Metal reducing agents (Fe and Al) as possible agents to measure the dimensions of the hydrogen sulfide (H₂S) pool of precursors in wines

Abstract

Reductive wine fault is characterized by the presence of odors such as rotten eggs or spoiled camembert cheese, originating from hydrogen sulfide (H₂S) and methanethiol (MeSH) [1]. These compounds stabilize in polysulfide forms, creating a complex pool of precursors that will revert to both molecules when the environment becomes anoxic [2]. A recent study [3], confirmed by ongoing research [4], suggests that wines could contain between 1 and >2.6 mg/L of H₂S precursors. However, there are no methods to determine the dimensions of this pool within a reasonable timeframe, as its development through spontaneous reduction takes more than 3 months at 75ºC. The aim of this study was to evaluate whether metallic reducers, such as Fe and Al, could serve as an effective alternative to evaluate the pool of H₂S and MeSH precursors.

Methods: Wines (reals and models) were incubated in containers holding vials with CuCl-based trapping solutions. Traps were analyzed by GC-SCD to determine H₂S and MeSH levels; and replaced by fresh trapping solutions [3]. Effects of wine pH (3 to 5.5), SO₂ (0-150 mg/L), temperature (ambient and 50°C), metal dose (between 8 and 40 cm of Fe wire per 80 mL), and acetaldehyde concentration (1-4 g/L for Fe and 1-10 g/L for Al) were evaluated.

Results: In all cases, SO₂ was a strong interference, as it is reduced to H₂S by both metals. However, its reduction can be partially limited by the addition of acetaldehyde, pH adjustment, and regulating the amount of metal present. Al was particularly sensitive to SO₂ interference, as its production could not be completely avoided even with 10 g/L of acetaldehyde. Additionally, Al was highly inefficient in reducing H₂S precursors, leading to the conclusion that it is not suitable for this assay. Contrary, Fe was significantly more effective, though it was also capable of reducing SO₂. At room temperature, at pH 4.5 with 2 g/L of acetaldehyde, the amount of SO₂ reduced to H₂S was marginal (<0.5 µg/L/day per 10 mg/L of SO₂ present). However, under these conditions, the pool of precursors could not be effectively reduced, yielding no more than 0.4 mg/L of H₂S and 16 µg/L of MeSH after 10 days of incubation. At 50°C, it was necessary to add 4 g/L of acetaldehyde to limit the reduction of SO₂ to H₂S (<2 µg/L/day per 10 mg/L of SO₂ present). Under these conditions the total reduction of the pool of precursors appears to be achieved within 10 days, allowing for a reasonable estimation of the H₂S and MeSH pool of precursors.

Acknowledgement

MYCIN PID2021-126031OB-C21 project and grant PRE2022-102031, funded by MICIU/AEI/10.13039/501100011033 and by ESF+.

References

[1] Franco-Luesma, E., Saenz-Navajas, M.-P., Valentin, D., Ballester, J., Rodrigues, H., Ferreira, V. (2016). Food Res. Int., 87, 152–160.

[2] Ferreira, V., Franco-Luesma, E., Vela, E., López, R., Hernández-Orte, P. (2018). J. Agric. Food Chem., 66, 2237-2246.

[3] Ferreira, V., Sánchez-Gimeno, D., Ontañón, I. (2023). Food Chemistry, 421.

[4] Ainsa-Zazurca, S.; Ontañón, I.; Ferreira, V.; In preparation.