Addition of alkali to the hydrothermal–mechanochemical treatment of Eucalyptus enhances its enzymatic saccharification

• NaOH enhanced cleavage of ester bonds between lignin and hemicellulose.
• Hydrothermal–mechanochemical treatment with added NaOH produced finer nanofibers.
• Specific surface area of already fibrillated substrate increased by 76% with NaOH.
• Increased specific surface area correlated with increased enzymatic digestibility.

The effects of alkali on hydrothermal–mechanochemical treatment (hydrothermal treatment combined with wet-milling) were examined with the aim of improving pretreatment of lignocellulosic biomass before enzymatic saccharification. After enzymatic saccharification, the highest glucose yield was obtained by autoclaving at 170 °C in the presence of 20% NaOH per substrate weight. The wood fiber was unraveled into finer nanofibers by hydrothermal–mechanochemical treatment, thus increasing the specific surface area of the substrate from 11 to 132 m2/g. Adding 20% NaOH to the treatment further increased the specific surface area of the already fibrillated substrate by 76% (232 m2/g) due to lignin removal and ester bond cleavage between lignin and hemicellulose. This increase in specific surface area was closely related to the increase in enzymatic digestibility; therefore, NaOH addition may have enhanced the effect of hydrothermal–mechanochemical treatment.