Hydrogen sulfide is the rotten egg character that develops in some wines. Sulfides are a common byproduct of the fermentation process, but can become problematic if produced in excess quantities by the yeast. This character tends to develop in red wines that are not given adequate aeration of their cap during fermentation. The formation of H2S is a reductive process so the air exposure helps prevent its formation. The compound is also very volatile so aeration allows H2S to be blown-off to some extent. Excessive H2S formation can develop into other unpleasant smelling mercaptan compounds that are much more difficult to remove from wine. Splashing aeration is recommended as the first procedure to treat this character in fermenting wine. Regular racking of wines post-fermentation is done to prevent its formation during ageing. If H2S persists or develops during ageing post-fermentation, treatment with copper sulfate is required.
