Cyclic Voltammetry Investigation of the Phenolic Content of Croatian Wines

Abstract

Cyclic voltammograms taken to 500 mV in a model wine solution (12% ethanol, 0.033 M tartaric acid, pH 3.6) at a glassy carbon electrode were used to determine the concentrations of antioxidants, expressed in catechin equivalents (CE), present in three red and two white Croatian wines. The order of increasing total phenol content for wines obtained from the electrochemical Q₅₀₀ measure (the charge passed to 500 mV) was comparable to the one obtained by the conventional Folin-Ciocalteu procedure and points to the reliability of cyclic voltammetry in estimation of total phenols. The anodic oxidation current peak, E_p,a, observed around 400 mV (versus Ag/AgCl) was attributed to compounds with an ortho-dihydroxy-phenol group, while the peak detected at 650 mV in red wines was ascribed to malvidin anthocyanins, ferulic acid, and t-resveratrol. Red wines had formal redox potentials of E^0', about 30 mV more negative than the white wines and, therefore, should exhibit stronger antioxidative properties. Electrochemical parameters, derived from cyclic voltammograms, were used to perform the diagnostic tests regarding the reversibility of catechin and oxidation of wine phenolics. For catechin, a clear picture of the reversibility of the oxidation reaction was obtained from a linear relationship between the oxidation current peak, I_p,a, and catechin concentration, c. For wine phenolics, the difference between the oxidation peak, E_p,a, and the half peak potential, E_p/2, was used as the diagnostic criterion. The value, close to 59 mV, indicates a one-electron transfer process.