Model Solution Studies Informing Interpretation of Wine Redox Potentials According to a Mixed Potential Model

The oxidation–reduction potential, or redox potential for short, has a long history in wine science and fermentation monitoring. Theoretical descriptions include the redox potential as a global measure of the ratio of redox couples, a mixed potential model based on all compounds electrochemically active at platinum electrodes, and as a potential determined only by the compounds involved in rate-limiting fermentation processes. Redox potentials for pH 3.3 model solutions with 0% ethanol, to represent juices prior to fermentation, and 12% ethanol for finished wines, have been examined, for both air-saturated and deaerated solutions, and with the addition of various redox-active species. Supporting cyclic voltammograms indicate which compounds are active at particular potentials. Ethanol and oxygen are shown to be directly active at platinum electrodes, while shifts in redox potential were observed with the addition of metal ions, caffeic acid, glutathione, hydrogen sulfide, or glucose, without fully accounting for the redox potential values of real wines. Theoretical curves based upon the Butler–Volmer equation are used to illustrate how mixed potentials are generated.