Metal-mediated reduction as a tool for estimating hydrogen sulfide (H₂S) and methanethiol (MeSH) precursor pools in wines

Abstract

Hydrogen sulfide (H₂S) and methanethiol (MeSH) are the primary compounds responsible for the reduction fault in wine. These molecules exist in various oxidized forms, creating a complex “pool” of precursors1,2. In anoxic environments, such as those found after bottling, this pool can trigger the release of H2S and MeSH. Recent studies indicate that these precursors can reach levels between 1 and 3 mg/L3; however, the industry lacks a rapid analytical method to quantify this risk before the wine is released. The objective of this study was to evaluate whether metallic reductants, such as Fe and Al, could serve as an effective alternative to assess the pool of H₂S and MeSH precursors. Real and synthetic wines were incubated in containers holding vials with CuCl-based trapping solutions for H₂S and MeSH. Throughout the incubation period, the traps were analyzed by HS-GC-SCD and replaced by fresh ones. The effects of pH, SO₂, temperature, metal dose (8 and 40 cm of Fe wire per 80 mL of wine), acetaldehyde concentration to prevent the reduction of SO₂ to H₂S, and dilution of the wine with different acids at various concentrations for Fe were evaluated. A strong interference from SO₂ was observed, as both metals reduced it to H₂S, which could only be partially limited. Al was inefficient in reducing H₂S precursors and was therefore unsuitable for this assay. In contrast, Fe was significantly more effective. At room temperature, pH 4.5, and with 2 g/L acetaldehyde, the amount of SO₂ reduced to H₂S was marginal. However, under these conditions, the precursor pool could not be effectively reduced. At 50 °C, the addition of 4 g/L acetaldehyde was necessary to limit the reduction of SO₂ to H₂S (< 2 µg/L/day for every 10 mg/L of SO₂ present in synthetic wine). Under these conditions, and through dilution with acetic acid, complete reduction of the precursor pool was achieved in some wines within 10 days, allowing a reasonable estimation of the H₂S precursor pool. However, in another set of wines, full reduction of the pool was not achieved due to the massive reduction of SO₂ to H₂S. In conclusion, while iron-mediated reduction offers a potential pathway to estimate H₂S precursor pools, the interference from SO₂ remains the limiting factor. Our results suggest that SO₂ removal appears necessary to develop a truly reproducible and effective method for predicting the reduction potential of wine.

References

1. Ferreira, V.; Franco-Luesma, E.; Vela, E.; López, R.; Hernández-Orte, P. (2018). J. Agric. Food Chem., 66, 2237-2246.
2. Franco-Luesma, E.; Saenz-Navajas, M.P.; Valentin, D.; Ballester, J.; Rodrigues, H.; Ferreira, V. (2016). Food Res. Int., 87, 152–160.
3. Ainsa-Zazurca, S.; Ontañón, I.; Ferreira, V. (2025). J. Agric. Food Chem. 2025, 73, 19684−19692.