Analysis of voltammetric fingerprints of different white grape musts reveals genotype-related oxidation patterns

Abstract

Must oxidation is a complex process involving multiple enzymatic transformations, including the oxidation of phenolics containing an ortho-diphenol function. The latter process has a primary influence on wine aroma characteristics and stability, due to the central role of ortho-diphenols in the non-enzymatic oxidative reactions taking place during winemaking and in finished wine. Although oxidation of must is traditionally avoided, in recent years its contribution to wine quality has been revisited, and in some cases improvements to wine aroma have been observed with the application of controlled must oxidation. Nowadays there is a great interest in the wine industry towards the identification of specific markers or patterns to characterize and classify the response of grape must to oxidation. In this study, the response of several grape genotypes to application of controlled doses of oxygen was investigated over three consecutive vintages. Healthy grapes were harvested at maturity form a single experimental vineyard and crushed in controlled conditions. The must obtained was submitted to three consecutive oxygen saturations (approx. 8 mg/L of oxygen). Oxygen consumption kinetics were measured using a chemioluminescence multisensor apparatus. Upon consumption of each saturation, samples were submitted to spectrophotometric analyses to assess oxidation-induced changes to relevant parameters such as absorbance at 280 nm, 320 nm and 420 nm. Voltammetric analyses were also carried out using a Nomasense Polyscan potentiostat with screen printed electrodes to assess the evolution of the entire must oxidizable fraction, including ortho-diphenols. Depending on the vintage, between four and eight genotypes were analyzed, with each oxidation experiment carried out in four replicates. Oxygen consumption rates varied considerably among samples and vintages. Genotypes could be divided in slow (0.07-0.17 mg/L/min) and fast (0.35-0.43 mg/L/min) oxygen consuming, and this was not clearly associate with Folin-Ciocalteu index. Minor changes were observed at the end of each oxidation cycle for UV-Vis parameters such as Abs 280 and 320, while Abs 420 generally increased, in particular during the 2015 trial. Electrochemical analysis revealed major changes in the content and profile of oxidizable compounds, which decreased with each oxidation cycle. The patterns of such changes, namely the regions of the voltammogram mostly affected by oxidation, were found to be genotype-dependent, with vintage only having a minor influence. Specific oxidation patterns could be associated with either slow or fast oxygen consuming musts. These data indicate that the response of grape must to oxidation is linked to specific compositional characteristics (phenolic profiles, enzymes etc) which can be more effectively investigated and controlled by electrochemical methods rather than conventional spectrophotometric approaches.