Evaluation and comparison of the antioxidant activity of extractable and non-extractable polyphenols of sour orange peel

Purpose: Research on food polyphenols has experienced a huge development during the last century. Most studies on polyphenolic compounds have focused on extractable portions. However, an important polyphenolic fraction is ignored (non-extractable polyphenols) because they are trapped in the corresponding extraction residues. In this study, the antioxidant activity of extractable and non-extractable polyphenols of orange peel, extracted by aqueous solvent and acid hydrolysis (with two different solvent systems) respectively, are evaluated and compared. Method: Antioxidant activity was evaluated by four tests of radical inhibition of 2, 2-diphenyl-1-picrylhydrazyl (DPPH), 2, 2-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP) and iron chelating. The amount of polyphenolic compounds, flavonoids, oligomeric proanthocyanidins, and flavanones were respectively determined by folin-ciocalteu, AlCl3 and UV-visible spectrophotometer. Also, the anti-lipid peroxidation effect in rat liver was measured. Results: The results of antioxidant analysis showed that non-extractable polyphenolic extract, which extracted by methanol / sulfuric acid solvent had the highest level of antioxidant activity in sour orange peel. It can be due to the high polyphenolic compounds in this extraction compared to the others. Conclusion: All extracts had antioxidant activity and can be suggested as a potential natural source in food and pharmaceutical industries.