| Summary: | At the Tembec sulfite pulp mill, an existing ethanol fermentation facility currently ferments the hexose sugars from spent sulfite liquor (SSL) to ethanol using an industrial strain of Saccharomyces cerevisiae. While this industrial yeast strain achieves high fermentation efficiency on the hexose fraction, it is incapable of fermenting the pentose fraction (primarily comprised of xylose) present in the SSL. The plant Saccharomyces strain exhibits high ethanol tolerance, fast fermentation rates, fermentation at low pH, and resistance to inhibitory substances. Natural xylose-fermenting yeasts such as P. stipitis exhibit high ethanol yield on xylose, however, fermentation rates are slow, and tolerance towards inhibitors in SSL is low. A recombinant strain possessing both the advantageous characteristics of S. cerevisiae and the xylose-fermenting capability of P. stipitis could significantly increase the efficiency of ethanol production from lignocellulosic hydrolysate. Xylose fermentation would have a large impact on the economics of ethanol production in the facility. Tembec Inc. could increase the production of high-quality industrial ethanol by approximately 30% if the xylose present in softwood and hardwood SSL were cofermented with hexose sugars. Xylose-fermenting recombinant Saccharomyces yeasts (plasmid-bearing and chromosomal transformants of robust industrial yeast strains) were constructed by Prof. N. Ho, Purdue University, and tested on xylose-rich SSL at pilot scale. The pilot fermentation plant is an automated 1/1000-scale emulation of the full-scale fermentation operation at Tembec and operates continuously in complete yeast recycle mode under typical industrial (aseptic) conditions. While up to 100% of the xylose present was consumed in extended pilot trials with the plasmid-bearing strain (LNH32), no appreciable increase in ethanol yield over the current plant strain was demonstrated. The fermentation efficiency of LNH32 was comparable to results with the plant yeast strain, despite the fact that LNH32 utilized only 50% of the galactose. Incomplete fermentation of galactose by the LNH32 would seem to explain the lack of increased ethanol production, and due to xylose fermentation by the strain, a 15% increase in fermentation efficiency (based on available hexose sugars) would be realized over the plant yeast strain if LNH32 were capable of fermenting galactose. Finally, in laboratory trials with LNH-ST, a stable xylose-fermenting chromosomal transformant of a galactose-positive host strain, a 20% increase in ethanol yield over the parental strain and plant yeast strain was obtained in both hardwood and softwood spent sulfite liquors.
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