Metabolomic changes in Citrus reticulata peel after conventional and ultrasound-assisted solid-state fermentation with Aspergillus niger: A focus on flavonoid metabolism.

This study explored the changes in nutrients, metabolites, and enzyme activity in Citrus reticulata peel powders (CRPP) under conventional or ultrasound-assisted solid-state fermentation (SSF) using Aspergillus niger CGMCC 3.6189. Compared to nonfermented CRPP (NF-CRPP), ultrasound-assisted fermented CRPP (UIS-CRPP) significantly increased total protein and carotenoid levels by 85.26 % and 179.68 %, respectively, surpassing conventionally-fermented CRPP (FO-CRPP). Among the 521 identified differential metabolites, organic acids, lipids, and flavonoids were predominant. Flavonoid accumulation was primarily driven by the flavone and flavonol biosynthesis pathway, with 90.47 % and 90.00 % of differential flavonoids upregulated in FO-CRPP and UIS-CRPP, respectively. SSF significantly increased phenylalanine, tyrosine, and methionine levels, and tyrosine ammonia-lyase and β-D-glucosidase activities, with higher levels in UIS-CRPP. These findings suggest that conventional and ultrasound-assisted fermentation enhances flavonoid levels in CRPP by modulating key enzyme activities in flavonoid biosynthesis and biotransformation. Our study offers a feasible approach for producing value-added products from citrus peel waste.