This study investigated the dynamic changes and bioaccessibility of free and bound phenolics in rice bean (Vigna umbellata) during simulated gastrointestinal digestion. A total of 34 phenolic compounds were identified and quantified across oral, gastric, and intestinal phases by UPLC-MS/MS detected. Gastric digestion was identified as the critical stage for phenolic release, with multiple flavonoids increasing 2–3-fold, including rutin (205%), isoquercitrin (226%), and procyanidin B1 (134%). In contrast, in the intestinal phase, flavonoids including procyanidin B1, epicatechin, and quercetin became undetectable after extensive degradation, while phenolic acids such as p-hydroxybenzoic acid (157%) and trans-cinnamic acid (200%) accumulated gradually. Phloroglucinol showed a progressive accumulation increased continuously during digestion (10 to 36 mg/kg DW). Most bound phenolics remained remarkably stable, with over 85% retained throughout upper gastrointestinal transit, except for bound p-coumaric acid and phloroglucinol, which were gradually released. Notably, 3,4-dihydroxyphenylacetic acid was detected only in the bound form across all phases. These findings reveal the dual fates of rice bean phenolics, especially the bound fraction, and underscore the importance of their release and transformation during digestion when evaluating the bioactivity of rice bean polyphenols.

Differences in the composition, microstructure, infrared spectral characteristics, and enzymatic hydrolysis properties of rice bran protein extracted by four methods were investigated, and the antioxidant and anti-photoaging activities of peptides derived from the resulting hydrolysates were further evaluated. Rice bran protein obtained via ultrasonic pretreatment combined with alkaline solublilization and acid precipitation (URP) exhibited relatively high purity (59.17%) and extraction yield (47.61%), together with increased surface porosity, enhanced hydration capacity, and improved enzymatic hydrolysis performance. The URP hydrolysate (URPP) showed a protein content of 81.00%, a degree of hydrolysis of 33.57%, and marked antioxidant activity (ABTS, 684.21; ORAC, 2016.15 μmol TE/g sample). The identified peptides were predominantly short and enriched in hydrophobic amino acids. Structural analysis suggested that the indole N-H group of tryptophan may play an important role in the antioxidant activity of these peptides. Moreover, these peptides alleviated UVB-induced photoaging in HaCaT cells by reducing oxidative stress and inflammatory responses and downregulating the mRNA expressions of AP-1, MMP-1 and MMP-3. Overall, these findings reveal an association between the extraction method, structural characteristics, and enzymatic hydrolysis properties of rice bran protein and the biological activities of its derived peptides, providing a basis for the high-value utilization of rice bran protein and the development of antioxidant and anti-photoaging functional ingredients.