Optimization of spent sulfite liquor fermentation

The hemicellulose-derived sugars in spent sulfite liquor (SSL), a process stream of the sulfite pulping process, may be fermented to ethanol. Low nutrient concentration, the presence of inhibitors, and a large proportion of xylose limit fermentation of the sugars in SSL. Factorial design experiments were used to find optimum conditions for hexose and xylose fermentation using a robust SSL-adapted (non-xylose fermenting) yeast strain, and Saccharomyces cerevisiae 259ST, a genetically modified strain capable of fermenting xylose. Xylose fermentation was slower than hexose fermentation, and required the detoxification of SSL and/or the addition of hexose sugars to support a large yeast concentration. Xylose fermentation and the growth of the genetically modified yeast in SSL were closely correlated. Under fermentation conditions where no growth was observed (yeast concentration ∼2 g/L and pH < 5), no xylose was fermented in SSL. Under conditions favorable for growth (yeast concentration ∼6 g/L and pH > 5.3), an average of 5.5 g/L ethanol was obtained from xylose fermentation. SSL pretreatment and the addition of nutrients were also beneficial. For the SSL-adapted strain, ethanol yield was improved by increasing the yeast concentration, however while adding nutrients and raising the pH increased yeast growth, these changes resulted in a decrease in the ethanol yield from greater than 85% of theoretical to 70%.