engleski

Analytical evaluation of antioxidant potential of fruit teas

It has been proven that a diet rich in fruits and vegetables containing various classes of polyphenols decreases the risk of premature mortality from major clinical conditions, including cancer and heart disease. Green, black and white teas have been shown to exhibit good antioxidant properties both in vitro and in vivo. The purpose of this study was to employ various analytical techniques to characterize the antioxidants present in ten fruit tea infusions commercially available on the Croatian market (blueberry, mandarine and peach, rose hip, apple and cinnamon, pink grapefruit, forest fruit, orange and cinnamon, apricot, cherry, strawberry and vanilla). The electrochemical behaviour of teas diluted in acetate buffer (pH 3) was investigated using cyclic voltammetry on a glassy carbon electrode. Electrochemical parameters describing the oxidation and reverse reduction process of phenolics were extracted from cyclic voltammograms of teas diluted 50x recorded up to 950 mV. The first anodic peak appearing at approximately 130 mV was attributed to the oxidation of ene-diol of ascorbic acid. The major anodic peak present around 440 mV was attributed to powerful low formal potential phenolics characterized by an ortho-dihydroxy-phenol group. Oxidation process resulted in formation of a stabile quinone, which can be reduced in the reverse scan, appearing as a cathodic peak around 350 mV. The third anodic peak detected at potentials higher than 600 mV was ascribed to oxidation of the monophenol group or the meta-diphenols on the A-ring of flavanoids. Cyclic voltammetry and potentiometric titration were employed to elucidate antioxidant capacity and ascorbic acid content, respectively, of tested teas. The total phenol (TP) content was determined using the spectrophotometric Folin-Ciocalteu assay (FC) while the antioxidant activity (AA) was deduced from reactions with 1, 1-diphenyl-2-picryl-hydrazyl (DPPH) and 2, 2′ -azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) radicals. The highest TP content and AA were determined for Rose hip tea while Pink grapefruit showed the lowest values. High linear correlation coefficients between the TP content and AA of tested teas indicate the major contribution of phenolics to the AA. Determination of phenolic compounds contributing to the AA of tested fruit teas was performed by high pressure liquid chromatography (HPLC). The most present compounds were gallic and chlorogenic acids, and epigallocatechin-gallate. Spectrophotometric methods were employed to determine the influence of ascorbic acid and honey addition on the antioxidant properties of teas. Antioxidant activity increased up to 56 % with the addition of ascorbic acid while the addition of honey to teas with previously added ascorbic acid resulted in a significant decrease of AA evaluated by the DPPH method (22%).

antioxidants; fruit teas

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