Robert Coleman, Alexei Stuchebrukhov, and Roger Boulton* 
*Department of Viticulture and Enology, University of California, Davis, CA 95616 (rbboulton@ucdavis.edu)

The renewed interest in oxidation reactions in wine has prompted a re-evaluation of the existence of free metal ions, their oxidation states, and the extent to which they form complexes with major wine constituents. The nature and extent of complex formation will vary across pH and ultimately determines the prevailing redox potential at its resting state. The redox potential establishes the starting conditions for electron transfer reactions, whether during fermentation, during aging, or in bottled wine. Using available software (HySS 4.0.31 from Hyper-Quad), the distributions of Iron (III) and Cu(II) complexes involving 16 major ligands found in wine were estimated in the pH range 2.5 to 4.5, using published complex configurations and their binding constants. The model mixture included tartrate, malate, succinate, sulfate, sulfite, chloride, nitrate, phosphate, hydroxide, gallate, caffeate, catechin, quercetin, glutathione, cysteine, and methionine as ligands in concentrations that are expected in wine and resulted in 120 entities. The results confirm that there are only small fractions of these metals in their free forms at wine pH: most are present in complexes. Ongoing work will establish how redox reactions will modify these equilibria using newly developed software and advanced experimental approaches.

Funding Support: Stephen Sinclair Scott Endowment in Viticulture and Enology