Metal Complexes and their Role in Wine Chemistry

Abstract

<p>The role of metal ions, in particular copper(II), iron(II) and zinc(II) and their complexes, in wine based redox reactions, which can impact quality characteristics, have been investigated in this thesis. The redox reactions of interest in this investigation are oxygen decay and xanthylium cation production which can both result in detrimental outcomes. Therefore, a detailed investigation of metal speciation under wine like conditions and the role of metal complexes on wine based oxygen decay and pigment production processes is presented in this thesis. </p> <p>Acid dissociation constants for wine relevant organic acids, lactic, malic, tartaric and citric acids as well as acetic, formic, glyoxylic, oxalic and succinic acids, were determined in aqueous solution, at 25.0 ± 0.1°C and a constant ionic strength of 0.1 M KCl, using potentiometric titrations. The dissociation constants for each of the acids, with the exception of glyoxylic and oxalic acids, were also determined in 12.5% (v/v) ethanolic solution. The p<i>k</i>_a values for each of the acids were higher when determined in 12.5% ethanol solution when compared to aqueous solution.</p> <p>Binding constants for complex species formed between Cu(II), Fe(II) or Zn(II) and wine relevant organic acids were determined under similar conditions as that for the acid dissociation constants. All metals were found to form at least a metal ligand complex with each of the organic acids, with the Cu(II) complex species determined to have the highest binding constants followed by Zn(II) and then Fe(II). The binding constants determined in ethanolic solution were generally larger than the equivalent aqueous determination for all metal ions. Speciation utilising the determined binding constants shows that citric acid binds the highest percentage of metal ion in solutions containing only a single acid; however, in competition, due to the high concentration of tartaric acid in wine, tartrate complexes were found to dominate. </p> <p>Organic acids which form higher concentrations of Cu(II) complex species at pH 3.25, which consequently results in lower free Cu(II) concentrations, decreased the rate of (+)-catechin consumption and therefore the production of xanthylium ion pigments which have a peak absorbance at 440 nm. The same trend was observed for samples containing iron with higher absorbance at 440 nm observed for those containing lower concentrations of iron complexes.</p> <p>Higher oxygen decay rates in the presence of caffeic acid were observed in solutions with higher percentages of free Cu(II) present. The presence of iron, without copper, did not have a significant effect on the oxidation of caffeic acid or ascorbic acid. In contrast to caffeic acid oxidation, a positive correlation between the rate of ascorbic acid oxidation and increasing levels of complexed Cu²⁺ by diprotic acids was observed. For both ascorbic and caffeic acid samples, changing the organic acid in solution did not have a significant impact on the ratio of total consumed SO₂:O₂.</p>