Catalytic depolymerization of mannan composition of palm kernel cake (PKC) by mannanase was optimized to enhance the release of mannan-derived monomeric sugars for further application in acetonebutanol- ethanol (ABE) fermentation. Efficiency of enzymatic hydrolysis of PKC was studied by evaluating effects of PKC concentration, mannanase loading, hydrolysis pH value, reaction temperature and hydrolysis time on production of fermentable sugars using one-way analysis of variance (ANOVA). The ANOVA results revealed that all factors studied had highly significant effects on total sugar liberated (P < 0.01). The optimum conditions for PKC hydrolysis were 20% (w/v) PKC concentration, 5% (w/w) mannanase loading, hydrolysis pH 4.5, 45 degrees C temperature and 72 h hydrolysis time. Enzymatic experiments in optimum conditions revealed total fermentable sugars of 71.54 +/- 2.54 g/L were produced including 67.47 +/- 2.51 g/L mannose and 2.94 +/- 0.03 g/L glucose. ABE fermentation of sugar hydrolysate by Clostridium saccharoperbutylacetonicum N1-4 resulted in 3.27 +/- 1.003 g/L biobutanol. (C) 2016 Elsevier Ltd. All rights reserved.