Palm kernel cake (PKC) is a mannan rich by-product from palm oil processing with potential for conversion to fuels and chemicals in biorefineries. However, mannanase, the key enzyme required for the biodegradation of mannan tends to produce short chain mannose oligomers which need further conversion by mannosidases before the resulting monomers can be used by most ethanologens. In this study, Geobacillus therrnoglucosidasius TM242, an engineered thermophilic ethanologen capable of utilising short-chain oligosaccharides was used for production of ethanol from a PKC hydrolysate rich in mannose oligomers without mannosidase supplementation, with high ethanol conversion efficiency. The highest amount of fermentable sugars of 28.34 +/- 0.06 g/L was obtained from 5% (w/w) PKC pretreated by steam explosion at 4.5 bar for 15 min followed by enzymatic hydrolysis using 17.9 Ljig mannan of SEB mannanase and 10.4 FPU/g glucan of CTec2 cellulase. Fermentation of the PKC hydrolysate by TM242 at 60 degrees C for 48 h gave a maximum ethanol concentration of 9.9 +/- 0.4 giL, equivalent to 92 +/- 2% of the theoretical yield based on the total convertible sugars in PKC. This was considerably higher than that obtained using Saccharomyces cerevisiae (35.4%) using PKC hydrolysed under the same conditions or the autoclave treated PKC (P < 0.05). The process developed is attractive for ethanol production from mannan-rich wastes as it requires mild pretreatment conditions and avoids beta-mannosidase supplementation in the hydrolysis step with utilization of all sugar types and oligosaccharides in the substrate. (C) 2016 Elsevier Ltd. All rights reserved.