Integrated Soybean Hull Biorefinery with Citric Acid-Catalyzed Hydrothermal Pretreatment for L-Lactic Acid and Xylooligosaccharide Production

Abstract

The main objective of this study was to develop a citric acid-catalyzed hydrothermal pretreatment to reach the maximum sugar recovery from soybean hull (SH), in the context of a biorefi nery concept. A central composite design (CCD) was used to optimize pretreatment conditions through the analysis of citric acid concentration, time, and temperature effects. Optimized conditions were defined as 3.56% w/w, 46.1 min, and 135.7 °C. From the solid fraction, 99.5% and 83.0% of glucose and xylose conversion, respectively, were obtained after enzymatic hydrolysis. Sugars were fermented successfully to L-lactic acid by Lactobacillus pentosus. In fact, SH hydrolysate presented no trace of inhibitor compounds, and glucose was totally consumed during fermentation. After 24 h of fermentation, a conversion rate of 0.724 g/g of sugars to lactic acid and 80.5% of the stoichiometric theoretical yield were reached. In addition, from the liquid fraction, 47.8 mg/g of xylooligosaccharides (XOS) was produced. This work demonstrates the potential of SH to be exploited as feedstock, with mild and efficient pretreatment process, for medium- to high-value bioproducts production in biorefineries.