SO2, or sulfur dioxide, is a preservative used in winemaking that serves as both an antioxidant and an antimicrobial agent. It’s the reason your bottle of wine can sit on a shelf for years without turning into vinegar. Nearly every wine you’ve ever drunk contains some amount of it, whether it was added deliberately or produced naturally during fermentation.
What SO2 Does in Wine
Sulfur dioxide pulls double duty. First, it prevents oxidation. When wine is exposed to oxygen, a chain of chemical reactions can turn it brown and flat-tasting. SO2 intercepts hydrogen peroxide that forms during this process, stopping it from degrading the alcohol and other flavor compounds in the wine. Second, it works as an antiseptic, killing or inhibiting unwanted bacteria and wild yeast that could spoil the wine or produce off-flavors. These two properties together make it, as winemakers have known for centuries, essentially indispensable for producing stable, quality wine.
Free, Bound, and Total SO2
Not all SO2 in a bottle of wine is doing useful work at any given moment. Winemakers track three categories. Free SO2 is the portion actively available to protect the wine. It exists mostly as bisulfite, which handles the antioxidant duties, plus a small fraction called molecular SO2, which is the form that actually kills microbes. Bound SO2 is the portion that has chemically latched onto other compounds in the wine, like aldehydes or, in red wines, pigment molecules called anthocyanins. Once bound, it’s no longer protective. Total SO2 is simply the sum of free and bound.
This is why winemakers can’t just add SO2 once and forget about it. Over time, more of the free portion becomes bound, so levels need to be monitored and sometimes topped up, especially during aging and bottling.
Yeast Makes Some SO2 Naturally
Even wines labeled “no sulfites added” contain trace amounts of SO2. Yeast produces small quantities of it as a byproduct of fermentation. However, these natural levels are very low, typically below 5 to 10 mg per liter. Research has consistently shown this isn’t enough to protect the wine on its own. External supplementation is needed for any wine that will be stored or shipped, which is why truly zero-sulfite wine is rare and usually meant to be consumed quickly.
How Much Is in Your Glass
In the United States, wine labels must include a “Contains Sulfites” statement if total SO2 reaches 10 or more parts per million (ppm). Since almost all wines exceed that threshold, you’ll see the phrase on nearly every bottle. The U.S. legal maximum for total sulfites in wine is 350 ppm, though most wines fall well below that. Sweet white wines tend to have the highest levels because residual sugar makes wine more vulnerable to microbial spoilage. Dry reds typically sit at the lower end, often between 50 and 100 ppm, partly because tannins provide their own layer of protection.
For context, wine is actually a modest source of sulfites compared to many everyday foods. Dried apricots routinely contain sulfite levels of 2,000 mg per kilogram, with some batches exceeding that. A single fruit-and-nut bar tested in one survey contained 1,395 mg per kilogram of sulfite from its dried fruit ingredients. Frozen foods, juices, and cereals, by contrast, generally contain less than 10 mg per kilogram. If you eat dried fruit without problems, sulfites in wine are unlikely to be an issue for you.
Sulfite Sensitivity
Sulfite sensitivity is real, but it’s far less common than most people assume. The group most affected is people with asthma: an estimated 3 to 10% of asthmatic adults experience adverse reactions to sulfite additives. For the general population without asthma, sensitivity is uncommon. Symptoms can range from mild to severe and include skin reactions like hives or flushing, gastrointestinal symptoms like abdominal pain and diarrhea, and respiratory symptoms like wheezing or difficulty breathing. In rare cases, sulfites can trigger anaphylaxis. Skin sensitivity specifically shows up in roughly 1 to 5% of people who are patch-tested for it.
SO2 Probably Isn’t Causing Your Wine Headache
The widespread belief that sulfites cause wine headaches doesn’t hold up well under scrutiny. If sulfites were the culprit, you’d expect white wines (which contain more SO2) to cause worse headaches than reds. In reality, the opposite pattern is far more commonly reported. The more likely triggers are histamine and other biogenic amines found in wine, particularly red wine.
Histamine can induce headaches in people whose bodies don’t break it down efficiently, a condition called histamine intolerance. It’s linked to low activity of an enzyme called diamine oxidase. In these individuals, the histamine in red wine triggers a vascular headache by prompting the release of nitric oxide, which dilates blood vessels. Other amines like phenylethylamine may also play a role. So while sulfites get the blame, they’re largely innocent bystanders when it comes to the “red wine headache.”
Organic and Natural Wine
Organic wine rules vary by country. In the U.S., wines certified “organic” by the USDA cannot have any added sulfites, though they may still contain the trace amounts produced naturally by yeast. Wines labeled “made with organic grapes” can include added sulfites up to 100 ppm. In the European Union, organic wines are allowed added sulfites but at lower maximums than conventional wines, typically 100 ppm for reds and 150 ppm for whites. Natural wine, which has no legal definition in most countries, generally aims for minimal or no added SO2, though practices vary widely from producer to producer.
Wines made without added sulfites tend to be less stable and more sensitive to temperature changes during shipping and storage. This is why natural wine enthusiasts often recommend keeping bottles refrigerated and drinking them relatively soon after purchase.