Optimization study on acid hydrolysis of hardwood-derived hemicellulosic extract for alcohol fermentation using response surface methodology

In this study, the extraction conditions of hemicelluloses from mixed hardwoods have been tested. In particular, 3% total titrated alkaline (TTA) green liquor (GL) was used to dissolve certain hemicelluloses and lignin in a manner similar to black liquor extraction, but the pH was maintained at near-neutral conditions, so that oligosaccharides were not completely degraded into isosaccharinic acids. The effects of temperature, time, and acid concentration on the fermentable sugar yields for the extract during secondary hydrolysis were investigated. Chips of mixed northern hardwoods were cooked in a rocking digester at 160°C for 110 min in the GL at a concentration of 3% Na2O-equivalent salts on dry wood. The mass of wood extracted into the GL extract was approximately 11.4% of the debarked wood mass, which resulted in a dilute solution of oligomeric hemicellulosic sugars. Dilute sulfuric acid hydrolysis was performed under the following conditions: 100°C–130°C, 2%–4% H2SO4, and 20–120 min residence time. The maximum fermentable sugar concentration of xylose, mannose, and galactose (XyMaGa) obtained from the hydrolyzed extract was 5.5 g l-1, representing 91.7% of the maximum possible yield. A factorial design was used to study the effects of temperature and acid concentration on the maximum quantity of XyMaGa oligomers obtained in the extract during secondary hydrolysis. The maximum potential yield of XyMaGa reached 5.34 g l-1at 130°C, 3.27% H2SO4, and 50.2 min residence time.