Pretreatment of Sophora flavescens Residues to Produce Fermentable Sugars for Lactic Acid Production: Optimization and Mechanism Analysis

The effects of three different pretreatments (H2SO4-heat, NaOH-heat, and hydrothermal pretreatment) on the enzymatic hydrolysis efficiency of Sophora flavescens residues (SFRs) were studied. After NaOH-heat pretreatment, the cellulose content of SFRs increased from 46.6% to 59.4%, and hydrolysis efficiency was 2.3 times higher than that of direct enzymatic hydrolysis without pretreatment. Moreover, the optimum pretreatment conditions were NaOH concentration of 1.00%, solid-liquid ratio of 1:10, temperature of 80 degrees C, and pretreatment time of 60 min. Under these conditions, 85.0% hydrolysis efficiency was obtained. When SFRs with NaOH-heat pretreatment were used as feedstocks in fermentation, the maximum lactic acid concentration was 33.7 g/L, which was twice that produced by SFRs without pretreatment. Mechanism analysis suggested that the lignin of SFRs was removed by NaOH solution, and the deacetylation reaction of hemicellulose occurred, which made the original smooth and dense structure become rough and loose, thus improving the accessibility of cellulose to enzymes.