UPLC–QQQ–MS/MS-based widely targeted metabolomic analysis, antioxidant and α-glucosidase inhibitory activities of mulberry leaves processed by solid-state fermentation

Mulberry leaves contain various bioactive constituents and are widely used as traditional medicine and food in Asia. In this study, mulberry leaves were processed by solid-state fermentation (SSF) using a mixture microbial of Lactobacillus plantarum, Candida utilis, Trichoderma koningii, and Bacillus cereus. The effects of SSF on the metabolite profile, antioxidant activity, and α-glucosidase inhibitory activity (GIA) of mulberry leaves were investigated. UPLC–QQQ–MS/MS-based widely targeted metabolomic analysis showed the detection of 11 classes of metabolites. SSF drastically influenced the metabolites of mulberry leaves. A total of 441 differential metabolites were identified after SSF, with 165 metabolites remarkably up-regulated and 276 metabolites of mulberry leaves drastically down-regulated during fermentation. The main types of differential metabolites were phenolic acids and flavonoids. Furthermore, SSF significantly increased the levels of total phenolics and flavonoids, and enhanced the scavenging activities of DPPH radical and ABTS·+, reducing power, ferrous ion chelating ability, ferric reducing antioxidant power, and α-glucosidase inhibitory activity of mulberry leaves. In conclusion, our study revealed that SSF remarkably improved the phytochemical components of mulberry leaves, thereby contributing to the enhancement of antioxidant and α-glucosidase inhibitory activities.