Improving the quality of Glycyrrhiza stems and leaves through solid-state fermentation: flavonoid content, antioxidant activity, metabolic profile, and release mechanism

Glycyrrhiza stems and leaves (GSL) are waste products of Glycyrrhiza; however, it has been considered a potential source of flavonoid compounds. In this study, GSL was processed by solid-state fermentation (SSF) to improve the flavonoid content and antioxidant activity. Firstly, a near-infrared (NIR) prediction model for flavonoid content in fermented Glycyrrhiza stems and leaves (FGSL) was established. Next, the effects of SSF on the antioxidant activity and metabolite profile of GSL were investigated. Finally, the possible mechanism of ferment release of flavonoids was explored based on enzyme activity, thermogravimetric analysis, and FTIR spectroscopy. The results revealed that NIR spectroscopy can efficiently analyze flavonoid contents in GSL, with predicted determination coefficient (Rp²) and root mean square error (RMSEP) of 0.9874 and 0.125, respectively. SSF significantly increased the levels of flavonoids, and enhanced the scavenging activities of DPPH radical and hydroxyl radical and reducing power of FGSL. Widely targeted metabolomic analysis showed the detection of 461 differential metabolites were identified after SSF, with 141 metabolites remarkably up-regulated and 320 metabolites of FGSL down-regulated during fermentation. The main types of differential metabolites were phenolic acids and flavonoids, and the destruction of cellulose by SFF was crucial to the release of flavonoids. In conclusion, our study revealed that SSF remarkably improved the phytochemical components of FGSL by increasing enzyme activity and destroying cellulose structure, thereby contributing to the enhancement of antioxidant activity. This study provided a scientific basis for the production of high-value flavonoids from plant materials and offered a novel approach to elucidate the release and conversion of flavonoids.

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