The water phase antioxidant activity of extracts from 23 Bulgarian medicinal plants was studied in relation to their polyphenol content in comparison with mate, black tea, honeybush and rooibos foreign species. Antioxidant activity was measured by the ABTS (2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)) cation radical decolorization assay, and the total polyphenol content was assayed according to the Folin-Ciocalteu method. Five Bulgarian plant extracts exhibited higher antioxidant activity than that of mate, which is 21.7% of all Bulgarian herbs included in this study. These were Alchemilla vulgaris L. (4.79 +/- 0.14 mm), Sambucus ebulus L. (4.03 +/- 0.07 mm), Mentha spicata L. (3.90 +/- 0.03 mm), Fragaria vesca L. (3.74 +/- 0.06 mm), Crataegus monogyna Jacq. (3.63 +/- 0.05 mm). Another eight Bulgarian medicinal plant extracts exhibited an intermediate antioxidant activity - lower than that of mate and higher than that of honeybush, which makes 34.8% of all Bulgarian herbs included in the study. More than half of the herbal extracts included in the present study exhibited antioxidant activity higher than or comparable to the reference foreign plants. A positive correlation (r = 0.92) between antioxidant activity and polyphenol content was found, suggesting that the antioxidant capacity of the aqueous plant extracts is due to a great extent to their polyphenols.
Food preparations containing substances with antioxidant activity have considerably aroused the interest of consumers, which has led to the marketing of a number of dietary supplements such as resveratrol, hesperidin or chlorogenic acid. On the other hand, natural antioxidants present in citrus fruit juices, together with dietary supplements, could contribute to an enhancement of their antioxidant properties and, in turn, alter their physiological impacts. The aim of the study was to evaluate the effect of supplementation of freshly squeezed citrus juices with resveratrol, hesperidin and chlorogenic acid on the in vitro antioxidant potential of the juices. The solutions of juices, selected polyphenols and the mixtures of solutions of juices and solution of polyphenols were analyzed to determine total polyphenol concentration using the Fast Blue method and total antioxidant potential by the ABTS method. As a result three different types of interactions were established: synergistic of chlorogenic acid which additionally increased the antioxidant potential by 35-38% over theoretical predictions, additive-after addition of resveratrol, and antagonistic-in the case of hesperidin. While the antioxidant potential was substantially higher in the mixture of mandarin juice with chlorogenic acid, the measured total polyphenol concentration decreased. Also a considerable decrease in the concentration of polyphenols was measured in the mixtures of mandarin and orange juices with resveratrol, all these possibly due to the sensitivity to other substances present in juices of the analytical method applied. A correlation between antioxidant potential of all examined solutions and total polyphenol concentration was found (r 2 =0.8879).
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