Dr. Carien Coetzee
23 April 2020
FOCUS ON H2S: PART 2
In Part 1 of this Focus on H2S series, the role of copper for the remediation of hydrogen sulphide (H2S) and the difficulties in removing the resulting copper-H2S complex, were studied1. It is recommended to first read through Part 1 before continuing with the current blog post as it will provide important background information.
Under certain conditions, considerable amounts of H2S can be generated by the yeast during fermentation. To date, yeast mechanisms of H2S liberation during wine fermentation are well understood, however, the formation of H2S and other reductive sulphur compounds after fermentation and during the ageing period is still unclear.
This time of year, Sauvignon Blanc wines are often treated with copper(II) sulphate to remove or prevent the formation of reductive aromas, especially considering the reductive winemaking techniques employed that could promote latent H2S formation.
The question is thus,
Where does post-fermentation H2S come from?
Mechanisms for the latent formation of H2S
Several hypotheses for the latent formation of H2S have been formulated2. Many of these hypotheses are based on the de novo formation of H2S. (The term de novo is used to refer to the formation of a molecule from simpler molecules or precursors via a certain biochemical pathway. Therefore, it is produced anew and was not present before.)
Other research results suggest that H2S are released from metal complexes such as the copper-H2S complex already present in the wine as a result of the reaction between Cu2+ and H2S. The effectivity of the removal of the copper-H2S complex from the wine (by racking and/or filtration) is under question. In Part 1 of this series, the presence of this copper-H2S complex after racking and/or filtration is shown1. The implication is that H2S remains in the wine, albeit in an odourless, metal complexed form, which would not have been a problem was it not for the fact that the reaction of complex formation is reversable3. Certain conditions could thus result in the release of H2S from these metal complexes.
A group of researchers studied the possible post-fermentation H2S accumulation pathways to determine the main mechanism involved. The study focussed on the two likely mechanisms 1) the contribution of de novo formation of H2S compared to 2) the release of H2S from complexes2. The study is titled, “Reductive off-odors in wines: Formation and release of H2S and methanethiol during accelerated anoxic storage of wines”.
H2S is present in Free and BOUND forms
Just like sulphur dioxide, H2S is present in the wine in different forms. The free H2S is the unpleasant-smelling compound and is predominantly responsible for reductive aromas in wine. The complexed H2S is H2S that is in a bound form with other compounds, often metals such as copper. This complexed/bound form is odourless. The sum of the free and bound H2S equals the total H2S in the wine.
Testing the possible mechanisms
Twenty-four wines were analysed for their levels of free H2S and total H2S. Results showed that most of the wines contained only small amounts of free H2S (6-8% of the total), while the remaining H2S was present in the complexed/bound form. The concentrations of these bound forms in the wines were enough to potentially cause an aromatic problem if the H2S was to be released.
The wines were stored under strictly anaerobic conditions at 50°C for three weeks. Samples were analysed for free and total H2S at regular intervals.
For the white and rosé wines
- The levels of free H2S increased relatively rapidly (within one week). This increase coincided with a rapid decrease in bound H2S. Results showed that about 58% of the accumulated H2S can be attributed by the release from the complexed/bound forms.
- The amount of total H2S increased over time and was on average 9.4 μg/L higher than the initial concentration. The result suggests that there was de novo formation of H2S, leading to an increase in free H2S (and therefore total H2S).
During the storage of the white and rosé wines, both mechanisms, 1) de novo formation and 2) the release of H2S from complexes, was significant.
For the red wines,
- The average levels of free H2S increased continuously during the storage period. The average total increase in free H2S was above 16 μg/L (a significant amount). This increase coincided with a decrease in complexed/bound H2S. Initially, the percentage of bound H2S was 94% of the total H2S. By the end of the storage period, the percentage of bound H2S amounted to 23% of the total H2S.
- The total H2S content remained constant during the ageing period. This would indicate that there was no de novo synthesis of H2S in the red wines during the period tested.
The results would suggest that there was a significant transformation of complexed H2S into free odorous H2S in the red wines. Calculations estimate that of this 16.2 μg/L free H2S accumulated, about 14.7 μg/L (90%) can be attributed to the release of H2S from the complexed H2S.
The complexation of free H2S into a non-volatile metal complex (by adding copper(II)sulphate), leads the producer to believe that the problem is taken care of. However, the results reported in the study2 clearly indicates that the release of free H2S from complexes (copper as well as other metals) is the dominant source of latent H2S accumulation in white and red wines. This brings us back to Part 1 of the blog series: the importance of removing these copper complexes after treating the wine with copper(II) sulphate. Some wines are also naturally high in copper or could have elevated copper concentrations due to vineyard treatments. For these wines, bottling under a hermetic closure could be detrimental due to the release of H2S over time.
De novo synthesis also contributed to the accumulation of H2S in the white and rosé wine samples but played a minor role in red wines under the conditions of this study. The exact mechanisms involved in the de novo synthesis of H2S during the post-fermentation period as well as factors affecting the formation still needs further investigation. The study also looked at the formation of MeSH (another prominent reductive compound) and found that de novo formation is the dominant source of MeSH, particularly in white and rosé wines. More information on the accumulation of MeSH is available in the full text.
Even though the conditions off the study was quite extreme, the same pattern could be expected in a more realistic environment. In the current study, the accumulated H2S concentration was, for the majority of the wines, significant and would have a profound effect on the wine sensory profile.
(1) Clark, A. C.; Grant-Preece, P.; Cleghorn, N.; Scollary, G. R. Copper(II) Addition to White Wines Containing Hydrogen Sulfide: Residual Copper Concentration and Activity. Aust. J. Grape Wine Res. 2015, 21 (1), 30–39. https://doi.org/10.1111/ajgw.12114.
(2) Franco-Luesma, E.; Ferreira, V. Reductive Off-Odors in Wines: Formation and Release of H2S and Methanethiol during the Accelerated Anoxic Storage of Wines. Food Chem. 2016, 199, 42–50. https://doi.org/10.1016/j.foodchem.2015.11.111.
(3) López, R.; Lapeña, A. C.; Cacho, J.; Ferreira, V. Quantitative Determination of Wine Highly Volatile Sulfur Compounds by Using Automated Headspace Solid-Phase Microextraction and Gas Chromatography-Pulsed Flame Photometric Detection. Critical Study and Optimization of a New Procedure. J. Chromatogr. A 2007, 1143 (1–2), 8–15. https://doi.org/10.1016/j.chroma.2006.12.053.