Bioethanol production by Scheffersomyces stipitis in intensified bioreactors using xylose and glucose as mixed carbon sources

For feasible second generation ethanol production (2G), it is essential to achieve an efficient fermentation process and a viable strain. In this work, two fermentations at different temperatures (25 and 30 ºC), 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^-1 (1º, 2º and 3º recycles), 96 g L^-1 (4º recycle) and 120 g L^-1 (5º recycle) reduced sugars led to a final maximum ethanol concentration of 17.2 and 34.5 g L^-1, at 25 and 30 ºC, 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^-1 h^-1 at 30 ºC. 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 30 ºC with 83.4% of theoretical yield referenced on initial biomass.