Xylose consumption by Saccharomyces cerevisiae 424A(LNH-ST) during simultaneous saccharification and co-fermentation (SSCF) of AFEX (TM) pretreated switchgrass was inhibited by unhydrolyzed solids. Such inhibitory effects were not found in unhydrolyzed solids from AFEX (TM) pretreated corn stover (AFEX (TM)-CS). However, the xylose consumption was still unsatisfactory during 6 h pre-hydrolysis SSCF. By extending the pre-hydrolysis time to 24 h or longer, the xylose consumption was improved significantly. In order to better understand the reasons for such improvement, the hydrolysate slurries after 6 h pre-hydrolysis and 24 h pre-hydrolysis were studied and compared. We found that the glucose concentration after pre-hydrolysis was the critical factor that determined cell viability and hence xylose consumption during SSCF. Low temperature (30 degrees C) and ethanol inhibition were shown to be the factors limiting hydrolysis rate and hence productivity during SSCF. (C) 2012 Elsevier Ltd. All rights reserved.