engleski

ELECTROCHEMICAL AND SPETROPHOTOMETRIC INVESTIGATION OF ANTIOXIDANTS IN DARK FRUIT JUICES

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. Dark fruit products such as juice have been shown to contain up to 10-fold higher concentration of polyphenols than light-colored juices. The purpose of this study was to characterize the antioxidants present in six dark fruit juices commercially available on the Croatian market (black currant, blueberry, cranberry, pomegranate, strawberry and cherry) and to examine the redox behavior of standards (trolox and gallic acid) and polyphenols in juices. The electrochemical behaviour of juices diluted in acetate buffer (pH 3) was investigated using cyclic voltammetry on glassy carbon electrode. Electrochemical parameters describing the oxidation and reverse reduction process of phenolics were extracted from cyclic voltammograms of juices diluted 50x recorded up to 950 mV. The first anodic peak appearing at approximately 130 mV was attributed to oxidation of ene-diol of ascorbic acid. The major anodic peak present around 440 mV was attributed to powerful low formal potential phenolics (phenolic acids, flavanols, catechin monomers, proanthocyanidins, flavones, flavanones) 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 (p-coumaric acid, ferulic acid, malvidin anthocyanins, and t-resveratrol) or the meta-diphenols on the A-ring of flavanoids. Electrochemical parameters derived from cyclic voltammograms of 100x diluted juices recorded to 600 mV, were used to perform the diagnostic tests of oxidation/reduction reaction of phenolics in juice. Cyclic voltammetry and potentiometric titration were employed to elucidate antioxidant capacity and ascorbic acid content, respectively, of tested juices. The results were compared with those obtained using spectrophotometric Brand- Williams assay (DPPH) and Folin-Ciocalteu assay (FC). The influence of 29-day refrigerated storage on total phenol content and antioxidant capacity was also evaluated.

antioxidants; cyclic voltammetry

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