Release characteristic and mechanism of bound polyphenols from insoluble dietary fiber of navel orange peel via mixed solid-state fermentation with Trichoderma reesei and Aspergillus niger

Dietary fiber (DF) is a carrier of abundant bound polyphenols (BP). The present work was to investigate the release characteristic and mechanism of BP from the insoluble dietary fiber of navel orange peel (NOP-IDF) by mixed solid-state fermentation (SSF) with Trichoderma reesei and Aspergillus niger . The optimized fermentation conditions were set as follows: inoculation ratio of T. reesei to A. niger 3:1, delayed inoculation time 28 h and initial pH 6.5. The compositions of BP were identified and quantified by UPLC-ESI-QTOF-MS/MS and HPLC-ESI-QQQ-MS/MS, then the biotransformation pathways were partially speculated. Moreover, cellulase, xylanase and β -glucosidase activities were strongly correlated with the BP content. The SEM showed the cell wall and the crystalline structure of the NOP-IDF was effectively decomposed. Furthermore, mixed SSF significantly enhanced in vitro antioxidant activities of BP. In summary, our study demonstrated that mixed SSF was a feasible method for the BP release.