Sulfur dioxide, a compound formed from sulfur and oxygen, exists naturally in wine as a result of the fermentation process. Winemakers add sulfur dioxide (sulfites), too.
Winemakers use sulfur dioxide at various stages of the winemaking process because:
It stabilizes the wine (preventing it from turning to vinegar or deteriorating from oxygen exposure).
It safeguards a wine's flavor.
Sulfur dioxide inhibits yeasts, preventing sweet wines from refermenting in the bottle. It’s an antioxidant, keeping the wine fresh and untainted by oxygen. Despite these magical properties, winemakers try to use as little sulfur dioxide as possible because many of them share a belief that the less you add to wine, the better.
Approximately 5 percent of asthmatics are extremely sensitive to sulfites. To protect them, Congress passed a law in 1988 mandating that any wine containing more than 10 parts per million of sulfites carry the “Contains Sulfites” phrase on its label. Considering that about 10 to 20 parts per million occur naturally in wine, that covers just about every wine. (The exception is organic wines, which are intentionally made without the addition of sulfites; some of them are low enough in sulfites that they don’t have to use the mandated phrase.)
Sulfur has been an important winemaking tool since Roman times.Today — when winemaking is so advanced that winemakers need to rely on sulfur dioxide’s help less than ever before. Sulfur dioxide use is probably at an all-time low.
Actual sulfite levels in wine range from about 30 to 150 parts per million (about the same as in dried apricots); the legal max in the United States is 350. White dessert wines have the most sulfur — followed by medium-sweet white wines and blush wines — because those types of wine need the most protection. Dry white wines generally have less, and dry reds have the least.
Red wines contain far less sulfur than white wines. That’s because the tannin in red wines acts as a preservative, making sulfur dioxide less necessary.