What are the chemical precursors to hydrogen sulfide (H2S) formation in wine and how can its production be prevented?

Hydrogen sulfide (H2S) is a volatile sulfur compound that can cause unpleasant aromas in wine, often described as rotten eggs, garlic, or rubber. Several chemical precursors contribute to H2S formation during fermentation. One primary source is the reduction of elemental sulfur. Elemental sulfur is sometimes used in vineyards to control powdery mildew. If sulfur residues remain on the grapes, yeast can reduce it to H2S during fermentation. Another major precursor is the degradation of sulfur-containing amino acids, such as cysteine and methionine, by yeast. Under anaerobic conditions, yeast can cleave these amino acids, releasing H2S as a byproduct. Sulfate (SO42-) can also be reduced to H2S by yeast, especially under conditions of nitrogen deficiency. Yeast requires nitrogen to synthesize proteins. When nitrogen is limited, yeast can use sulfate as an alternative sulfur source, leading to H2S production. Sulfites (SO32-), added as a preservative, can be reduced to H2S by certain yeast strains, particularly under reductive conditions. To prevent H2S formation, several strategies can be employed. Avoid using excessive elemental sulfur sprays in the vineyard, particularly close to harvest. If sulfur is used, ensure thorough washing of the grapes before fermentation. Ensure adequate yeast assimilable nitrogen (YAN) levels in the must. YAN provides yeast with the nitrogen they need for protein synthesis, reducing the likelihood of sulfate reduction and H2S formation. Supplementing with diammonium phosphate (DAP) or other nitrogen sources can address nitrogen deficiencies. Maintain healthy yeast populations during fermentation. Stressed or sluggish fermentations are more likely to produce H2S. Proper aeration or oxygenation during fermentation can help prevent reductive conditions and H2S formation. Avoid extended lees contact after fermentation, as this can provide a source of sulfur-containing compounds that can be reduced to H2S. If H2S is present, copper sulfate (CuSO4) can be used to remove it. Copper reacts with H2S to form insoluble copper sulfide, which precipitates out of the wine.