Which specific parameter is most critical for controlling the formation of hydrogen sulfide (H2S) during wine fermentation?

The most critical parameter for controlling the formation of hydrogen sulfide (H2S) during wine fermentation is nitrogen availability, specifically the concentration of yeast assimilable nitrogen (YAN) in the must. Hydrogen sulfide (H2S) is a volatile sulfur compound that can produce undesirable aromas in wine, often described as rotten eggs or sewage. Yeast requires nitrogen to synthesize proteins and other essential cellular components. If the must is deficient in YAN, yeast may resort to alternative metabolic pathways to obtain sulfur, leading to the reduction of sulfates and sulfites into hydrogen sulfide. YAN consists primarily of free amino acids and ammonia. Winemakers typically measure YAN levels in the must and supplement with nitrogen sources, such as diammonium phosphate (DAP) or yeast nutrients containing amino acids, to ensure adequate nitrogen availability for the yeast. Maintaining sufficient YAN levels throughout fermentation prevents the yeast from becoming nitrogen-stressed and minimizes the risk of H2S production. While other factors like yeast strain, temperature, and sulfur dioxide (SO2) levels can influence H2S formation, YAN availability is the most crucial parameter to manage proactively. Monitoring YAN and making appropriate additions allows for a controlled and healthy fermentation, reducing the likelihood of off-aromas caused by H2S.