Hydrogen Sulfide

H2S forms when yeast is stressed by nutrient deficiency (low nitrogen) or during reductive winemaking (no oxygen). At low levels, a struck-match note may be acceptable. At higher levels, the rotten egg smell is unmistakable. If addressed early (copper fining, aeration), H2S can be eliminated. Untreated, it converts to mercaptans — far worse and irreversible.