A natural chemical compund that has been used as a wine preservative since antiquity. Sulfur in all its forms is harmless to people except for the tiny number of individuals who are severely allergic to it. The most common form of sulfur used in winemaking is sulfur dioxide (SO2), which is formed when elemental sulfur is burned in air. Added to wine (usually as a gas), sulfur dioxide prevents OXIDATION as well as bacterial spoilage, and it inhibits the growth of yeasts. As a result of this, sulfur dioxide can be used to stop FERMENTATION in order to produce a sweet wine, and it can be used to prevent MALOLACTIC FERMENTATION. A form of sulfur dioxide known as metabisulfite is often added to freshly picked grapes (and fruit juices in general) as a preservative. Sulfur dioxide’s disadvantage is that it has an unpleasant, burnt match odor, which can be smelled at low concentrations, although people vary widely in the thresholds at which they can detect it. The ability to detect sulfur dioxide also varies based on the type of wine, since in some wines the compound reacts to or combines with other compounds, rendering it more difficult to perceive. In any case, during the last few decades, winemakers the world over have sought to minimize the amount of sulfur dioxide they use in winemaking, mostly in response to health concerns voiced by wine drinkers. Nonetheless, it’s virtually impossible to produce a wine that is entirely sulfur-free because a small amount of sulfur dioxide is a by-product of the metabolic action of yeasts during fermentation (this is why bread, too, contains sulfur dioxide). As a result, United States law mandates that the term “contains sulfites” appear on all wine labels that contain more than 10 parts per million of sulphur dioxide (and most do), even when the wine has been produced without the addition of any sulfur dioxide at all. The word sulfites in the warning is a catch all term for sulfur in all its various forms, including sulfur dioxide, sulfurous acid, bisulfite ion, and sulfite ion, as well as other complex forms. It should be noted that sulfur in all its forms is harmless except for the small percentage of individuals who are severely allergic to it.