Developmental Dynamics of Bioactive Composition and Biological Activities of Methanolic Extracts from Orange (Citrus sinensis) Peel: Comprehensive Metabolite Profiling, Antioxidant Capacity, and Antibacterial Activity

Although the chemical composition and biological properties of orange peels undergo significant changes during fruit development, the extent of variation in metabolites and their activities remains poorly characterized. In this study, the fruit development characteristics, chemical composition, antioxidant capacity and antibacterial activities of orange peel extracts were evaluated across six sampling times corresponding to 120 to 250 days after flowering (DAF) using the Valencia cultivar. All pomological variables increased during fruit development except peel thickness and fruit juice pH. Both total phenolics and flavonoid content and antioxidant capacity were higher at early stages and their content and activity decreased as fruit development progressed. Of the 58 compounds (5 carotenoids, 19 flavonoids, 10 phenolic acids, 9 organic acids, 9 vitamins, 6 aroma-volatile compounds), 38 were quantitatively determined, with β-carotene, hesperidin, and ferulic acid being predominant compounds among carotenoids, flavonoids, and phenolic acids, respectively. Generally, the amount of individual carotenoids, flavonoids and phenolic acid content increased with fruit development, except epicatechin and syringic acid. Citric and malic acids were the predominant organic acids, while ascorbic acid was the major water-soluble vitamin, reaching a maximum concentration of 1508.3 mg kg−1 in Stage V. Among the volatile compounds, limonene was predominant. Mineral content of peels decreased steadily during fruit development and Ca, K and Mg were predominant elements. Antibacterial activity varied with developmental stage and microorganism; Bacillus cereus was inhibited by extracts from all stages, while the largest inhibition zone (27.67 mm) was observed against Enterococcus faecalis with the Stage II extract. Multivariate analysis further distinguished the developmental stages based on their metabolite profiles, with the first two principal components explaining 84.63% of the total variance. Overall, the chemical and biological properties of Valencia orange peel exhibited dynamic and compound-specific changes throughout development, highlighting the importance of developmental stage when evaluating the potential use of orange peel.