Development of industrial yeast strains for efficient xylose fermentation in lignocellulosic material

Fermentation at large industrial scale poses several challenges for the fermenting microorganism to handle. Thus, for an efficient production it is desirable to have robust and efficient strains, which can cope with the specific conditions in the process. For bioethanol production by yeast from lignocellulosic material, one of the largest challenges is the mixture of sugars and the content of inhibitory compounds in the material. Wild-type strains of Saccharomyces cerevisiae can only convert hexose sugars but not the pentoses, xylose and arabinose, which may be present in these materials. However, strains have been genetically modified to allow for xylose conversion, but their performance need to be improved in terms of rate and efficiency. During the pre-treatment of lignocellulosic material the inhibitory compounds are formed; furans, phenolics and organic acids. In an industrial setting, a robust strain back ground (industrial yeast strains) is a prerequisite, in which first pentose fermenting traits should be incorporated and further improvement of the tolerance to inhibitory compounds need to follow. In the present project, we have used directed evolution to simultaneously improve the inhibitor tolerance and xylose conversion capability of recombinant yeast strains with an industrial background. The strains showed increased xylose utilization and ethanol production which was for some strains coupled to decreased xylitol formation. The resulting properties of the strains are highly dependent on the mode of directed evolution applied, which may also give rise to quite a number of clones with different properties.