Effects of maturity stage and particle size on the functional properties of dietary fiber isolated from rice (Oryza sativa L.) bran

Rice bran, a by-product of milling, is rich in dietary fiber with well-documented health benefits, yet its use as a fiber source remains limited. This study therefore investigated insoluble dietary fiber from rice bran at two maturity stages, fully ripe (RB) and green (RB-G), focusing on the impact of particle size on chemical composition, physicochemical traits, functional properties, and bioactive compounds. FTIR and XDR analyses confirmed the presence of cellulose structures and crystalline regions in both fiber types. RB showed higher cellulose content (44–61 g/100 g) than RB-G (22–52 g/100 g), increasing with decreasing particle size, and markedly greater β-glucan levels (170–525 mg/g) than RB-G (59–102 mg/g). Finer particle sizes in both RB and RB-G demonstrated significantly improved functional properties, a twofold increase in water- and oil-holding capacities, as well as elevated bioactives. Importantly, different growth stages also influenced functional characteristics, with RB showing higher WHC and WSC, whereas RB-G exhibited superior OHC. In addition, GC-MS revealed a more diverse phytosterol profile in RB, while RB-G contained higher total phenolic content. These findings highlight the potential of RB and RB-G as valuable dietary fiber sources, suggesting promising applications as functional ingredients in health-promoting and sustainable foods. • Insoluble dietary fiber was isolated from ripe (RB) & green-stage (RB-G) rice bran. • RB had higher cellulose and β-glucan contents than RB-G. • GC–MS identified greater phytosterol diversity in RB than RB-G. • Particle size reduction improved water/oil holding and swelling capacities. • Distinct dietary fiber characteristics of RB and RB-G support food applications.