Copper and SO₂ used together – effect on H₂S

Dr. Carien Coetzee
Basic Wine
20 May 2020

FOCUS ON H₂S: PART 4
COPPER AND SO₂ USED TOGETHER – EFFECT ON H₂S

The mechanisms involved in the formation of hydrogen sulfide (H₂S) post-bottling are not yet as well defined as those involved during fermentation. However, recently researchers have been investigating key factors affecting H₂S concentration after fermentation and many of the findings are summarised and presented in this blog series (see Part 1, Part 2 and Part 3) covering post-bottling H₂S formation.

The use of copper (Cu²⁺) and sulphur dioxide (SO₂) as wine additives are not uncommon in the wine industry and can often be found to be present in the wine at the same time. The interactive effects between these two compounds are not something that is often considered and it is of critical importance that the winemaker understands the potential outcomes when the additives are used together.

In the current blog post, the interactive effects of Cu²⁺ and SO₂ and the effect on H₂S concentration will be addressed. The original research paper titled “Formation of Hydrogen Sulfide in Wine: Interactions between Copper and Sulfur Dioxide”, is published in an open-access journal and can be viewed here.

MATERIALS AND METHODS

Two wines, one white and one red were obtained from an Australian winery. Neither of these wines was treated with SO₂ or Cu²⁺ during the winemaking process.

The wines were placed in 20 mL crimp top vials and the following additions were made “at bottling”:

1. Control

• no additions

2. Cu²⁺

• 1 mg/L Cu²⁺ added

3. SO₂

• 100 mg/L SO₂ added

4. Cu²⁺ and SO₂

• 1 mg/L Cu²⁺ and 100 mg/L SO₂ added

The samples were stored at 20°C under nitrogen to prevent any oxygen exposure.

RESULTS

H₂S formation in the Control samples (blue line)

• For both the white and red wine control samples, there was no significant increase in H₂S concentration (Figure 1). The largest increase observed was for the white wine where an increase of 0.9 μg/L (which is quite low) was observed after six months of storage.

Figure 1. The effects of the treatments on the formation of H₂S over 12 months in (a) white wine and (b) red wine. Adjusted from Bekker et al., 2016¹. View permissions.

The effect of Cu²⁺ addition on H₂S formation (pink line)

• For the white wine sample, an increase in H₂S concentration was observed after six months of storage (from 1 μg/L to 11 μg/L) (Figure 1(a)). However, the concentration decreased again after 12 months of storage ending in a concentration similar to the starting concentration of 1 μg/L. For the most part, the presence of Cu²⁺ alone in the white wine sample did not result in the accumulation of H₂S.
• In the red wine sample, a significant constant increase in H₂S formation was observed from zero to six months (from 2 μg/L to 29 μg/L) followed by a decrease in H₂S concentration from six to 12 months ending at 12 μg/L (Figure 1(b)). In this case, the addition of Cu²⁺ alone had a significant effect on the formation of H₂S.

The effect of SO₂ addition on H₂S formation (green line)

• For both the white and red wine samples, the addition of SO₂ alone had no significant effect on the formation of H₂S after 12 months of storage (Figure 1).

The effect of simultaneous addition of Cu²⁺ and SO₂ on H₂S formation (purple line)

• In the white wine sample, the combination of Cu²⁺ and SO₂ resulted in a significant and large increase in H₂S content. After one month’s storage, the concentration already increased from 1 μg/L to 17 μg/L (Figure 1 (a)). The concentration then increased further to 27 μg/L and 51 μg/L after three and six months, respectively. After 12 months, the concentration decreased to 36 μg/L which is still a significant concentration likely to alter the aroma perception of the wine.
• In the red wine sample, an increase in H₂S was also observed. The concentration increased relatively quickly compared to when Cu²⁺ was added alone, already reaching 18 μg/L after one month’s storage (Figure 1 (b)). After six and 12 months, the concentration increased further to 26 μg/L and 27 μg/L, respectively.

It is clear that the addition of a combination of Cu²⁺ and SO₂ “at bottling” resulted in significant H₂S increases for both the white and red wines tested. The increase observed in these samples was also much faster with a large increase of H₂S already after one month of storage.

INTERACTIVE EFFECTS EXPLAINED

The question is: why is H₂S formed post-bottling, and how does Cu²⁺ and SO₂ influence this formation? Unfortunately, the answer is not simple and the exact mechanisms involved in the formation of latent H₂S formation is still unclear, however, some possible mechanisms are explained in Part 2 of this series.

However, what we do know is that oxygen exposure limits the formation of H₂S post-bottling^2–4. The basic reaction involves the oxidation of certain wine compounds, such as phenolics, leading to the formation of reactive compounds such as o-quinones. These o-quinones react readily to various wine constituents, but especially to sulphur containing compounds such as SO₂, H₂S and even the fruity volatile thiols.

Through reacting with the o-quinone, the inherent properties of the compounds are changed often accompanied by the removal of the aroma. For instance, when an o-quinone reacts with SO₂, it will result in the formation of a hydroxysulphonate and form part of the total SO₂ content. The o-quinone will also react readily to H₂S, binding the stinky compound and therefore effectively removing it (and the accompanying aroma) from the wine.

The researchers delved deeper into the study by investigating the reaction speed of SO₂ with o-quinones and comparing that to the reaction speed of H₂S with o-quinones. They found that both compounds technically react at the same speed. However, in a wine medium, the concentration of the SO₂ present is much higher compared to H₂S, therefore, there are far more SO₂ molecules present which will quickly react with the o-quinones, leaving the H₂S unreacted and the aroma unchanged.

Now let’s consider the results found in the current study.

In the samples (both white and red wines) to which Cu²⁺ and SO₂ were added “at bottling”, latent H₂S formation took place leading to an increase in the concentration. The fact that SO₂ was present, means that any o-quinones formed at this stage immediately reacted with the abundance of available SO₂. Therefore, none of the o-quinones molecules was available for the reaction with H₂S resulting in the accumulation thereof. The presence of SO₂ thus indirectly caused an increase of H₂S.

In the absence of SO₂, the o-quinones formed, reacted with the H₂S in the wine medium (perhaps even as H₂S is formed), therefore reducing the concentration and preventing the accumulation of post-bottling H₂S. There might have been more o-quinones forming in the white wine which could explain the difference observed between the white and red wines.

In principle, the H₂S is preserved and protected by SO₂ (in the same way that it protects other positive aroma compounds). The presence of SO₂ thus allows the accumulation of H₂S and is thus not directly involved in the formation. It seems that Cu²⁺ and SO₂ operate independently on separate pathways, but the effects of the additives are cumulative, leading to increased H₂S concentration.  It is important to note that in the samples to which only SO₂ were added, there was no H₂S accumulation. Therefore, as mentioned in the previous blogs in this series, the presence of Cu²⁺ is an integral part of the formation of post-bottling H₂S.

CAN SO₂ AND Cu²⁺ FORM H₂S DIRECTLY?

Other than the preservation of H₂S by SO₂, another mechanism has been proposed previously^5,6 based on the direct formation of H₂S. This pathway involves the formation of H₂S through the reduction of SO₂ by Cu²⁺, however, the existence of this pathway in a real wine needs to be confirmed.

A simple test was done to establish if this direct reaction is a realistic possibility¹. The researchers of the study added both Cu²⁺ and SO₂ to a model wine medium and monitored the formation of H₂S. Results showed that no H₂S was formed. This could either mean that the reaction does not take place in wine, or it could indicate the necessity of other wine constituents (absent in the model wine) needed for this reaction to occur.

CONCLUSION

The results reported in the study suggests that when wines are produced without early Cu²⁺ and SO₂ additions but treated with these additives before bottling, an increase in H₂S may be observed post-bottling.

The antioxidant and antimicrobial benefits of SO₂ offsets the risk of developing reduced aromas. And this study showed that even late SO₂ additions close to bottling did not pose a risk of increased H₂S unless there was a high concentration of residual Cu²⁺ present in the wines.

It is therefore critical that the winemaker limits the residual Cu²⁺ to a minimum in the finished wine.

REFERENCES AND PERMISSIONS 

(1)           Bekker, M. Z.; Smith, M. E.; Smith, P. A.; Wilkes, E. N. Formation of Hydrogen Sulfide in Wine: Interactions between Copper and Sulfur Dioxide. Molecules 2016, 21 (9). https://doi.org/10.3390/molecules21091214.

(2)           Ugliano, M.; Kwiatkowski, M.; Vidal, S.; Capone, D.; Siebert, T.; Dieval, J.-B.; Aagaard, O.; Waters, E. J. Evolution of 3-Mercaptohexanol, Hydrogen Sulfide, and Methyl Mercaptan during Bottle Storage of Sauvignon Blanc Wines. Effect of Glutathione, Copper, Oxygen Exposure, and Closure-Derived Oxygen. J. Agric. Food Chem. 2011, 59 (6), 2564–2572. https://doi.org/10.1021/jf1043585.

(3)           Bekker, M. Z.; Day, M. P.; Holt, H.; Wilkes, E.; Smith, P. A. Effect of Oxygen Exposure during Fermentation on Volatile Sulfur Compounds in Shiraz Wine and a Comparison of Strategies for Remediation of Reductive Character. Aust. J. Grape Wine Res. 2016, 22 (1), 24–35. https://doi.org/10.1111/ajgw.12172.

(4)           Day, M. P.; Schmidt, S. a; Smith, P. a; Wilkes, E. N. Use and Impact of Oxygen during Winemaking. Aust. J. Grape Wine Res. 2015, 21 (S1), 693–704. https://doi.org/10.1111/ajgw.12199.

(5)           Ribéreau-Gayon, P.; Dubourdieu, D.; Donèche, B.; Lonvaud, A. Handbook of Enology. The Chemistry of Wine Stabilization and Treatments., 2nd ed.; John Wiley & Sons Ltd: Chichester, 2006; Vol. 2. https://doi.org/10.1002/0470010398.

(6)           Lopes, P.; Silva, M. a.; Pons, A.; Tominaga, T.; Lavigne, V.; Saucier, C.; Darriet, P.; Teissedre, P.-L.; Dubourdieu, D. Impact of Oxygen Dissolved at Bottling and Transmitted through Closures on the Composition and Sensory Properties of a Sauvignon Blanc Wine during Bottle Storage. J. Agric. Food Chem. 2009, 57 (21), 10261–10270. https://doi.org/10.1021/jf9023257.

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