Abstract

A study assessed several approaches to managing reductive off-odours in wine, with an emphasis on long-term effectiveness. Modifications to copper fining, including fining agents and filtration, were ineffective and sometimes counterproductive. Nitrogen sparging provided only short-term improvement, as it did not reduce the underlying precursor pool. Among the strategies tested, strong aeration was the only method shown to maintain long-term stability against the recurrence of H2S and MeSH.

Introduction

Reductive aromas associated with hydrogen sulphide (H2S) and methanethiol (MeSH) remain a significant challenge in modern winemaking. While these compounds are natural by-products of yeast metabolism, their accumulation can lead to undesirable sensory characteristics, such as rotten-egg, burnt-match, or struck-flint off-odours. In many cases, these aromas do not appear immediately, but develop during storage under low-oxygen conditions, including tank maturation, bottle ageing, or the use of reductive closures.

Copper fining has long been the industry’s primary corrective measure. Although often effective in the short term, it is now well recognised that copper does not remove sulphur from the wine, but rather binds the sulphur compound, thereby temporarily suppressing its sensory expression. Wines treated with copper frequently redevelop reductive characters later due to the release of the odour-active sulphur compounds from copper complexes, raising concerns about long-term efficiency and stability of the copper treatment.

For this reason, simply binding sulphur compounds is insufficient to ensure long-term stability. To prevent the later reappearance of reductive aromas, the bound copper-sulphur complexes themselves need to be effectively removed from the wine, rather than merely formed. If these complexes remain in the wine, they can act as a reservoir of odour-active sulphur, releasing H2S and related compounds back into solution when conditions become sufficiently reductive during storage or bottle ageing. Long-term management of reductive faults therefore requires strategies that reduce or eliminate this bound sulphur pool, not just its immediate sensory impact.

Remediation strategies evaluated

Recent research conducted by a group from Spain investigated alternative or complementary remediation strategies to copper fining. These remediation strategies included:

Copper-based treatments

  • Standard copper fining (0.5 mg/L Cu).
  • Copper fining followed by protein fining (0.5 mg/L Cu + 120 mg/L potato protein).
  • Copper fining followed by bentonite fining (0.5 mg/L Cu + 1 g/L bentonite).
  • Copper fining followed by filtration (0.5 mg/L Cu + 0.45 μm / 0.20 μm filtration).

Non-copper-based treatments

  • Physical removal of volatiles via inert gas (nitrogen sparging).
  • Chemical transformation of volatiles and complexes through oxidation (strong aeration).

The primary objective was to assess each strategy’s ability to 1) reduce the immediate sensory effects of the reductive compounds, and 2) to remove the copper-sulphur complexes and thereby prevent the recurrence of reductive off-odours at a later stage.

Results

Copper-based treatments

All copper-based treatments were immediately effective in reducing free H2S to undetectable or near-undetectable levels. However, this apparent success was accompanied by a significant and consistent increase in the bound H2S fraction (copper-sulphur complexes) across all treated samples.

Long-term stability tests showed that the addition of potato protein or bentonite after copper fining (in an attempt to precipitate the copper-sulphur complexes) fails to improve the long-term efficacy of copper fining. The fining agents successfully reduced residual copper levels in the wines; however, this did not improve the stability of the wine over time. In several cases, wines fined with fining agents showed a greater tendency to re-accumulate H2S during ageing. This decreased long-term effectiveness is likely attributable to the fact that both fining agents removed a significant amount of the added copper, which would otherwise have bonded with the volatiles released from the bound form over time.

It can be concluded that the addition of fining agents in an attempt to precipitate the copper-sulphur complexes fails to improve the long-term efficacy of copper fining. By removing active copper from the wine, these fining agents reduce the treatment’s effectiveness in controlling the long-term recurrence of sulphur compounds.

Copper fining combined with filtration

Copper fining, followed by filtration, was investigated as a potential means of removing bound copper-sulphur complexes. Results indicate that the filtration process was entirely unsuccessful in its primary goal, as it failed to remove any copper from the red wine used in the study. In some cases, filtration appeared to introduce mild oxidative effects, which were followed by increased sulphur release during subsequent reductive ageing. This suggests that filtration may inadvertently shift sulphur into the bound complexed forms that are later released under oxygen-deprived conditions.

Nitrogen sparging

This strategy aimed to physically strip volatile sulphur compounds from wine by sparging it with nitrogen gas. The nitrogen sparging of wines proved highly effective at immediately removing the free, volatile forms of H2S and MeSH. In some cases, sparging with nitrogen reduced the H2S content to near-detection levels.

When these wines underwent further reductive ageing, many re-accumulated sulphur compounds. Therefore, although the free forms were effectively removed, the bound forms were not affected by the sparging actions, leading to the latent release of volatile sulphur compounds over time.

While nitrogen sparging provides an immediate, temporary reduction in reductive off-odours, its long-term efficacy is limited. The process fails to solve the underlying problem of precursor compounds and requires multiple, time-consuming cycles for only marginal gains.

Chemical transformation through oxidation (strong aeration)

Of all the remediation approaches tested, controlled but strong aeration was the most effective, substantially decreasing the free, volatile sulphur compounds and, crucially, neutralising a large portion of the precursor pool responsible for subsequent sulphur release. After subsequent reductive ageing, aerated wines accumulated much less sulphur compounds when compared to the non-aerated control.

This recommendation, however, must be treated with caution. While oxidation can resolve reductive aromas, uncontrolled aeration increases the risk of browning, aroma loss and other oxidative faults. Further work is therefore needed to define safe, controlled protocols suitable for routine cellar use.

Practical implications for the cellar

This research provides several important insights for winemakers managing reductive sulphur faults:

  • Copper fining remains a short-term corrective tool, but it does not eliminate the bound and complexed forms, and therefore, the treatment cannot guarantee long-term stability.
  • Copper fining followed by fining the wine with either potato protein or bentonite does not improve outcomes and may even increase the risk of sulphur reappearance at a later stage.
  • Nitrogen sparging is effective for immediate aroma correction, particularly before bottling, but offers limited long-term protection.
  • Strong aeration emerges as the most promising and effective strategy for the long-term remediation of reductive off-odours. It was the only treatment capable of addressing the root cause of recurrence by chemically transforming the sulphur compounds and their precursors, thereby ensuring the wine’s stability over time, provided it is carefully managed to avoid oxidative damage.

Ultimately, the findings reinforce a shift in thinking: durable control of reductive faults depends less on suppressing the immediate volatile expression and more on reducing the underlying pool of non-volatile sulphur complexes, including copper-bound forms that could potentially release the off-odours at a later stage. For winemakers, this means carefully weighing the timing, intensity, and cumulative effects of interventions – particularly copper use and oxygen exposure – to achieve wines that remain stable beyond the cellar door.

Reference

Sánchez-Gimeno, D., Vela, E., Ferreira, V. & Ontañón, I., 2024. Alternative strategies for eliminating hydrogen sulphide and methanethiol from wine: results and learnings, OENO One 58(4), 1 - 13. https://doi.org/10.20870/oeno-one.2024.58.4.7655.

For more information, contact Carien Coetzee at [email protected].

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