In an understandable effort to avoid oxidation and the development of volatile acidity, many winemakers limit the exposure of wine to oxygen. If their efforts are too successful, however, it can lead to the development of reductive odors (such as hydrogen sulfide and mercaptans). Reduction is the opposite of oxidation. Some oxygen is needed in during wine production, and if it is not available, then hydrogen molecules can combine with the sulfur in the wine to produce hydrogen sulfide.
This is the compound that can give a wine a “rotten egg” smell. This aroma can be dispelled by aeration, but if it is not eliminated, it can combine with carbon molecules in the wine, leading to the development of mercaptans. These skunk-like aromas are nearly impossible to remove. Some tasters do not see low-level presence of mercaptans as a fault, however, and believe that they add to the complexity of the wine.
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