In the production of ethanol from lignocellulosic material, it is necessary to reach a high ethanol concentration after fermentation. Simply increasing the substrate concentration leads to stirring problems and inhibition of the enzymes and yeast in the process. Batch simultaneous saccharification and fermentation (SSF) of steam-pretreated spruce with 13.7% water-insoluble solids (WIS) (25% total solids (TS)) was run in a stirred-tank reactor as well as in two reactors designed to handle solid or semi-solid material. In all reactors, the overall ethanol yields were only between Sand 6%. Fermentation of the liquid fraction of the steam-pretreated spruce slurry resulted in an overall ethanol yield of 85%. 22 h of prehydrolysis at 48 degrees C prior to SSF at 32 C significantly increased the overall ethanol yield to 72% (final ethanol concentration of 47.8 g/L), using the whole slurry of steam-pretreated spruce at a dry matter content of 13.7% WIS (25% TS). (C) 2013 Elsevier Ltd. All rights reserved.