Ethanol Fermentation From Hexose And Pentose By Kluyveromyces Marxianus

Fuel ethanol is an alternative to fossil fuels as a new energy, lignocellulosic bio mass, which is large-scale availability and low cost, has been regarded as an ideal feedstock to produce fuel ethanol. However, the production of lignocellulosic ethanol is limited by some obstacles:the rate of xylose fermentation is slow and ethanol yield is low; the optimum temperature for enzymatic hydrolysis is typically different from that of fermentation, resulting in low efficiency of simultaneous saccharification and fermentation. In this work, the key technologies for lignocellulosic ethanol production were addressed.In the study, profiles of hexose (glucose, mannose and galactose) and pentose (xylose and arabinose) utilization by Kluyveromyces marxianus under a microaerobic condition were studied, compared with an industrial ethanol fermentation strain, Saccharomyces cerevisiae6525. K. marxianus DL1showed the characteristic of fast growth and higher ethanol yield and productivity. When using glucose as a substrate, the maximum ethanol production of K. marxianus DL1reached7.96g/L at10h with ethanol yield of0.351g/g and ethanol productivity of0.80g/(L·h), which were11%and135%higher than those of S. cerevisiae6525, respectively. Although the ethanol production and yield of K. marxianus DL1were similar to those of S. cerevisiae6525, the ethanol productivity of K. marxianus DL1were1.2and2times more than those of S. cerevisiae6525using mannose and galactose as the carbon source, respectively. The best results were gained with K. marxianus DL1using pentose as substrates. K. marxianus DL1, which was the only strain among five strains, could ferment xylose to ethanol and arabinose to arabitol. The maximum ethanol production and productivity were1.71g/L and0.095g/(L·h) at72h. The concentration of arabitol was2.28g/L at168h.Then, we examined the five strains above utilizing hexose (glucose, mannose and galactose) and pentose (xybse and arabinose) under an anaerobic condition. The advantages of K. marxianus DL1were still evident. K. marxianus DL1could ferment glucose to a maximum ethanol concentration of14.69g/L at16h corresponding to ethanol yield of0.357g/g and ethanol productivity of0.92g/(L·h), which were1.3and1.1times than those of S. cerevisiae6525. Unlike microaerobic conditions, the ethanol production and yield of K. marxianus DL1were higher than those of S. cerevisiae6525, the ethanol productivity of K. marxianus DL1were1.3and1.1times more than those of S. cerevisiae6525using mannose and galactose as the carbon source, respectively. Utilization patterns of pentoses were similar to those in the mircoaerobie conditions. It was found that K. marxianus DL1could ferment xylose to ethanol and arabinose to arabitol under anaerobic conditions, and that xylose was fermented to7.68g/L ethanol, giving an ethanol yield and productivity of0.199g/g and0.064g/(L·h). Arabinose was fermented to5.96g/L arabitol.Based on the above experiment, the effect of temperatures (30℃、35℃、39℃、41℃) and medium compositions (organic medium and synthetic medium) on K. marxianus DL1using xylose as the substrate were investigated. As the temperature gradually increased, a low ethanol and xylitol production and xylose utilization rate were observed. The ability of K. marxianus DL1metabolizing xylose on an organic medium was superior to that on a synthetic medium at35℃. K. marxianus DL1showed the characteristics of fast growth and higher ethanol and xylitol production on an organic medium. The remaining xylose concentration on an organic medium was3.5g/L at the end of fermentation while it increased to15.5g/L on a synthetic medium, the maximum xylitol and ethanol concentrattion on an organic medium were1.5and1.2times than on a synthetic medium.In the end, simultaneous saccharification and fermentation of corn stover for ethanol production was performed with K. marxianus DL1. The results showed that K. marxianus DL1could coferment glucose and xylose produced from cellulose and hemicellulose, and high-temperature fermentations were advantageous for simultaneous saccharification and fermentation by K. marxianus DL1. At35℃, the rate of cellulase is fast and the maximium ethanol concentration reached5.26g/L which was11.4%higher than that at30℃.