A sulfite is a chemical compound built around sulfur and oxygen that acts as a preservative in food and drink. Your body actually produces sulfites on its own as part of normal metabolism, and they’ve been used in food preservation for centuries, dating back to ancient winemaking. Today they show up in everything from dried fruit to french fries, and while they’re harmless for most people, a small percentage of the population (particularly those with asthma) can have serious reactions to them.
The Basic Chemistry
At its core, a sulfite is an ion made of one sulfur atom bonded to three oxygen atoms. It’s the conjugate base of sulfurous acid, which forms when sulfur dioxide dissolves in water. In practice, the term “sulfite” covers a family of related compounds: sulfur dioxide gas, sodium sulfite, sodium bisulfite, sodium metabisulfite, and potassium metabisulfite, among others. These are all called “sulfiting agents” because they release sulfite ions when dissolved.
The important thing to understand is that sulfites are chemically reactive. They readily donate electrons to other molecules, which is what makes them useful as preservatives. This reactivity is also why your body has a dedicated enzyme to break them down.
How Sulfites Preserve Food
Sulfites do two main jobs in food: they prevent browning and they slow microbial growth.
When you slice an apple and it turns brown, that’s an enzyme called polyphenol oxidase at work. It converts colorless compounds in the fruit into darker pigments. Sulfites shut this process down by acting as a reducing agent, chemically reversing the browning reaction before it can produce visible color. This is why sulfite-treated fruits and vegetables keep their fresh appearance far longer than untreated ones.
On the antimicrobial side, sulfites discourage the growth of bacteria and certain yeasts. This is why winemakers have relied on them since antiquity. A small dose of sulfur dioxide in a barrel of wine keeps unwanted bacteria from turning it into vinegar, while allowing the desired yeast strains to do their work.
Which Foods Contain the Most Sulfites
Sulfite levels vary enormously across foods, and the ones that surprise people most aren’t wine. Dried fruits top the list, with concentrations reaching up to 3,000 parts per million (ppm). French fries can contain up to 1,800 ppm. By comparison, white wine typically contains around 150 ppm, and red wine sits lower at 50 to 100 ppm.
Red wine actually needs fewer sulfites than white wine because the tannins and pigments in red grapes provide some natural antioxidant protection. White wine, lacking those compounds, relies more heavily on added sulfites to stay fresh. This is worth knowing because many people blame red wine sulfites for their headaches, when white wine generally contains more. Other sulfite-containing foods include bottled lemon juice, pickled vegetables, shrimp, and maraschino cherries.
How Your Body Handles Sulfites
Your liver contains an enzyme called sulfite oxidase that converts sulfites into sulfate, a harmless compound your body can easily excrete. This is actually the final step in breaking down the sulfur-containing amino acids (cysteine and methionine) that you get from protein in your diet. The same enzyme also detoxifies any sulfites you consume from food or drink.
For most people, this system works efficiently and without any noticeable effects. Your body processes dietary sulfites quickly and moves on. In extremely rare cases, people are born without functional sulfite oxidase. This inherited deficiency is a severe condition that causes profound neurological damage in infancy, because sulfite accumulates and damages brain tissue, particularly the fatty insulation around nerve cells. This genetic condition is entirely different from the more common sulfite sensitivity that some adults experience.
Sulfite Sensitivity and Symptoms
Roughly 3 to 10% of people with asthma experience adverse reactions when they consume sulfites. In the general, non-asthmatic population, true sulfite sensitivity is extremely rare. The connection to asthma isn’t fully understood, but the airways of sensitive individuals appear to react to sulfite exposure the way they react to other asthma triggers.
Symptoms range widely in severity. Mild reactions include flushing, hives, and abdominal pain or diarrhea. More serious reactions can involve a drop in blood pressure, severe wheezing, and in rare cases, full anaphylaxis. The reaction can come on quickly after eating or drinking a sulfite-containing food, which makes identification easier than with some other food sensitivities. If you have asthma and notice that wine, dried fruit, or restaurant salad bars consistently trigger breathing difficulties, sulfites are a reasonable suspect.
Labeling Rules in the U.S.
The FDA requires any packaged food containing 10 ppm or more of sulfites to declare them on the label. This threshold applies to the finished product, not the amount added during processing. On wine bottles, you’ll see “contains sulfites” if the level hits that mark, which it almost always does since even natural fermentation produces some sulfites.
The maximum allowable sulfite level in U.S. wine is 350 ppm, though most wines fall well below that ceiling. For context, that upper limit is still less than a quarter of what dried apricots can contain. The labeling requirement on wine but not on, say, bags of dried fruit has contributed to a disproportionate public focus on wine sulfites, even though wine is a relatively modest source.
Fresh fruits and vegetables sold raw can no longer be treated with sulfites in the U.S. The FDA banned that practice in 1986 after reports of severe reactions, particularly from salad bars at restaurants where sulfites were sprayed on produce to keep it looking fresh.
Sulfites vs. Sulfates vs. Sulfa Drugs
These three terms get confused constantly, but they’re chemically unrelated. Sulfites are the preservative compounds described throughout this article. Sulfates are a fully oxidized, stable form of sulfur found naturally in mineral water, Epsom salts, and many medications. Your body converts sulfites into sulfates as part of normal detoxification. Sulfa drugs are a class of antibiotics with a completely different chemical structure. An allergy to sulfa antibiotics does not mean you’ll react to sulfites in food, and vice versa.