Rice bean (Vigna umbellata) is a medicinal and dietary legume rich in polyphenols. In this study, the free and bound phenolics in rice bean were extracted by water, 80% methanol, and acid, base, and composite enzymatic hydrolysis, respectively. The polyphenol profiles of the extracted fractions were analyzed. The outcome demonstrated that base hydrolysis was the most effective way to liberate bound phenolics from rice bean (14.18 mg GAE/g DW), which was 16.68 and 56.72 folds higher than those extracted by acid and enzymatic hydrolysis, respectively. The bound polyphenols released by base hydrolysis contributed to 71.15% of the total phenolic content. A total of 35 individual phenolics was identified, of which isoquercitrin, procyanidin B1, rutin, taxifolin, and catechin were the main monomeric phenolics in the free fraction, while gallic acid, protocatechuic acid, p-hydroxybenzoic acid, catechin, and phloroglucinol were the main monomeric phenolics in the bound fraction. In comparison to the free phenolics extracted by water and 80% methanol and the bound phenolics extracted using acid and composite enzymatic hydrolysis, the bound phenolics from base hydrolysis had a superior antioxidant capacity. The antioxidant activity of rice bean is primarily attributed to individual phenolics such as catechin, abundant both in free and bound fractions, and also p-hydroxybenzoic acid, gallic acid, and protocatechuic acid in bound fractions. The bound phenolics of rice bean were first reported and showed large differences with the composition of free phenolics. This work suggests that the bound fraction of rice bean must be taken into account in assessing its potential benefits to health.

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.