12-04: A new yeast producing ß-glucosidase and tolerant to lignocellulose hydrolysate inhibitors for cellulosic ethanol production using SSF

Conventional cellulose-to-ethanol conversion requires cellulose degradation in order to be utilized for growth and fermentation by common ethanologenic yeast. Cellulose is commonly enzymatically degraded into cellobiose by cellulase and subsequently cellobiose broken down into glucose by ß-glucosidase. Thus, both cellulase and ß-glucosidase enzymes are required for a complete cellulose hydrolysis in a simultaneous saccharification and fermentation (SSF). In addition, due to a higher temperature required for an optimal enzyme hydrolysis and a lower temperature for the yeast fermentation, temperatures have to be compromised in SSF. This not only complicates the fermentation procedures but also increases the cost significantly. In this study, we report a new yeast that is able to produce ß-glucosidase and ethanol from cellulose so no additional ß-glucosidase needs to be added into the SSF reaction procedure. Through evolutionary engineering efforts, we further improved the yeast for tolerance to higher temperatures and inhibitors associated with lignocellulose hydrolysate such as furfural and HMF. Using the newly designed yeast, an ethanol yield of 23 g/L was obtained with a 25% solid load of xylose-extracted corncobs at 37C by SSF without the addition of ß-glucosidase. The yeast showed strong tolerance and in situ detoxification capabilities of furfural and HMF. Its fast growth rate and inhibitor detoxification of the fermentation medium exceeded Saccharomyces cerevisiae. The SSF was completed in five days without application of ß-glucosidase compared with S. cerevisiae in seven days with the addition of the enzyme. The new yeast has a potential for lower-cost cellulosic ethanol production.