hydrogen sulfide or H2S is the foul-smelling gas, reminiscent of rotten eggs, which even at very low concentrations (parts per billion) is easily recognized in wine because it is highly volatile and has a very low sensory detection threshold. Although it can form at any stage of wine production, hydrogen sulfide is produced most commonly during alcoholic fermentation, either in the early to middle, vigorous, phase or towards the end. Hydrogen sulfide production during the vigorous phase, which coincides with rapid yeast growth, is associated with a deficiency in the amount of nitrogen (often referred to as yeast-assimilable/available nitrogen or YAN) in the grape must or juice. The nitrogen content of grapes is highly variable and depends on many factors, including grape variety, soil type and nutrient status, climate, and vineyard management practices, such as type and amount of nutrient application, irrigation, and use of cover crops. Nitrogen-deficient grape musts are often supplemented with nitrogen, typically in the form of diammonium phosphate (DAP), which usually suppresses the appearance of hydrogen sulfide. Addition of sulfur dioxide, often made to the must shortly before inoculating with yeast, as an antioxidant and to inhibit ambient yeast and bacteria, can also increase the amount of hydrogen sulfide produced by some yeast strains.
