Fermentation of oat and soybean hull hydrolysates into ethanol and xylitol by recombinant industrial strains of Saccharomyces cerevisiae under diverse oxygen environments

In this study, we evaluated the capacity of recombinant industrial Saccharomyces cerevisiae YRH 396 and YRH 400 strains to ferment sugars from oat hull and soybean hull hydrolysates into ethanol and xylitol. The strains were genetically modified by chromosomal integration of Pichia stipitis XYL1/XYL2 genes and the overexpression of S. cerevisiae XKS1 genes, in order to have the ability to metabolize xylose, one of the main sugars in lignocellulosic biomass. The strains YRH 396 and YRH 400 were tested by fermenting acid and enzymatic hydrolysates of oat and soybean hull, with different concentrations of sugars, in orbital shaker under conditions of anaerobiosis and oxygen limitation. The YRH 396 strain showed the best kinetic parameters for the production of ethanol and xylitol, thus its metabolism was further studied in bioreactor cultivations. Under anaerobiosis, the maximum consumption of xylose was approximately 35% when using hydrolysates containing similar concentrations of glucose and xylose, whereas when hydrolysates mainly composed of xylose were used, this strain showed a consumption of 73% of the xylose, reaching yields of ethanol of 0.33 g g(-1). In bioreactor cultivations under oxygen limitation, xylose consumption reached approximately 65%, and the main product was xylitol, reaching a final concentration of 8.17 g L-1. These results suggest that, in addition to an adaptive evolution process, molecular modifications are necessary for an industrial application of these type of genetically modified strains.