.8 g/L to 2.1 g/L after fermented 72 hours, while theethanol concentration reached 31.1 g/L.Cofermentation of glucose and xylose in corn stover enzymatic hydrolyzate by S.cerevisiae ZU-10 was investigated. Using the hydrolyzate treated by alkali as substrate, which contained 66.9 g/L glucose and 32.1 g/L xylose, the ethanol concentration reached 40.7 g/L after the fermentation by immobilized cells. During repeated ethanol fermentation of 12 batches of corn stover enzymatic hydrolysate by immobilized cells, glucose was completely utilized and more than 90% xylose was consumed during each batch, and the average ethanol concentration achieved 40.4 g/L with 0.41 ethanol yield. Results showed that the enzymatic hydrolysate of corn stover could be utilized by immobilized cells for the ethanol fermentation steadily and efficiently.