Enhanced Cofermentation of Glucose and Xylose by Recombinant Saccharomyces Yeast Strains in Batch and Continuous Operating Modes

Agricultural residues, such as grain by-products, are rich in the hydrolyzable carbohydrate polymers hemicellulose and cellulose; hence, they represent a readily available source of the fermentable sugars xylose and glucose. The biomass-to-ethanol technology is now a step closer to commercialization because a stable recombinant yeast strain has been developed that can efficiently ferment glucoseand xylose simultaneously (coferment) to ethanol. This strain, LNH-ST, is a derivative of Saccharomyces yeast strain 1400 that carries the xylose-catabolism encoding genes of Pichia stipitis in its chromosome. Continuous pure sugar cofermentation studies with this organism resulted in promising steady-state ethanol yields (70.4 percent of the theoretical based on available sugars) at a residencetime of 48 h. Further studies with corn biomass pretreated at the pilot scale confirmed the performance characteristics of the organism in a simultaneous saccharification and cofermentation (SSCF) process: LNH-ST converted 78.4 percent of the available glucose and 56.1 percent of the available xylose within 4 d, despite the presence of high levels of metabolic inhibitors. These SSCF data werereproducible at the bench scale and verified in a 9000-L pilot scale bioreactor.