Lignocellulosic biomass is recalcitrant to enzymatic hydrolysis due to its structural complexity. Controlled pH, liquid hot water pretreatment of yellow poplar improves its enzymatic digestibility. Liquid hot water pretreatment conditions were optimized for maximum conversion of yellow poplar to fermentable sugars while minimizing sugar loss to degradation products, at an enzyme loading of 15 FPU cellulase/g glucan. At the optimized pretreatment conditions about 44% of acid soluble lignin initially present in the poplar was solubilized into the liquid fraction of the pretreated poplar. Hot washing of the pretreated poplar enhanced glucose yield two-fold as compared to the pretreated but unwashed poplar, by removing the solubilized lignin that can re-attach to the pretreated poplar surface or nonspecifically bind with enzymes, thereby lowering the sugar yields. Enzymatic digestion of the pretreated and hot washed poplar at 15 FPU cellulase per gram of glucan and a solids loading of 150 g dry solid/L and resulted in 50% of glucose and 60% of xylose yields after 48 hours. Fermentation of the hydrolyzate using a xylose co-fermenting yeast strain 424A (LNH-ST) gave a metabolic yield of 0.45g ethanol/g consumed sugars. Possible mechanisms and process conditions for optimizing aqueous hot water pretreatment of poplar.

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