Sulfur Dioxide (SO2) is the most common chemical compound used in winemaking. Its preservative and anti-bacterial qualities were discovered thousands of years ago, and because it is non-toxic it has been used in foods and wine since antiquity.Sulfur is naturally present on the grape skins, and though it is typically not enough for ensure against spoilage for winemaking purposes, it means that no wine is entirely sulfite-free.
SO2 is added periodically throughout the winemaking process and plays a critical role in quality winemaking. Sulfur serves two main purposes. It prevents the wine from reacting with oxygen which can cause browning and off-odors (oxidation), and it inhibits the growth of bacteria and undesirable wild yeasts in the grape juice and wine. The grapes are not rinsed before crushing because some of the grapes have already leaked juice and this would potentially dilute this high quality free-run juice. The unwashed skins have bacteria and wild yeast on them that can unpredictably influence fermentation and some winemakers therefore choose to use sulfur dioxide before they are loaded into the crusher/destemmer. Sulfur may be added again at the time of racking or anytime the wine could potentially be exposed to oxygen. Because a small percentage of the population is potentially allergic to sulphur dioxide, winemakers are required by law to keep the levels of SO2 in their wines below 200 parts per million for dry wines (many wines have far less), and only a little higher for desert wines. The laws have become more stringent with regards to admissible sulfur content in all food products, however, regardless of the international regulations, winemakers avoid the addition of sulfur because of its unpleasantly pungent smell and impact on the natural development of wine.
Without a complete understanding of the amount typically employed in winemaking and the reasons for its use, Americans have reacted strongly to sulfur content in wines as they have to any substances or foods that could potentially cause allergic reactions. For this reason "Contains Sulfites" is now required language on all wine labels sold in the US. Because all wines contain naturally occurring sulfur compounds, no other country, aside from Australia, has this requirement, and many, in fact, find this regulation laughable.