Ethanol production thermotolerant yeast

Cao et al. [60-62] examined a fractionation option that used corn cob and aspen woodchip as the substrates. In this biomass fractionation scheme (Fig. 6), the majority of lignin, alkaline extractives, and acetate were solubilized and separated from cellulose and hemicellulose fractions by alkaline treatment. Hemicellulose was then hydrolyzed to its sugar constituents with dilute acid (0.3 M HCl). Hemicellulose carbohydrates were then fermented to ethanol by a xylose-fermenting yeast strain (Fig. 7). The cellulose fraction, after separation from lignin and hemicellulose, was used as the substrate in the SSF process for ethanol production using a thermotolerant yeast strain as the biocatalyst (Fig. 8). 

In designing an efficient SSF system for the conversion of cellulose to ethanol, the fermentation temperature should be compatible with the saccharification temperature that is generally between 45 and 55 °C. The optimal temperature for the most commonly available cellulase is about 50 °C. Therefore, the use of high-temperature-tolerant microbes is desirable for the application of the SSF process to ethanol production. Typical industrial ethanol-producing yeast strains are mesophiHc with an optimal fermentation temperature of 30-37°C. Only a few yeast strains that are thermotolerant, as well as good ethanol fermenters, have been described. However, some thermophilic bacterial species are known to produce ethanol from cellulosic-derived carbohydrates [68,69]. 

Ballesteros et al. [72] studied SSF of pure cellulose with K. maxianus and K. fragilis at 45 °C with cellulase loading of 15 FPU/g substrate. Both yeast strains produced close to 38 g/1 ethanol in 78 h. The results also confirmed the importance of using thermotolerant yeast in SSF processes in order to improve hydrolysis rates and achieve higher ethanol production. Possible benefits of using a thermotolerant yeast to carry out ethanol fermentation at a supraoptimal temperature are ... 

Golias H, Dumsday GJ, Stanley GA, Pamment NB. (2002). Evaluation of a recombinant Kfeb-siella oxytoca strain for ethanol production from cellulose by simultaneous saccharification and fermentation comparison with native cellobiose utilizing yeast strains and performance in co-culture with thermotolerant yeast and Zymomonas mobilis. J Biotechnol, 96,155-168. 

Yanase S, Hasunuma T, Yamada R, Tanaka T, Ogino C, Fukuda H, Kondo A. (2010a). Direct ethanol production from cellulosic materials at high temperature using the thermotolerant yeast Kluyveromyces marxianus displaying cellulolytic enzymes. Appl Microbiol Biotechnol, 88, 381-388. 

Yu, J., Xuzhang, Tan, T. (2008). Ethanol production by solid state fermentation of sweet sorghum using thermotolerant yeast strain. Fuel Processing Technology, 89, 1056—1059. 

Computational methods can also be focused on one selected pathway with finer detail, as opposed to the broad-sweeping computational methods described in Section 18.2.1.3. A kinetic model of xylose utilization by S. cerevisiae for ethanol production aimed to identify which portion of the poorly functioning pathway should be improved [85]. This analysis concluded that higher xylulokinase activity was needed. The authors experimentally verified that increasing xylulokinase activity via the expression of the E. coli xylB improves ethanol production and xylose consumption [85]. Since this initial report, a variety of other studies have reported strategies for increasing xylulokinase activity that also improve xylose utilization [86, 87], including those implemented in the thermotolerant yeast Hansenula polymorpha [88]. 

Pasha, C., Nagavalli, M., Venkateswar Rao, L., 2007. Lantana camara for fuel ethanol production using thermotolerant yeast. Letters in Apphed Microbiology 44 (6), 666—672. Available at http //dx.doi.Org/10.llll/j.1472-765X.2007.02116.x. 

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