[Fe(III)]:[Fe(II)] Ratio and Redox Status of Red Wines: Relation to So-Called “Reduction Potential”

Abstract

Wine oxidation is catalyzed by iron (Fe). Fe(II) is oxidized by O₂ to produce Fe(III), which oxidizes reductants, such as polyphenols, thereby returning to the ferrous state. Therefore, Fe-redox cycles and the [Fe(III)]:[Fe(II)] ratio depend on the relative rates of Fe(II) oxidation and Fe(III) reduction. Under reducing conditions, Fe(II) dominates, but with increasing O₂ exposure, the proportion of Fe as Fe(III) increases. Reduction potentials have been used for many years to determine the redox state of wines. However, it has now been realized that these potentials are mainly due to oxidation of ethanol on platinum electrodes and not due to the oxidative processes that normally occur in wine. It is proposed that [Fe(III)]:[Fe(II)] ratios may provide an alternative way of estimating the redox state. These ratios were obtained with ferrozine to determine Fe(II) and total Fe concentrations spectroscopically in white wines. However, ferrozine cannot be used in red wine because of color interference. A similar method using 2-(5-Bromo-2-pyridylazo)-5-[N-propyl-N-(3-sulfopropyl)amino] phenol disodium salt dihydrate (Br-PAPS), which is water soluble and the Fe(II) complex of which absorbs outside the red wine absorption range, was therefore developed with minimal wine disturbance so as not to alter ratios. High Fe(II) content (~97%) was observed in bottled wine with screwcap, technical, and plastic closures. Lower proportions of Fe(II) were found with natural corks and wine boxes. The [Fe(III)]:[Fe(II)] ratio increased on O₂ exposure. Calculated reduction potentials of the Fe couple did not correspond to those that would be measured for wines, providing further evidence that these wine potentials are not true reduction potentials.