Effects of Citrus Peel Powder Addition on the Antioxidant Activity and Phenolic Compound Composition of Rice Morning Bread

This study investigated how the incorporation of citrus ( Citrus unshiu Marcov.) peel powder (0, 3, 6, and 9 % w/w) influences the antioxidant capacity and phenolic profile of rice morning bread. Powder addition significantly increased the total phenolic, flavonoid, and carotenoid contents; 9 % supplementation resulted in a 3.7-fold increase in the total phenolic content compared to the control. The antioxidant activities measured by DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)), and FRAP (ferric ion reducing antioxidant potential) assays increased accordingly and were strongly correlated with the total phenolics (r ≥ 0.98, p < 0.05). High-performance liquid chromatography with diode-array detection analysis quantified 15 phenolic acids and two flavonols. The levels of gallic, vanillic, syringic, chlorogenic, and trans-ferulic acids, together with rutin and quercetin, increased markedly as supplementation rose. These results suggest that citrus peel powder can serve as an effective clean-label ingredient for enhancing the bioactive compound content of rice-based bakery products, while also offering a sustainable and value-added use for citrus processing by-products.