Fe(II):Fe(III) Ratio and Redox Status of White Wines

Abstract

Iron plays a key role in wine oxidation. Polyphenols that contain catechol systems are the main reductants, and it has been proposed that the oxidation of these substances is mediated by the redox cycling of the Fe(III)/Fe(II) couple. At any time, the Fe(II):Fe(III) concentration ratio should depend on the rate of Fe(II) oxidation by oxygen relative to that of Fe(III) reduction by polyphenols. Fe(III) oxidation of polyphenols, although facilitated by sulfite, is somewhat slower than the reaction of Fe(II) with oxygen, which is strongly accelerated by Cu. Alongside this process, Fe(III) inhibits is own formation. Therefore, the Fe(II):Fe(III) concentration ratio is determined by the interplay of a number of competing reactions. However, because of the relative speed of Fe(II) oxidation, oxygen should be a major determinant of this ratio. A simple spectroscopic method involving ferrozine is used to measure Fe(II) concentration in wines collected under nitrogen with minimal disturbance so as to determine Fe(II) levels in the original wine container. However, Fe(III), which becomes a strong oxidant in the presence of ferrozine, oxidizes catechols in wine conditions. Therefore, Fe(II) concentration, which increases as a result of catechol oxidation, was monitored over time and extrapolated back to the moment of ferrozine addition. Total Fe concentration was determined by adding ascorbic acid to reduce the Fe(III). As expected, the Fe(II):Fe(III) ratio was higher in wines bottled with screw caps than in those bottled with natural cork or filled in boxes. Exposure of wines to oxygen lowered the ratio, which reached equilibrium after some days of aerial saturation. However, the ratio attained differed in the different wines, and this difference likely depends on wine constituents that alter the relative rate of Fe(II) oxidation to that of Fe(III) reduction.