| Summary: | For a biochemically based biomass-to-ethanol process, one of the advantages of using softwoods as the substrate is the predominance of hexose sugars, which means that most of the sugars should be readily fermented by Saccharomyces cerevisiae. However, one of the biggest challenges with fermenting softwood derived sugars is the presence of both process derived and naturally occurring inhibitory compounds that are detrimental to both the growth and metabolism of yeasts. The presence of inhibitory compounds together with “low” initial sugar concentrations typically result in poor ethanol yields and titres which limit the economic viability of the process. In the work reported here, we tried to improve the fermentation of Douglas-fir derived sugar streams by enhancing the sugar concentration of the upstream processes (steam pretreatment and enzymatic hydrolysis) while using a combination of strategies to efficiently ferment the resulting liquor. These included the use of industrially relevant Saccharomyces cerevisiae strains, high yeast cell density, nutrient supplementation and liquor detoxification. To obtain as high a sugar concentration as possible, a high consistency steam pretreatment and subsequent enzymatic hydrolysis of the combined cellulose and hemicellulose fractions was carried out. Although this “softwood derived liquor” had a final sugar concentration of 18% wt/wt, it also had a very high concentration of inhibitory compounds including phenolics, furan derivatives and organic acids. When the fermentation profile obtained after growth on this liquor was compared to those obtained after growth on glucose and an enzymatically hydrolysed dissolving pulp, it was apparent that these inhibitory compounds severely restricted the growth and fermentation of all of the S. cerevisiae strains. Although the Tembec T2 strain that had previously been adapted to growth on spent sulfite liquor demonstrated the best fermentation performance, a detoxification stage was still required before reasonable (77.2%) ethanol yields could be obtained. Even with a prior detoxification stage, a high initial cell density of OD=13 was required before effective fermentation could be achieved. A combination of sulfite detoxification and high cell density fermentation resulted in a final ethanol concentration of about 5.0% (wt/vol) and volumetric productivity 4.9g/l/h.
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