Control of Hydrogen Sulfide Formation during Fermentation

Hydrogen sulfide and related sulfur compounds produced internally by yeast during fermentation are considered highly undesirable if they are excreted and exceed threshold concentrations in the finished wine, as they cause an unpleasant "reduced character". The threshold for hydrogen sulfide is on the order of parts per billion and that for some of the other reduced sulfur compounds derived from hydrogen sulfide ranges from parts per million to parts per billion. Factors which affect production of these compounds include, but are not limited to, the nutritional status of the grapes, yeast strain, and fermentation conditions (Rauhut, 1993). Formation of excessive reduced character in table wines produced from sound grapes using good winemaking practices is a perplexing problem because it can occur unpredictably, and spoil affected wines. Yeast produces this class of compounds internally as a byproduct of the normal and necessary synthesis of S-adenosyl-methionine and the sulfur-containing amino acids, methionine and cysteine (Thomas and Surdin-Kerjan, 1997). Excessive synthesis leads to excretion of hydrogen sulfide into the wine. Although excess hydrogen sulfide formation has generally been correlated with nitrogen deficiency (Jiranek et al., 1995a, 1995b), deficiencies in vitamin B₆ and pantothenic acid, and residual sulfur on grapes from the application of fungicides have also been implicated. In one study, formation of hydrogen sulfide was found not to correlate with variation in activity of the formative enzyme, sulfite reductase, which reduces sulfite to hydrogen sulfide (Jiranek et al., 1996). This suggests that the amount of extracellular hydrogen sulfide formed is controlled by other rate-limiting steps: its utilization in sulfur amino acid biosynthesis, its active or passive transport from the cell, or consumption through other biochemical pathways. Formation still occurs unexpectedly, and often enough to be considered a persistent problem (Boulton et al. 1996; Park et al. 2000)