| The following studies were intended to contribute both practical and fundamental information on the process of wine oxidation, and the chemical mechanisms in which key contributing substances such as oxygen (reactive oxygen species), phenolics, and metals intervene. At first, studies on the effects of adding small amounts of oxygen in commercial red wines were performed and novel dissolved oxygen distribution patterns were precisely recorded. At the same time, minimal changes on the wine's phenolic profile were observed, suggesting that these reactions might depend on the presence and concentration of other important substances. Additionally, the presence of wine free radical signals, measured with electron paramagnetic resonance (EPR) spectroscopy, was evaluated. In agreement with the literature, free radical signals were observed only when wine samples were concentrated, or when purified wine phenolic fractions were measured. Nevertheless, it was noted that these signals were enhanced in oxygen rich environments and suppressed under nitrogen rich ones. All of this, led us to advance a theory of oxidation in which oxygen, phenolics, and metal ions are essential in forming a strong free radical, namely hydroxyl radical ( ˙OH), in a "Fenton" reaction, that would be capable of oxidizing most hydroxyl-substituted compounds, thus generating a wide variety of carbonyl compounds, and possibly having an impact in wine color chemistry and flavor. To verify this theory, the ˙OH oxidation of glycerol and ethanol was studied in model solutions and wines. In spite of the strong protective effect displayed by ethanol against this type of wine oxidation, significant amounts of the two main oxidation product of glycerol, namely glyceraldehyde and dihydroxyacetone, were observed. In addition to that, a moderate increase in color was measured when these carbonyl compounds where present in large amounts. The polymers formed, and the reactions mechanism of flavanols, (+)-catechin and (-)-epicatechin, and malvidin-3-glucoside linked by glyceraldehyde were characterized using electrospray tandem mass spectrometry. Therefore, it was shown that ˙OH is the likely compound responsible for the oxidation of glycerol and ethanol in wine, and it was suggested that other hydroxyl-substituted wine substances such as sugars and organic acids probably undergo the same kind of oxidation reactions. |
