Valorization of Citrus Reticulata Peels for Flavonoids and Antioxidant Enhancement by Solid-State Fermentation Using Aspergillus niger CGMCC 3.6189

The bioactive components and bioactivities of citrus peel can be enhanced with microbial fermentation. Accordingly, this study investigated the ability of Aspergillus niger CGMCC3.6189 to accumulate flavonoids in Citrus reticulata peel powder (CRPP) by solid-state fermentation (SSF). Under the optimal SSF conditions including 80% moisture, 30 °C, pH 4.0, 4 × 10⁷ spores/g d.w. CRPP, and 192 h, the total phenolic content (TPC), total flavonoid content (TFC), and 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activities of fermented CRPP significantly increased by 70.0, 26.8, 64.9, and 71.6%, respectively. HPLC analysis showed that after fermentation, the contents of hesperidin, nobiletin, and tangeretin were significantly increased from 19.36, 6.31, and 2.91 mg/g to 28.23, 7.78, and 3.49 mg/g, respectively, while the contents of ferulic acid and narirutin were decreased under the optimal fermentation conditions. Fermentation time is one of the most important factors that affect the accumulation of flavonoids and antioxidant activity; however, extended fermentation time increased the darkness of CRPP color. Therefore, our study provides a feasible and effective SSF method to increase the bioactive components and the antioxidant activity of CRPP that may be used in food, nutraceutical, and medicinal industries.