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•An integrated process was used to treat poplar to improve enzymatic digestibility.•A maximum enzymatic hydrolysis efficiency of 96.1 % was achieved after pretreatment.•35.2–56.8 % of hemicelluloses were also obtained and well characterized.•The dissolution of hemicelluloses was monitored by confocal Raman microscopy.
Herein, a two-step hydrothermal pretreatment combined with alkali extraction method was applied to deconstruct the poplar cell walls for enzymatic hydrolysis. Results revealed that 88.1 % of hemicelluloses and 77.6 % of lignin were removed during the integrated treatment performed at 180 °C and a maximum enzymatic hydrolysis efficiency of 96.1 % was achieved. Confocal Raman microscopy suggested that the removal of hemicelluloses from cell walls was inhomogeneous, and most hemicelluloses were released from the secondary wall. In addition, 35.2–56.8 % of hemicelluloses were isolated from the integrated treatment. Detailed structural analysis revealed that the water-soluble hemicelluloses possessed more branched structure than the alkali-soluble hemicelluloses and the hemicelluloses isolated from the poplar were mainly composed of a linear backbone of (1→4)-β-d-Xylp with 4-O-Me-α-d-GlcpA attached as side chains. This work provides an efficient pathway to transform poplar into fermentable sugars and hemicelluloses with considerable yield.