Sweet potato is an important dietary and economic material in China (accounting for 85% of global production in 2005) and Southeast Asia The limitation of using root and tuber of sweet potato mash at high solids content is attributed to its high viscous nature The aim of this study was to investigate the influence of different viscosity reduction factors and found optimal parameters via a surface response design The optimal xylanase enzyme dose pretreatment time and temperature were 1 56 AGU/g 876 min and 44 1 C respectively Using pretreatment sweet potato mash on the optimized condition the final viscosity 498 1 cp and ethanol yield of 135 1 g/kg was obtained by Saccharomyces cerevisiae which was equivalent to 90 7% of the theoretical yield (C) 2010 Elsevier B V All rights reserved