Chemistry of Manganese and Interaction with Iron and Copper in Wine

Iron (Fe) plays a key role in wine oxidation. The reduction potential of the Fe(III)/Fe(II) redox couple in wine conditions allows Fe(II) to react with O₂ and Fe(III) to react with polyphenols with the assistance of sulfite. Copper (Cu) accelerates Fe(II) oxidation and thus can greatly accelerate wine oxidation. Some studies suggest that manganese (Mn), which is present at concentrations similar to those of Fe in wine, may also participate in the catalytic process. This study was therefore undertaken to examine the possible interaction of Mn with Fe and Cu in wine. The reduction potential of the Mn(III)/Mn(II) redox couple is considerably higher than that of the Fe couple. As a result, Fe(III), H₂O₂, and O₂ cannot oxidize Mn(II). Furthermore, Mn(III) is a stronger oxidant than Fe(III) and rapidly oxidizes tartaric acid. Thus, Mn cannot redox-cycle in the same way as the Fe couple. Despite this, Mn(II) accelerates Fe(II) oxidation in an air-saturated wine model, and it is proposed that Mn(II) reacts with an intermediate Fe(III)-superoxo complex to generate Mn(III). This accelerates the oxidation of 4-methylcatechol (4-MeC; a model polyphenol) in the presence of Fe and Cu in the model wine. Mn is a powerful catalyst of sulfite autoxidation, involving a radical chain reaction initiated by traces of Fe. As a radical scavenger, 4-MeC prevents chain propagation and, as with Fe, polyphenols prevent Mn-catalyzed sulfite autoxidation in wine. Results show that Mn accelerates the air oxidation of white wine, which is most evident at high Fe and Cu concentrations.