In the initial reactions of the wine oxidation pathway, oxygen consumption and phenol oxidation are coupled through the redox cycling of iron between two oxidation states: oxygen oxidizes iron(II) to iron(III) while phenols reduce iron(III) back to iron(II). Wine has been observed to consume oxygen more slowly the more oxygen it has consumed. Given the dependence of oxygen uptake on the availability of iron(II), it was hypothesized this decline is due to exhaustion of wine’s capacity to reduce iron(III). Experiments have been done to better understand iron reduction and its relationship to oxygen consumption, as well as how it may be used to gauge wine ageability.
The reduction of iron was monitored in model wines to determine how reaction rates vary with composition. 4-methylcatechol and pyrogallol, models for o-dihydroxy phenols (e.g. catechin) and 1,2,3-trihydroxy phenols (e.g. epigallocatechin) respectively, differed in their ability to reduce iron, indicating the phenolic compounds in wine are not equally oxidizable. Furthermore, the inability of 4-methylcatechol to reduce iron in the absence of a nucleophile suggests wine oxidation may depend on the availability of nucleophiles (e.g. SO2, glutathione), the depletion of which could hinder further reactions.
Oxygen consumption was found to occur more quickly as pH increased from 3.0 to 4.0, though unexpectedly the opposite was observed for iron reduction. However, rates of oxygen consumption never exceeded those of iron reduction, indicating wine phenols, in the presence of sufficient nucleophiles, maintain a supply of iron(II) that does not limit oxygen consumption, i.e. wine is constantly “primed” to receive more oxygen. This suggests wine ages at a rate limited not by the reactions therein, but by oxygen ingress, thus ageability may be conceptualized not in terms of a rate, but rather the capacity for oxidation.
The ferric reducing antioxidant power (FRAP) assay was modified to quantify the “lifespan” of wine in terms of its capacity to reduce iron(III) and supply the iron(II) required for oxygen consumption. A 2016 UC Davis Petite Syrah was stirred continuously under constant air exposure for several days to test whether its iron-reducing capacity would decrease with oxygen exposure, though this treatment ultimately did not produce any significant changes to FRAP measurements. It is possible FRAP is not a suitable metric for wine ageability as previously hypothesized, though additional trials are necessary to confirm this, before work can be done towards an alternative definition and measurement of ageability. However, it is also possible continuous stirring does not give rise to the same chemical changes as would years of aging, thus FRAP analysis of vertical series of wines aged “naturally” may reveal iron-reducing capacity to be a function of wine age.
The unexpected slowing of iron reduction/phenol oxidation with increasing pH may be explained by increased complexation of iron by wine acids (e.g. tartaric), though this remains an active area of investigation. It is additionally worth studying the effects of iron complexation on other reactions in wine, such as the Fenton reaction and subsequent oxidation of ethanol into acetaldehyde.