XYLOSE FERMENTATION BY SACCHAROMYCES CEREVISIAE USING AS SUBSTRATE THE HEMICELLULOSIC LIQUID STREAM FROM HOT-COMPRESSED WATER TREATMENT OF SUGARCANE BAGASSE
R.S.PAREDES*, L.F. VILELA, E.P.S. BON, E.C.A. ELEUTHERIO
Universidade Federal do Rio de Janeiro
Instituto de Química
Programa de Pós-graduação em Bioquímica
The xylo-oligosaccharides rich liquid stream from hot-compressed water (HCW) treatment of sugarcane bagasse was submitted to acid hydrolysis and the resulting C5 sugars syrups used as fermentation medium for recombinant Saccharomyces cerevisiae (xylA of Burkholderia cenocepacia), able to ferment xylose anaerobically. Statistical experimental design was used to determine the HCW temperature and residence time for maximum hemicellulose-derived sugar recovery to the liquid stream. The HCW reactor was fed with 10% (w/v) bagasse, and the treatment was performed under agitation using an initial pressure of 20 bar (N2) at 170 °C for 40 minutes. The recombinant yeast was submitted to evolutionary engineering using a fermentation medium containing 12.02 ± 0.11 g/L xylose and 3.06 ± 0.06 g/L biomass derived acetic acid. After 40 passages, the evolved strain was able to consume the 8.73 g/L of xylose and produced 2.58 g/L of ethanol. The low ethanol yield could be explained by the presence of acetic acid in the C5 biomass syrups since this inhibitor decreases the yeast fermentation performance. As such and aiming to improve the xylose consumption and ethanol production, the selected yeast strain was submitted to a subsequent evolutionary engineering procedure via the stepwise increase of acetic acid up to 4 g/L. The best selected yeast strain (adapted in 3 g/L of acetic acid) showed an improved capacity to consume xylose as well as to produce ethanol (11.3 g/L and 5.01 g/L, respectively), after 48 h of fermentation, reaching 0.44 g ethanol/g xylose (72% of maximum theoretical yield) without xylitol accumulation. The utilization of the HCW liquid fraction, a byproduct of biomass pretreatment which is produced in high amounts, is vital to reduce the costs of second generation ethanol.
Financial Support: CNPq, FINEP, CAPES; Area: Biochemistry
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