Fermentation of Xylose and Glucose Mixture in Intensified Reactors by Scheffersomyces stipitis to Produce Ethanol

In this work, two fermentations at different temperatures (25 and 30oC), with cell recycling, were accomplished to produce ethanol, using a mix of commercial substrates, xylose (70%) and glucose (30%), as organic source for Scheffersomyces stipitis. Five consecutive fermentations of 80 g L (1o, 2o and 3o recycles), 96 g L (4o recycle) and 120 g L (5o recycle)reduced sugars led to a final maximum ethanol concentration of 17.2 and 34.5 g L, at 25 and 30oC, respectively. Glucose was the preferred substrate; moreover xylose startup degradation was initiated after a remaining glucose presence in the medium. Results showed that yeast acid treatment, performed before each cycle, provided improvements on cell viability, accompanied by ethanol productivity of 2.16 g L h 1 at 30oC. A maximum 36% of xylose was retained in the fermentation medium and after five-cycle fermentation an ethanol yield of 0.43 g ethanol/g sugars was observed. S. stipitis fermentation capacity and tolerance showed better results at 30oC with 83.4% of theoretical yield referenced on initial biomass. Keywords—5-carbon sugar, cell recycling fermenter, mixed sugars, xylose-fermenting yeast.