The wine: a never-ending source of H2S and methanethiol

Abstract

Volatile sulfur compounds (VSCs), mainly hydrogen sulfide and methanethiol (H2S and MeSH), are the responsible for reductive off-odor in wine. These compounds can remain in the wine under different chemical forms: free forms, bound to metal cations or as oxidized precursors (polysulfides and polysulfanes). Some remediation treatments, such as aeration, micro-oxygenation, copper fining and addition of oenological products are frequently used by the winemakers to eliminate the reductive problems however, they are not completely effective and sometimes this problem can reappear after a certain period of time. Recently, another options (e.g. filtration, purge…) have been also tested but their efficacy at long term is not much better. These strategies act on the free and bonded forms, therefore it has been hypothesized that exist a huge reservoir of VSCs (in oxidized forms) which is not removed by the remediation treatments and that could explain their inefficacy. Nowadays, it does not exist any reliable method to know the amount of oxidized forms in wine which could be the source of H2S and MeSH. This knowledge could help to understand better the problem of reduction of wines and improve the remediation strategies. For that reason, the objective of this work was developing a new system to monitor the release of VSCs during the storage of different wines under anoxia. This system is based on the use of reversible trapping solutions to retain the VSCs at the same time that they are produced in the wine. Different metal cations, in terms of ability and speed have been studied as potential trapping agents. The reversibility of the process to quantify H2S and MeSH was also evaluated. After the system was optimized, it was applied to several wines stored at different temperatures under anoxic conditions. Cu (I) was chosen as the best option to use in the trapping solution and a dilution with brine and addition of tris(2-carboxyethyl)phosphine (TCEP) was selected to revert the trapping process and quantify the analytes. The linearity and the reproducibility of the system was evaluated and satisfactory results were obtained. The stability of the trapping solutions was also studied to know when they should be replaced in the system to avoid problems in the determination of the analytes. The rate of formation of the VSCs on the real wines depended on the storage temperature, ranging the maximum for each wine from 3 µg/hour to 10 µg/hour of H2S at 75ºC and from 0.1 µg/hour to 0.4 µg/hour at 50ºC. In the case of MeSH, the rate was one order of magnitude lower than for H2S. The total amount of VSCs produced was different for each wine and for each temperature, reaching more than 2 mg/L of H2S at 75ºC and more than 200 µg/L at 50ºC after one month of storage. This system could be useful to predict the tendency of a wine to develop the problem of reduction and evaluate the efficacy of different remediation strategies.