Kinetics of xylooligosaccharides acid hydrolysis for ethanol production

Sugarcane bagasse is the main agricultural residue for second generation bioethanol production in Brazil, however its natural recalcitrance acts as a barrier against direct enzymatic cellulose deconstruction. A pretreatment step is necessary to make it more susceptible to enzymatic saccharification. Hydrothermal pretreatment — a water-based environment-friendly pretreatment — breaks down the lignocellulosic structure of bagasse and solubilizes part of the hemicelluloses. However, the hemicelluloses hydrolysis during this pretreatment is not complete and leads to the formation of xylooligosaccharides which cannot be directly fermented by most part of the microorganisms. In order to explore the hemicelluloses fraction for bioethanol production, a kinetic study of xylooligosaccharides acid hydrolysis obtained after hydrothermal pretreatment was performed. Maleic, oxalic (two dicarboxylic acids) and sulfuric acids were compared in terms of monomeric xylose yield and selectivity as a function of severity factor. Sulfuric acid outperformed the dicarboxylic acids in kinetics terms: the highest xylose yields — up to 96 % — were achieved with shorter reaction times — less than one hour. Maleic acid, however, showed superior selectivity by hydrolyzing xylooligosaccharides — up to 98 % of xylose yield — with minimum xylose degradation. After acid hydrolysis, the liquors were evaluated for fermentability by a wild-type of yeast, Scheffersomyces stipitis NRRL Y-7124 (formerly known as Pichia stipitis). At the end, an overall mass balance considering the fermentation of hexoses and pentoses, selected from the best condition after hydrolysis study and fermentation, was conducted to evaluate the process of second generation bioethanol production using hydrothermal pretreatment.