Enhanced Extraction of Flavonoids from Licorice Residues by Solid-State Mixed Fermentation

Purpose Industrial and agricultural processing of licorice has produed a considerable quantity of residues, which are normally discarded randomly or treated by landfill. The residual active components in licorice residues including flavonoids were wasted. More seriously, the obstruction of lignocellulose in the residues has brought a huge challenge for efficient extraction of remaining flavonoids. Here, solid-state fermentation was thus developed to treat licorice residues for enhancing the flavonoids extraction. Methods Focusing on the production of cell-wall degrading enzymes including pectinase, exocellulase and laccase, Lichtheimia ramose and white rot fungus were selected for solid-state mixed fermentation. The extraction of flavonoids was enhanced through optimizing the condition of solid-state mixed fermentation. Results Under the inoculation ratio 1:1 of Lichtheimia ramose and white rot fungus with an inoculum concentration 10%, moisture content of 50% (w/w), nitrogen source content 20% (urea: NH 4 NO 3 1:2), optimal activities of exocellulase, pectinase and laccase were obtained, which were 60.31 U/g, 65.99 U/g and 62.30 U/g respectively. The FT-IR confirmed the effective removal of lignin and hemicellulose. The SEM revealed a rough morphology with cracks and appearance of ruffles in fermented licorice residues, implying the destruction of lignocellulose by fermentation. After mixed fermentation, three new flavonoids including glabrone, isoflavone and licochalcone A were detected and identified by UHPLC-Q-TOF–MS. The extracted liquiritigenin increased by 75% compared with unfermented licorice residues. Conclusion Mixed fermentation efficiently enhanced the release of flavonoids from licorice residues through destroying the compact structure of lignocellulose, demonstrating the application potential of solid-state fermentation technology in improving the extraction of active components from medicine residues. Graphical Abstract