Two-Step Hot-Compressed Water Treatment of Douglas Fir for Efficient Total Sugar Recovery by Enzymatic Hydrolysis

The non-catalytic hydrothermal pretreatment of softwood is generally less effective for subsequent enzymatic hydrolysis. In this study, the efficacy of hot-compressed water (HCW) treatment of Douglas fir was investigated between 180 °C and 260 °C, allowing solubilization of the cellulose components. The enzymatic digestibility of cellulosic residues increased significantly under HCW conditions > 250 °C, and the enhanced glucan digestibility was closely related to the decomposition of the cellulose component. Combination of the first-stage HCW treatment (220 °C, 5 min) to recover hemicellulosic sugars with the second-stage HCW treatment (260 °C, 5 min) to improve cellulose digestibility gave a total sugar recovery of 56.2% based on the dried raw materials. This yield was 1.4 times higher than that from the one-step HCW-treated sample (260 °C, 5 min). Additionally, an enzymatic hydrolysate from the two-step HCW-treated sample exceeded 90% of the ethanol fermentation yield based on the total sugars present in the hydrolysates. These results suggest the potential of the two-step HCW treatment of softwood as a pretreatment technology for efficient total sugar recovery and ethanol production.