Mechanism and Functional Activities of Dietary Fiber-Bound Polyphenol Released from Pomegranate Pomace in Mixed Solid-State Fermentation with Aspergillus niger and Rhizopus oryzae

The insoluble dietary fiber (IDF) of pomegranate peel residue was treated by mixed solid-state fermentation (SSF) of Aspergillus niger and Rhizopus oryzae. The content of bound polyphenols (BP) reached a maximum of 26.50 mg GAE/g DW after 5 days of fermentation, which was 3.22-fold higher than that of the unfermented group. The BP release during fermentation was strongly correlated with the activities of CMCase, xylanase and β-glucosidase. A total of 35 polyphenolic compounds and catabolic metabolites were identified, and potential biotransformation pathways were proposed. Structural characterization of IDF before and after fermentation via SEM, XRD, and TGA revealed that the degradation of cellulose and disruption of hemicellulose and hemicellulose–lignin complexes played a key role in BP release. Moreover, the postfermentation BP in vitro showed enhanced antioxidant capacity and pronounced inhibition of α-amylase/α-glucosidase. Collectively, the mixed SSF significantly promote the BP release, facilitating pomegranate peel valorization.