Metal Complexes and their Role in Wine Chemistry

To further understand the speciation of transition metals in wine, the stability constants of Cu(II), Fe(II) and Zn(II) complex species formed with a range of organic acids found in wine were investigated in both aqueous and 12.5% ethanol solution (25°C, I = 0.1 M KCl). When higher concentrations of metal complexes are formed at the wine pH of 3.25, which consequently results in a lower free metal ion concentration, decreased production of xanthylium ion pigments and decreased rates of caffeic acid oxidation are observed. In contrast, higher concentrations of metal complexes generally resulted in increased ascorbic acid oxidation.
When higher concentrations of metal complexes are formed at the wine pH of 3.25, which consequently results in a lower free metal ion concentration, decreased production of xanthylium ion pigments and decreased rates of caffeic acid oxidation are observed. In contrast, higher concentrations of metal complexes generally resulted in increased ascorbic acid oxidation.