LC-MS based metabolomics and target analysis to study the chemical evolution of wines stored under different redox conditions

Abstract

Oxygen is a key player in oenology, since its effects can be a blessing, benefiting wine quality, or a curse causing irreversible damage. Therefore, many modern winemaking choices tend to favor reduction, even if the severe lack of oxygen can be responsible for a loss in quality due to the formation of Volatile Sulfur Compounds (VSCs) able to cause aroma depreciation, such as H2S and MeSH. The aim of this study was to measure the changes caused to the metabolic space of several red and white wines stored under different levels of oxidative or reductive conditions.

Twelve wines (8 reds and 4 whites) were stored in strict anoxic conditions at 25 ºC (1, 2 and 3 months) and also at 35 ºC for 3 months. Aliquots of the same wines were also micro-oxygenated at 25 ºC during 3 months at different doses of oxygen. The redox potential of all samples was measured and then they were analyzed with an untargeted approach protocol by using an UPLC-HRMS-QTOF instrument to register their metabolic fingerprint; and with a targeted method by using a GC-SCD instrument to analyze the free and Brine Releasable (BR) forms of VSCs. A typical in-house workflow for the data analysis of the metabolic data was used for the quality control of the data-set and for the biomarker discovery and annotation.

The redox potential measurements indicated the reliability of the sample set, since as expected it increased in the presence of oxygen and decreased in anoxic conditions. The LC-MS untargeted analysis generated a dataset of over 10000 features, which after the statistical analysis our attention was focused to approximately 150 tentative markers. These markers were classified in four groups depending on their behaviour under the different conditions. Between the markers were annotated various anthocyanins, such as peonidin 3-glucoside-catechin which decreased under oxidative conditions but remained stable in reduced samples. By contrast, malvidin 3-glucoside decreased also under anoxic conditions although at lower rates with respect to oxidative conditions. Some sulfonated indoles were identified as markers of oxidation conditions. As far as the free VSCs, the highest concentrations were determined in the more reduced samples. The study provides a new understanding about the role of oxygen and of its absence in wine aging.

Acknowledgements

Work funded by the Spanish MCIU AGL2017-87373-C3-1R. I.O. received the grant (CB 8/18) from “Programa Ibercaja-CAI Estancias de Investigación” funded by Universidad de Zaragoza, Fundación Bancaria Ibercaja and Fundación CAI.