Dr. Carien Coetzee
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”:
- no additions
- 1 mg/L Cu2+ added
- 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.
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-bottling2–4. 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 previously5,6 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.
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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