Sulfur dioxide is a preservative that is added to nearly all wines, both at the crusher and at the end of primary fermentation, or after malolactic if desired. SO2 (which creates in wine a family of free and bound sulfites) is an intentionally added preservative, whereas hydrogen sulfide is a stinky aroma defect that is not chemically related and is never added to wine. Excessive sulfur dioxide has a sharp odor similar to a freshly struck match, and is not to be confused with hydrogen sulfide, which has a rotten egg odor. It is unprofessional to speak of adding or smelling “sulfur,” as this is
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