Oxidation vs reduction: the fate of tannins, pigments, vscs, color,SO₂ and metabolomic fingerprint

Abstract

The management of oxygen during winemaking and aging is a big issue in order to achieve high quality wines. The correct amount of O2 improves aroma, astringency, bitterness and color, however an excess of oxygen promotes the appearance of yellow and brown colors and oxidative off-odors, while its absence leads to the formation of reductive aromas. Even thought our knowledge about the reactions occurring during wine oxidation are very rich and detailed, the scientific data about the wine behaviour under reductive storage is limited. The main objective of this work was to study the metabolomic changes of eight red wines caused by the storage under different oxidative and reductive conditions.

Eight red wines were stored under eight different conditions, which include a) micro-oxygenators at 25 ºC for 3 months; b) anoxic atmosphere at 25 °C for 1, 2 and 3 months; c) anoxic atmosphere at 35 °C for 3 months; and d) control.  The following physicochemical analysis were made: LC-MS based metabolomic fingerprint, CIELab color, analyses of volatile sulfur compounds, redox potential, and basic oenological analysis. 

Changes of concentration of H2S and methanethiol (higher amount of free forms under reductive conditions) and redox potential results showed the reliability of the sample set. Color of samples evolved in a different way depending on the storage conditions, getting darker the reduced samples. Metabolomic study revealed reactions with SO2 and direct linked tannin-anthocyanin (T-A) adducts were favoured under anoxia but in the presence of oxygen, reactions with acetaldehyde and ethyl-linked T-A and tannin-tannin (adducts) were the favoured. The reaction mechanism of these reactions favoured in absence of oxygen could explain the observed changes during reductive storage.