Controlling the Fenton Reaction in Wine

The fate of hydrogen peroxide in a model wine system was studied under a competitive scenario in the presence of ferrous ions and sulfur dioxide. The metal-catalyzed reduction of hydrogen peroxide (H2O2), referred to as the Fenton reaction, yields hydroxyl radicals capable of oxidizing ethanol to acetaldehyde and is now thought to be a key step in nonenzymatic wine oxidation. It appears that sulfur dioxide (SO2) exerts its protective function in wine by scavenging hydrogen peroxide in oxidizing wine, thereby diverting peroxide from the Fenton route. In this study, the factors affecting the rate and outcome of hydroxyl radical-mediated ethanol oxidation were examined under wine conditions. The exclusion of oxygen in the model wine led to conditions wherein ferric ions (50 mu M) were rapidly reduced, presumably by 1-hydroxyethyl radicals. This resulted in the complete stoichiometric conversion of H2O2 (300 mu M) to hydroxyl radicals, giving an equimolar concentration of acetaldehyde (similar to 300 mu M). Surprisingly, the yield of acetaldehyde was markedly depressed in the presence of oxygen. The addition of a model phenol, 4-methylcatechol (4-MeC; 12 mM), did not protect the ethanol from hydroxyl radical-mediated oxidation under the conditions tested but rather appeared to slightly increase the rate of the Fenton reaction, perhaps by forming a complex with the added iron. The competition for H2O2 in the presence of Fe(II) ions and SO2 was also examined, and the effect of added 4-MeC, as well as dissolved oxygen, was investigated. Higher concentrations of 1-hydroxyethyl radicals, which were trapped by N-tert-butyl-alpha-phenylnitrone (PBN) and detected by electron paramagnetic resonance spectroscopy, were observed when oxygen was excluded and when 4-MeC was included.