Effect of alkali on hydrothermal-mechanochemical treatment of woody biomass and mechanism of improvement in enzymatic saccharification

Development of pretreatment for improving the enzymatic digestibility of cellulosic biomass has been studied broadly. In our laboratory, unraveling the fiber of cellulosic biomass by using hydrothermal-mechanochemical treatment has been examined. It was revealed that higher sugar yields were obtained with the combination of autoclaving and milling treatments than with each treatment applied independently. In this study, we added alkali to the hydrothermal-mechanochemical treatment and the effect of alkalis [Na₂SO₃, Ca(OH)₂, and NaOH] was examined with characterizing the pretreated substrates with the aim of optimizing 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 alkali to the treatment further increased the specific surface area of the already fibrillated substrate (maximum 232 m²/g with 20% NaOH) due to lignin removal and ester bond cleavage between lignin and hemicellulose. As to Ca(OH)₂ especially, despite two-thirds of the lignin remaining, the glucose yield was more than 90%. Thus, it is expected that delignification contributes to making the substrate fragile rather than reducing inhibition of enzymatic activity. Moreover, the increase in specific surface area of the substrate was closely correlated with increase in enzymatic digestibility. Therefore, it is suggested that addition of alkali enhanced the effect of hydrothermal-mechanochemical treatment.