Key factors affecting radical formation in wine studied by spin trapping and EPR spectroscopy

The nonenzymatic oxidation of wine has profound effects on its sensory attributes and, thus, perceived quality. While wine oxidation has been studied for centuries, only recently has the role of free radical intermediates in wine aging been seriously addressed. In this study, the effect of various wine components on the formation or suppression of free radicals was investigated. Electron paramagnetic resonance (EPR) spin trapping was used to detect, quantify, and identify factors that alter the major radical species formed in actual wine systems. In all cases and across all treatments, the 1-hydroxyethyl radical was the sole spin adduct detected, suggesting that the Fenton reaction (i.e., formation of the hydroxyl radical and its subsequent oxidation of ethanol) is the major route for oxidation in wine. The addition of either iron, copper, or iron and copper in combination to a red wine resulted in a marked increase in observed spin adducts, demonstrating that trace levels of metals are essential catalysts in the oxidation of wine. The addition of catechin to a white wine containing excess sulfur dioxide had no effect on the initial rate of radical formation, but was prooxidative in the latter stages of the experiment. Finally, sulfur dioxide was shown to inhibit radical formation in a concentration-dependent manner.