Electrochemical approach for discriminating and measuring predominant flavonoids and phenolic acids using differential pulse voltammetry:: towards an electrochemical index of natural antioxidants

This paper deals with possibilities of employing differential pulse voltammetry (DPV) at a glassy carbon electrode in the analytical characterization of natural phenolic antioxidants in samples containing phenolic classes such as cinnamic acids, flavan-3-ols, and flavonols. A selective electrochemical oxidation of predominant flavonoids at pH 7.5 and an electrochemical oxidation of the total phenolic acids and flavonoids at pH 2.0 was carried out. Our electrochemical approach, based on DPV, has proved to be an easy, fast, and reliable method for discriminating flavonoids and phenolic acids. It has also helped establish the relationship between their voltammetric behavior and their in vitro "antioxidant power". The analytical possibilities of the proposed method have been explored using both, representative standards and real samples where the predominant flavonoid and phenolic acid classes are present in different concentrations. The new approach has also proved satisfactory for the study of apple and pear juices as well as fresh apples and pears in different matrices such as peels and pulps. In general terms. "high antioxidant power" was always related to predominant phenolic classes (acids and flavan-3-ols) while the "low antioxidant power" was a result of a contribution of predominant phenolics (flavan-3-ols) and fingerprint phenolics: phlorizdin and arbutin. Likewise, the proposed electrochemical strategy showed a satisfactory analytical performance. A reproducibility of oxidation potential (less 5%) as well as in the height of peaks and calibration slopes (with R.S.D.s ranging between 0.3 and 8.3%) were also obtained. Concomitantly, the newly proposed voltammetric method showed recoveries between 83.5 and 108.6%. (C) 2004 Elsevier B.V. All rights reserved.