Controlling Wine Oxidation: Redox Cycling of Iron and the Formation of Acetaldehyde

Over time and with oxygen ingress, the chemical composition of wine evolves through a cascade of reactions comprising many pathways, one of which yields acetaldehyde, an oxidation product which not only directly impacts wine aroma but also facilitates further reactions altering other sensory properties such as color and mouthfeel. Key reactions in this pathway are catalyzed by the redox cycling of iron between two oxidation states, iron(II) and iron(III), thus the majority of the work presented herein focuses on control of iron’s reactivity. A spectrophotometric method of iron speciation was developed as a means of monitoring the reciprocal processes of iron reduction and oxidation in wine. The method relies on the combined use of two opposing chelating agents, ferrozine and ethylenediaminetetraacetic acid (EDTA), iron(II)- and iron(III)-selective respectively, to halt redox reactions, thereby providing a stable measurement of iron(II):iron(III) ratios in wine. This method was used to investigate the redox cycling of iron in model wine as it occurs in the primary reactions of the oxidation pathway: the oxidation of iron(II) by oxygen and the reduction of iron(III) by phenols. The rates of both processes varied with changes to model wine composition, though the former was found to be consistently slower. This suggests that iron(II) oxidation is the rate-determining reaction for the wine oxidation pathway, and wine ages at a rate limited not necessarily by its chemical composition but by oxygen ingress. Despite this, different wines subject to the same oxidative conditions will often vary in their response to oxygen, thus the reactions of hydrogen peroxide, a key branchpoint further down the oxidation pathway, were investigated, specifically as influenced by complexation of iron by major wine acids (tartaric, malic, and citric). The rate of the iron(II)-catalyzed Fenton oxidation of ethanol into acetaldehyde was found to occur more quickly toward the upper limits of typical wine pH, an effect most apparent with citrate complexation of iron. This appeared to affect the efficacy of sulfur dioxide as an antioxidant in competing with iron(II) for hydrogen peroxide. In addition, acetaldehyde formation in wine was found also to depend on oft-overlooked biotic factors post-fermentation. In a series of experiments pertaining to micro-oxygenation, found critical to the process were residual sugar and populations of Saccharomyces cerevisiae, without which acetaldehyde production would not occur. The question of why acetaldehyde does not accumulate with oxygenation alone warrants further investigation