This treatment used in organic farming adds a rotten egg taste to wine

To protect their vineyards from fungi, winegrowers can use copper-based treatments such as Bordeaux mixture, which is also permitted in organic farming.

A research team from INRAE and the Institute Agro shows that the acquisition of copper resistance by yeast used in winemaking has resulted in the excessive production of hydrogen sulfide (H2S), a compound with a rotten egg odor that alters the sensory quality of wine. This is due to the multiplication of a gene involved in copper resistance, which leads to excess H2S production.


In-depth analysis of 51 yeast strains shows that for those with more than 10 copies of this gene, H2S production decreases or even disappears. These results, published in Scientific Reports, open new research avenues on yeast strains to avoid H2S production during winemaking.


The aromatic profile of wines is the primary quality sought by consumers. One of the key steps in winemaking is the fermentation of grape must during which the yeast Saccharomyces cerevisiae, used since antiquity to produce wine, as well as beer or bread, transforms the sugar contained in the grape must into alcohol.

During this stage, the yeasts also synthesize hydrogen sulfide (H₂S) to produce the sulfur amino acids essential for their development. However, this compound, with a very unpleasant rotten egg odor, can be produced in excess by S. cerevisiae and thus alter the taste of wine. The production of H₂S is very energy-intensive for the yeast, raising the question of why certain S. cerevisiae strains produce it in excess.

Scientists sought an explanation for this phenomenon by exploring the link between H₂S synthesis in yeast and two traditional practices of vine and wine professionals: treating the vineyard with Bordeaux mixture and sulfiting the grape must.

Bordeaux mixture is a copper-based antifungal treatment used to protect vines from fungi, including in organic farming. Sulfiting the grape must involves adding sulfite during winemaking, an antiseptic and preservative compound that helps preserve wine quality.

Discovery of the crucial role of a yeast gene involved in copper fixation

First, the scientists showed that wine-derived Saccharomyces cerevisiae yeasts had increased H₂S production compared to strains from oak bark or isolated from wine film. This production was amplified by the addition of sulfite, which serves as a precursor for H₂S synthesis.

Wine-derived Saccharomyces cerevisiae yeasts have adapted to the presence of copper due to the common use of Bordeaux mixture in vineyard treatment for over a century. This adaptation is characterized by the multiplication of the gene CUP1, which allows the production of the Cup1 protein capable of fixing copper, giving the yeast the ability to survive in a copper-rich environment. This protein is rich in sulfur amino acids, which require H₂S for their production.


The adaptation of wine yeast Saccharomyces cerevisiae to copper due to antimycotic treatments with Bordeaux mixture has led to overproduction of H₂S during alcoholic fermentation. This excessive production, exacerbated by the addition of sulfite commonly used in oenology, significantly alters wine quality.

51 yeast strains analyzed

To confirm these results, scientists showed that exposing yeasts to copper triggers higher H₂S production. An in-depth analysis of 51 yeast strains reveals a complex link between the number of copies of the CUP1 gene in their genome and H₂S production.

For strains containing approximately 2 to 10 copies of CUP1, the most frequent case in wine yeasts, the research team observed an increase in H₂S production. In contrast, for strains with a higher number of copies, the H₂S concentration decreases, even disappearing for strains with more than 20 copies of the gene.

This phenomenon is explained by an excessive demand for sulfur amino acids required for the synthesis of Cup1 proteins, thereby limiting the presence of H₂S in the wine used by S. cerevisiae to produce the amino acids.

The acquisition of copper resistance in wine S. cerevisiae yeasts has resulted in the overproduction of H₂S. This excessive production, amplified by the addition of sulfite commonly used in winemaking, can significantly alter wine quality. In response to these sensory defects, racking^[1] practices in the winemaking process have likely been intensified to limit the negative impact of H₂S on the aromatic profile of wines.

The results of this study also open up a new research avenue into yeast strains highly resistant to copper (having a large number of copies of the CUP1 gene) to limit H₂S presence in wine. In the context of changing practices and new health or climate constraints, these results show that understanding yeast resistance and adaptation mechanisms is crucial for optimizing wine quality.

Note:
[1] During racking, the winemaker decants the wine in progress to eliminate unwanted deposits. This also helps aerate the wine and limit excess H₂S.

Reference:
De Guidi I. et al. (2024). Copper-based grape pest management has impacted wine aroma.
Scientific reports 14, 10124, DOI: https://doi.org/10.1038/s41598-024-60335-9