Role of polyphenols in copper complexation in red wines.

Four monovarietal red wines, Tinta Roriz, Tinta Barroca, Touriga Nacional, and Touriga Francesa, were studied for their complexometric properties toward copper. For the purpose, ion selective electrode potentiometric titrations were carried out, and the data were treated by two methods suitable for heterogeneous ligands:  the Scatchard plot, which provided the complexation capacity (CCtotal = maximum number of moles of sites per liter), and the differential equilibrium function (DEF), which provided the conditional stability constants of the sites effectively involved in the metal complexation (KDEF) at a given titration point. The different wines displayed similar properties:  CCtotal ranged from 4.5 × 10-3 (Tinta Barroca) to 4.9 × 10-3 M (Touriga Francesa), and log KDEF values were between 4.2 and 6.5. The range of log(|Cu|bound/CCtotal) = log θ embraced in the titration was between −0.2 and −1.9. With regard to the natural levels of copper, from 4.4 × 10-7 (Tinta Roriz) to 1.3 × 10-6 M (Touriga Naciona...