How small amounts of oxygen introduced during bottling can influence the metabolic fingerprint of white wines

The impact of minute amounts of headspace oxygen on the post-bottling development of wine is generally considered to be very important, since oxygen, packaging and storage conditions can either damage or improve the quality of wine, in terms of its characteristics. Anecdotal evidence of the negative influence of oxygen on the quality of white wines is reflected in the generalised use of inert bottling lines, where the headspace between the white wine and the stopper is filled with an inert gas. This project aimed to address some basic questions about the chemistry of the interaction between wine and oxygen, crucial for decisions regarding optimal closure. While it is known that similar amounts of oxygen affect different wines to a variable extent, our knowledge of chemistry is not sufficient to construct a predictive method. To obtain the widest coverage of the metabolic space, an untargeted method was used with a LC-MS instrument. The experimental design included 12 different wines produced using five different cultivars.10 samples of each wine were bottled using the standard industrial process, with inert headspace and variable exposure to oxygen (1.30 - 4.25 ppm O2), along with a further 10 bottles produced using the same line, but without inert gas and with extra headspace (5.93 - 8.38 ppm O2). After bottling, all the wines were stored for 60 days at room temperature. After storage, the wines were analysed using an untargeted LC-ESI-QTOF MS method, already optimised for wine analysis and adopted for other wine metabolomics experiments [1]. Instrumental analysis produced a dataset with over 20,000 features, and data analysis showed the presence of about 25 putative markers induced by different amounts of oxygen. The stability of the analysis in the experiment was sufficient to highlight several metabolites whose concentrations were influenced by the minute amounts of oxygen entering the wine during bottling in real conditions. The approach made it possible to compare the fate of metabolites already known to be influenced by oxygen, such as the exogenous antioxidants sulphur dioxide and ascorbate, in the same experiment, and to compare them with other endogenous wine constituents in order to obtain new information about wine chemistry.