Development of industrial yeasts for the bioethanol production from lignocellulose

Increasing attention has been recently devoted to the production of bioethanol from lignocellulosic biomass. However, lignocellulose is expensive to process because of the need for costly pre-treatments and large dosages of commercial enzymes. Moreover, pre-treatment of lignocellulose results in the formation of inhibitors affecting the following fermentation phase. Ethanol production from lignocellulosic pre-treated materials has been widely reported, but only limited efforts has been conducted on selecting yeasts able to both tolerate inhibitors and efficiently ferment sugars. This study aimed at the design of robust yeasts suitable for the lignocellulose-to-bioethanol route. One hundred and twenty Saccharomyces cerevisiae yeasts were screened for their fermentative abilities at 25 and 40 \ub0C in minimal media with high glucose and/or xylose concentrations. Their inhibitor tolerance was then assessed in rich and defined broths having increasing concentrations of inhibitors (weak acids, furans and aldehydes). The effects of pH and high sugars levels on yeast inhibitor tolerance were also considered. A number of S. cerevisiae strains showed outstanding ethanol yield from glucose at both temperatures. Few yeasts were able to grow well once exposed to high inhibitors levels. Their tolerance was influenced by the pH in the medium and high glucose and xylose concentration enhanced their ability to withstand weak acids. The newly isolated S. cerevisiae F17 and MEL2 were studied for their fermentative abilities in a defined medium containing glucose, xylose and cocktails of inhibitors. Both yeasts exhibited interesting ethanol yield within 48h, producing about 48 g/L ethanol. The preliminary results of this study are encouraging towards the development of yeasts suitable for the industrial processing of biomass into ethanol. The selected strains have been adopted for the production of ethanol from real lignocellulosic hydrolysates and an evolutionary engineering approach is ongoing to further improve their robustness