Alterations in structural and functional properties of insoluble dietary fibers-bound phenolic complexes derived from lychee pulp by alkaline hydrolysis treatment

Dietary fiber and phenolics were mainly focused on their benefits separately. The alterations in structural and functional properties of insoluble dietary fibers (IDF) -bound phenolic complexes derived from lychee pulp were investigated. The structure of the IDF-bound phenolics complexes became loose after alkaline hydrolysis treatment, which was observed by scanning electron microscopy. It would be related to the removal of phenolics such as ferulic acid (an autofluorescent phenolic) after alkaline hydrolysis treatment, which was characterized by a confocal laser scanning microscope. Furthermore, the FT-IR spectroscopy confirmed that the C–O bond was broken after alkaline hydrolysis treatment. Eleven phenolics were qualitatively and quantitatively analyzed in alkaline solution by high-performance liquid chromatography. The content of ferulic acid was between 4.79 and 5.30 μg/g. It was confirmed that ferulic acid does exist as a dietary fiber-bound phenolic complex and can be hydrolyzed by alkaline solution. Moreover, the total phenolic content (TPC), total flavonoid content (TFC), and results of FRAP and ABTS assays of "Jizui" were the highest in IDF, which were 7.91 mg/g, 5.57 mg/g, 13.49 mg/g, 9.98 mg/g, respectively. The results demonstrate that phenolics are bound to IDF, and the antioxidant activity of dietary fiber would be related to bound phenolics.