Selection and development of industrial yeasts suitable for the bioethanol production from lignocellulose.

In recent years increasing attention has been devoted to the production of bioethanol from lignocellulosic biomass, as low-cost and greatly available feedstock. However, despite these advantages, lignocellulose is very expensive to process because of the need for costly pretreatments and large dosages of commercial enzymes. Moreover, pre-treatment of lignocellulose results in the formation of inhibitory compounds affecting the following fermentation phase (Hamelinck et al., 2005). Therefore, more efficient and cost-effective methods for the hydrolysis and fermentation of lignocellulosic biomass to ethanol are needed and the selection of yeasts capable of withstanding also the inhibitors released after lignocellulose pre-treatment is crucial. Ethanol production from lignocellulosic hydrolysates has been widely reported in literature, but only limited research has been conducted on selecting yeasts able to both tolerate inhibitors and ferment sugars with high yield. This study aimed at the selection and design of robust yeasts suitable for the industrial scale bioethanol production. One hundred and twenty yeast strains, mainly newly isolates belonging to Saccharomyces cerevisiae, were screened for their fermentative abilities at different temperatures in minimal media supplemented with high glucose and/or xylose concentrations. The yeasts were then assessed for inhibitor tolerance in rich and defined broths having increasing concentrations of weak acids (acetic, formic, lactic acid) and furans (furfural and 5-hydroxymethyl-2-furaldehyde). The effects of pH value 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 25 and 40°C. Moreover, their fermentative abilities were not affected by the presence in the medium of high xylose concentrations, known to interfere with the glucose uptake and fermentation in S. cerevisiae (Hahn-Hägerdal et al., 2007). Few yeasts were also 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 seemed to enhance their ability to withstand weak acids. The newly isolated S. cerevisiae F17 and S. cerevisiae MEL2 were further studied for their fermentative abilities in a defined medium supplemented with 100 g/L glucose, 50 g/L xylose and increasing concentrations 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 a robust yeast 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. Their genetic modification for the expression of different genes (glycoside hydrolases and pentose metabolising enzymes) are also in progress.