If alcohol is a preservative, why do winemakers add sulfur dioxide?

Dear Dr. Vinny,

If the alcohol in wine acts like a preservative and prohibits the growth of pathogens, why do winemakers use SO2, or sulfur dioxide, as a preservative?

—Ruwan F., Melbourne, Australia

Dear Ruwan,

Wine comes with a few natural preservatives, including tannins, acidity, alcohol and sulfites. These all act together to give wine a long life, and help keep it from turning to vinegar—especially as long as the bottle is sealed and the wine is protected from oxygen.

You’re right that there aren’t any human pathogens that could stay alive in wine, but there are other concerns of spoilage bacterias and yeasts that—while they can’t make you sick—can make a wine seem off, or become fizzy. That’s where sulfur dioxide comes in. Sulfur dioxide is naturally found in wines and is a byproduct of fermentation, but most winemakers choose to add a little extra to prevent the growth of undesirable yeasts and microbes, as well as to protect against oxidation.

Why are we talking about this? My guess is that it’s part of the confusion about sulfites, why they are in our wine (and why there are warning labels about it). Sulfur dioxide is a type of sulfite, and wine labels alert us to the presence of sulfites. For those sulfite-sensitive, it’s best to avoid products high in sulfites, including dried fruit and molasses, otherwise they run the risk of something like an asthma attack. While some wines might be made without any added sulfites, it’s unlikely that there are any wines that are entirely sulfite free.

—Dr. Vinny