Oenotannins addition in wine: can be the modulation of redox potential predictable?

The purpose of this research was to study the interaction between oenotannins and wine matrix in order to design a targeted oenotannins addition for modulating the redox status of wine. It is in fact known that oenotannins can regulate the redox potential of musts and wines since they are electroactive substances (1). To this aim, the present study was divided in two steps: (i) twenty different commercial tannin preparations (including condensed and hydrolysable tannins) were characterized in order to prepare three oenotannins mixtures suitable for diverse modulations of the redox potential in wine (increase, decrease and stabilize). In particolar, tannins were dissolved in a model wine solution (12% alcohol, pH 3.5 with tartaric acid) and analyzed to determine the total polyphenol content, anti-radical activity, polyphenolic composition by HPLC-MS, and reactivity with proteins. The effect of tannins on the redox potential modulation was monitored for 48 hours after addition to the model wine solution, in standard condition (20 °C, no oxygen) as suggested by Vivas et al. (2); (ii) the three oenotannins mixtures obtained were added to eight different red and white wines with the aim of modulating their redox potential. Wines were chemically analyzed and the redox potential monitored for three months. The results here obtained highlighted that commercial oenotannins, when added to the model wine solution, showed a different impact in the redox potential: condensed tannins from grape seed, quebracho and tea showed the ability to raise the redox potential; hydrolysable tannins from gallnuts and tara showed an important increase in redox potential, as well as for some oenotannins derived from oak. In this last case, the ability to modulate the redox potential (increase or decrease) was linked to the wood origin, toasting and drying prior the tannins extraction. The three oenotannins mixtures added to the red and white wines affected their chemical composition as well as their redox potential. A PLS1 model was successfully built using the wine chemical parameters and the redox potential monitored for three months, in order to predict the redox status of wines through the oenotannins addition.