Introduction

Many white wines, especially those prized for their bright, tropical aromas, can sometimes develop unwanted smells that people describe as “rotten egg”, “cabbage”, or “cooked vegetables”. These faults are commonly linked to a group of highly reactive, very smelly molecules known as volatile sulphur compounds (VSCs). Two of the main culprits are hydrogen sulphide (H2S) and methanethiol (MeSH). They often form during fermentation as byproducts of yeast metabolism, and can become more prominent under winemaking and storage conditions that restrict oxygen exposure.

Why do such conditions matter? Modern white winemaking has long embraced “reductive” practices by limiting oxygen exposure during processing, storage and bottling to protect the delicate, fresh aromas that define varieties like Sauvignon blanc and Chenin blanc. Key players in those fruity profiles are varietal thiols, particularly 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA). Even at tiny concentrations (ng/L) they contribute to appealing notes such as passionfruit, grapefruit and guava. Unfortunately, the same low-oxygen strategies that help preserve these thiols can unintentionally set the stage for “reductive” off-odours to emerge, either in the bottle or during storage.

Wine professionals often respond to “closed” or “reductive” aromas by swirling the glass after it has been poured. The effect of swirling is simple: it increases the wine’s surface area in contact with air, encouraging aroma release and some oxygen pickup. In tasting rooms and restaurants, the advice to swirl is almost instinctive.

At the same time, sensory scientists know that VSCs can mask or suppress fruity aromas, which means even a small reduction in H2S could let the wine’s “tropical” character shine through. Yet the literature still has important gaps: for example, how do moderate vs. higher free H2S concentrations affect the perceived fruity profile when varietal thiols are present?

To answer these questions, this study looked at both chemistry and sensory perception in model wine with different varietal thiol and H2S levels. Using a model wine, we were able to manipulate the varietal thiol and free H2S at low and high levels and assessed sensory changes after different glass swirling regimes.

Materials and methods

The study was built around pairing varietal thiols (3SH and 3SHA) at two levels with free H2S at two levels, then examining how glass handling (swirling and covering/uncovering) influenced both chemical composition and sensory perception. We tested these conditions in a model wine (to tightly control the matrix, (12% v/v ethanol, pH 3.5 and 5 g/L tartaric acid). Varietal thiols were added at low thiol (T): 3SH at 600 ng/L and 3SHA at 42 ng/L, as well as high thiol levels (hT): 3SH at 1 500 ng/L and 3SHA at 125 ng/L. Hydrogen sulphide was added at low H2S (H2S): 4 μg/L and high H2S levels (hH2S): 8 μg/L. These varietal thiol ranges reflect concentrations reported for South African Sauvignon blanc and Chenin blanc wines, helping ensure the work is relevant to wines as they are produced and enjoyed. Combining each thiol level with each H2S level yielded four core treatments, and each was subjected to different glass-handling approaches prior to sensory assessment.

Swirling and serving protocols for sensory testing

Tastings were conducted in a sensory facility using ISO-standard wine glasses with 25 mL of wine served at 20 ± 2°C. To replicate realistic service, samples were covered with Petri dish lids, and the time between pouring and panel evaluation was tightly controlled. Three handling styles were compared:

1. No swirl (glass covered)

This treatment will be indicated as “T” or “T + H2S” (and the analogous combinations for hT/hH2S). The procedure entailed pouring the model wine and covering for 15 minutes before smelling.

2. Swirl then cover (S)

This treatment will be indicated as “T + H2S + S”. The procedure entailed pouring the model wine and swirling for 30 seconds in an uncovered glass (about 1.5 - 2 circular motions per second), then covering for 15 minutes before smelling.

3. Swirl then leave open (S + O)

This treatment will be indicated as “T + H2S + S + O”. The procedure entailed pouring the model wine and swirling for 30 seconds in an uncovered glass, then leaving it uncovered for 15 minutes, and covering briefly just before sensory evaluation.

The 15-minute period allowed headspace and liquid to reach equilibrium for highly volatile compounds like H2S, which mirrors what often happens in real tasting scenarios where a poured sample sits for a short time before evaluation.

For strict oxygen management, the wine was stored in stainless steel kegs, transferred under inert gas (nitrogen) into brown glass bottles (flushed with argon), sealed with crown caps, and kept cold (4°C). Additions of 3SH/3SHA and H2S were performed in an anoxic glove box (<1% O2), and bottles were re-sealed immediately to prevent oxygen exposure.

Sensory panel and training

A trained panel of 10 assessors (all females; ages 30 - 63) performed the sensory work, which included rigorous training and assessing the intensity of the model wine treatments using descriptive analyses.

Results and discussion

General effects of varietal thiols and H2S on wine aroma

Using a model wine enabled us to assess different levels of varietal thiols and H2S in the same wine matrix in a very controlled manner. The varietal thiol concentrations (3SH and 3SHA) were chosen to reflect realistic levels found in South African Sauvignon blanc and Chenin blanc wines. The low set (T) and high set (hT) were designed to balance the odour-active values (potency relative to threshold) of 3SH and 3SHA, thereby encouraging a stable and representative “fruity” baseline against which the effects of H2S could be observed.

Panellists consistently associated the thiol-only wines (no added H2S) with fruity descriptors, including passionfruit, pineapple, peach and apple, confirming that the chosen thiol levels effectively produced the expected aromatic profile. Introducing H2S shifted the balance toward “reductive” notes such as boiled egg and general sulphur/reductive, showcasing the familiar masking effect that sulphur compounds can have on fruit expression.

Sensory outcomes depend on both thiol and H2S levels

The sensory results show that the degree to which swirling improves aroma depends on the balance between thiol concentration and H2S concentration:

1. Low thiols + Low H2S

When thiols were at the lower set (T), and H2S was low, the H2S increased boiled egg and reductive notes compared to the thiol-only control. A brief swirl began to restore “passionfruit” even before a strong decrease in reductive notes was evident. However, a more complete removal of the “boiled egg” and “reductive” descriptors usually require the swirl followed by leaving the glass open (S + O). Put simply, when thiols were modest, and H2S was present at lower levels, extra headspace time after swirling made a noticeable difference in the model wine.

2. Low thiols + High H2S

At higher H2S levels, the key fruity character of passionfruit was strongly suppressed. Interestingly, simply swirling the wine was enough to restore the passionfruit aroma to levels similar to the control wine, even before a clear reduction in sulphur off-odours. However, to significantly reduce the boiled egg and reductive aromas, the wine glass had to be left open after swirling.

3. High thiols + Low H2S

With higher thiols (hT) and low H2S, the fruit profile was more resilient. The presence of low H2S did not knock out “passionfruit”, and swirling (even when the glass was then covered) was often enough to nudge the wine back toward its fruity identity, decreasing the “boiled egg” note and reinforcing attributes like passionfruit and fresh green/grass. In other words, when the thiol intensity is strong, and H2S is low, the benefit of a quick swirl can be both perceptible and fast.

4. High thiols + High H2S

When both thiols and H2S were high, the masking from H2S became more pronounced. A simple swirl alone often did not reduce “boiled egg” or “reductive” notes enough to pull the wine perceptually out of the reductive zone. It took swirling and then leaving the glass open to bring those off-odours down toward the thiol-only control.

To summarise, these outcomes suggest that swirl + open is a robust strategy for wines with noticeable H2S-driven reductivity, especially when H2S levels are high. When H2S is lower, and thiols are higher, a swirl + cover may already be enough to substantially improve the sensory balance.

Practical implications for tasters

For tasters and sommeliers, the take-home message is straightforward:

If a wine smells reductive, swirl vigorously for ~30 seconds, then leave it uncovered for a short period (around 15 minutes is what was used here, but even a few minutes can help). This maximises volatilisation of H2S, particularly when its levels are moderate to high. When the fruity profile is strong (e.g., high thiols) and reductivity is only mild, a brief swirl may already be enough.

However, this work was conducted on a model wine. We wanted to assess if the same trends would be found in real wine, as well as try to elucidate the mechanism involved in the reduction in the reductive aromas when wine is exposed to glass swirling.

This will be discussed in a follow-up article.

Reference

Lesković, M., Bekker, M.Z., Shoba, S., Brand, J. & Du Toit, W., 2026. Effect of glass swirling on the removal of ‘reductive’ off-odours caused by H2S in wine. Oeno One (Vol. 60, no. 1). https://doi.org/10.20870/oeno-one.2026.60.1.9616

For more information, contact Wessel du Toit at [email protected].

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