Chitosan Nanoparticles Loaded with Alkaline-Released Rice Bran Hydroxycinnamic Acids: Physicochemical Characterisation, Antioxidant Activity and In Vitro Bioaccessibility

Food-grade nanocarriers are widely investigated for improving the stability and bioaccessibility of labile phenolics. Ferulic and p-coumaric acid, the principal hydroxycinnamic acids of rice bran, were recovered from this rice milling by-product and encapsulated in chitosan nanoparticles by ionic gelation with sodium tripolyphosphate. Because they occur mainly esterified to cell-wall polysaccharides, aqueous ethanolic extraction, acid hydrolysis and alkaline hydrolysis were compared as release routes; alkaline hydrolysis gave the highest total phenolic content (4.58 mg gallic acid equivalents per g), cupric reducing capacity and by far the highest levels of both acids. This extract was encapsulated at three phenolic loadings; the 1.5 mg GAE/mL loading was the highest loading that remained nanoscale with polydispersity below 0.4. The resulting particles had a mean diameter of 184.7 nm, a polydispersity index of 0.35 and a zeta potential of +32.8 mV, retained on loading, and appeared under electron microscopy as discrete spheroidal particles of 100–200 nm that were absent from the blanks. Encapsulation efficiency was highest for this formulation, 34.6% by Folin–Ciocalteu, and 42.6% and 36.4% for p-coumaric and ferulic acid by UHPLC-DAD. A synthetic mixture matched to the extract was encapsulated more efficiently (52.3% and 45.3%, respectively), supporting an inhibitory effect of the extract matrix. FTIR, XRD and TGA were consistent with entrapment, reduced crystallinity and altered thermal decomposition relative to chitosan. Per unit of phenolic content, the reducing capacity of the nanoparticles matched that of the free extract, so that encapsulation preserved intrinsic antioxidant activity. Under simulated digestion, intestinal phase recovery of both acids was approximately 4-fold that ofthe free extract (p < 0.001), with antioxidant recovery likewise being higher. Chitosan nanoparticles are therefore promising for valorising rice bran phenolics in functional foods and nutraceuticals.