Two-Step Hydrothermal Pretreatments for Co-Producing Xylooligosaccharides and Humic-like Acid from Vinegar Residue

Abstract

This study proposes an efficient strategy for co-producing high-value-added xylooligosaccharides (XOS) and humic-like acid (HLA) from vinegar residue based on two-step hydrothermal pretreatments. During the first-step hydrothermal pretreatment (170 °C, 50 min), 29.1% of XOS (X2-X6) was obtained. The XOS yield was further improved to 36.2% with endoxylanase hydrolysis, thereby increasing the value of (X2-X4)/XOS from 0.8 to 1.0. Subsequently, the second-step hydrothermal pretreatment was investigated to produce HLA from the solid residue of the first-step hydrothermal pretreatment. The highest HLA yield was 15.3% in the presence of 0.6 mol/L of KOH at 210 °C for 13 h. In addition, 31.7% of hydrochar by-product was obtained. The mass balance results showed that 1000 g of vinegar residue produced 67.9 g of XOS, 91.6 g of HLA, and 189.5 g of hydrochar. Therefore, this study provides a promising pathway for comprehensive use of lignocellulosic biomass in producing XOS and HLA.