Study on the composition and antioxidant activity of free and bound polyphenols in pomegranate peel

Objective:To ascertain the contents, compositions and antioxidant activities of the free and the bound polyphenols of pomegranate peel for providing theoretical basis of extraction and utilization. Methods:Four kinds of peel power were prepared from pomegranate fruits of 3 cultivars respectively. Their free and bound polyphenols were extracted with ultrasonic-assisted extraction method, and caustic stripping method respectively. The polyphenol contents and compositions of the extracts were determined by 3 methods. The antioxidant activities of them were assayed by 4 methods. Results:The total polyphenol, total flavonoid, and total tannin contents of the free polyphenol extracts were in the ranges of 107.55~157.60 mg GAE/gDW, 24.24~44.54mgQUE/gDW and143.03~231.44mgTAE/gDW respectively. The total polyphenol, total flavonoid, and total tannin contents of the bound polyphenol extracts were in the ranges of 7.35~30.89mgGAE/gDW, 2.25~11.56mgQUE/gDW and 9.01~18.86 mg TAE/g DW respectively. The polyphenol compositions of whole peel, outer peel, inner peel, and juicing marc of whole peel powers were significantly different with each other (p<0.05) , and the power of juicing marc of whole peel had the lowest free polyphenols content and the highest bound polyphenols content among them.Meanwhile, the compositions of the extracts from same kind of peel power from the 3 cultivars were also significant different, which reflected the impact of the cultivar was also significant. In addition, the free polyphenol extract from any peel power had similar antioxidant capacity with the bound polyphenol extract from the same peel power. And the antioxidant capacity were in consistent significantly with both the total polyphenol contents and the total tannin contents of the extracts (p <0.01) . Conclusion :The raw material using for extracting pomegranate fruit peel polyphenols could be chosen from whole peel, the inner peel or the marc of peel according to their actual reserves and hygiene situation. Extraction of the free polyphenols could get most part of the polyphenols in the material. The antioxidant potential of the bound polyphenols of pomegranate peel was similar with the free ones at same dose, but the bound polyphenols content of peel was very lower generally.