Optimization of monosaccharide recovery by post-hydrolysis of the water-soluble hemicellulose component after steam explosion of softwood chips

SO2-catalyzed steam explosion effectively fractionates softwood carbohydrat es, releasing 80-90% of the hemicellulose into solution and enhancing the e nzymatic hydrolysis of the water-insoluble cellulose fraction. When steam p retreatment conditions were modified to ensure maximum hemicellulose recove ry, a significant Dart of hemicellulose-derived carbohydrates were solubili zed in an oligomeric form that was not readily fermented. When Douglas fir wood chips were used as the substrate, sulfuric acid-catalyzed post-hydroly sis at 120 degrees C for 1 h allowed most of the original hemicellulose to be recovered in the monomeric form. Although higher catalyst concentrations (1-3%) resulted in complete depolymerization of the carbohydrates, noticea ble losses due to sugar degradation were observed. Lower concentrations of the added catalyst increased the monomer yields, and resulted in less secon dary degradation. Partial oxidation of the added SO2 to sulfuric acid durin g steam explosion resulted in the depolymerization of the hemicelluloses du ring post-hydrolysis, even without further acid addition. Hexoses present i n the post-hydrolyzates could be readily fermented to ethanol. (C) 1999 Els evier Science Ltd. All rights reserved.