17757: Can some Saccharomyces cerevisiae strains tolerate the inhibitors present in the hemicelluose rich water soluble fractions of steam pretreated softwoods?

The high hexose content of the hemicellulose rich, water soluble fractions (WSFs) of steam pretreated softwoods makes them an attractive substrate for “conventional” ethanol producing yeasts. However, the WSF also contains a range of inhibitory products, some naturally occurring in wood, such as extractives and others that are produced during steam pretreatment, such as furans. Although various strategies, such as detoxification of this WSF have been tried, the natural resistance, or robustness, of some yeasts has not been extensively studied. To try to assess the level of tolerance of some potentially robust strains, four S. cerevisiae strains, Tembec T1 and T2, BY4742, and Y1528, were evaluated for their ability to ferment the sugars present in the WSF of steam pretreated Douglas-fir. Although none of the strains were able to ferment “un-conditioned” WSFs to any appreciable levels, when the hexose content of the WSF was increased by spiking with exogenous glucose, close to theoretical yields of ethanol could be achieved by several of the strains after 48 h. In terms of ethanol productivity, T1 and T2 were better than BY4742 while Y1528 was poorer with regard to both ethanol productivity and yield. Overliming with Ca(OH)₂ to pH 11 increased the fermentability of the WSF for most strains. It was apparent that the industrial strains T1 and T2 were more robust than the “wild type” strains. It was also evident that increasing sugar concentrations helped all of the S. cerevisiae strains to better deal with the toxicity of the hemicellulose containing WSFs.