Modified carbon paste electrodes for the voltammetric evaluation of early post-bottling oxidation of white wines: a proof of concept

The process of bottling entails exposure of wine to variable amounts of oxygen. Depending on oxygen levels, this can contribute to wine evolution during bottle storage, including occurrence of oxidative off-flavours. Due to the large compositional differences existing among wines, the type and extent of the chemical transformations due to oxygen exposure are wine dependent. Analytical approaches typically involving GC-MS and/or LC-MS allow detailed characterisation of these changes. However, at a winery level, rapid assessment of the oxidative changes due to oxygen exposure at bottling remains challenging. In this study, the use of modified carbon paste electrodes for the voltammetric assessment of early post-bottling oxidation has been investigated. Ten commercial white wines from different grape varieties were bottled under controlled conditions to obtain two different degrees of oxygen exposure, ideally corresponding to good (NoOx) vs. average (Ox) bottling conditions. Free SO2, total SO2 and ascorbic acid at bottling were in the range of 37-42 mg/L, 97-123 mg/L, and 37-44 mg/L respectively. Either a lower or a higher oxygen ingress closures were used for NoOx and Ox wines respectively. After two months of cellar storage, samples analysed by linear sweep voltammetry using modified disposable carbon paste screen printed electrodes. Analyses were carried out in the 0-1200 mV range, with the obtained voltammograms providing a global profile of the pool of major white wine oxidizable substrates such as ortho-diphenols, phenolic acids, ascorbic acid and SO2. NoOx wines showed relatively similar voltammetric profiles, although with different anodic current values, indicating differences in the concentrations of oxidizable substrates. Compared to NoOx samples, Ox wines generally exhibited a decrease in anodic current, with patterns that were wine depended. Oxidation fingerprints were obtained as difference of NoOx minus Ox voltammograms, revealing the existence of 4 different fingerprints across the 10 wines studied. Comparison of these oxidation fingerprints with those of a white wine spiked with pure compounds was attempted to assess the contribution of major wine compounds to the 4 fingerprints observed. Voltammetric data were also cross-compared with those obtained on the same wines using an untargeted LC-ESI-QTOF MS method, in a further attempt to evaluate the suitability of a rapid voltammetric approach to the evaluation of wine post-bottling oxidation.