Sulphites: The Wine Ingredient That Doesn't Deserve Its Reputation

You've seen it on the back of every bottle you've ever bought. Contains sulphites. It sounds like a warning, and it reads like one too, which is probably why it makes people nervous. But sulphites are genuinely one of the least interesting things to worry about in your wine glass.

What Are Sulphites?

Sulphites (technically sulphur dioxide, or SO₂) are a simple compound that winemakers have used since at least Roman times. Pliny the Elder wrote about burning sulphur in wine vessels nearly two thousand years ago. It's not a modern chemical additive. It's one of the oldest tools in winemaking.

SO₂ does three things. It fights off spoilage bacteria and wild yeast that could turn your wine into vinegar. It protects wine from oxygen, because too much exposure turns the colour brown and leaves you with something that tastes like bruised apples. And it blocks the enzymes that want to start browning the juice the moment grapes are crushed.

Sulphites keep your wine tasting the way the winemaker intended.

Every Wine Has Them

The yeast that ferments grape juice into wine naturally produces sulphites as a byproduct, typically 6–40 mg/L. There is no such thing as a sulphite-free wine. Even a bottle labelled "no added sulphites" contains them. The label exists because winemakers add a bit more to keep the wine stable on its journey from the winery to your glass.

How much gets added depends on the style. Dry reds need the least, because their tannins already act as natural preservatives (typically around 50–75 mg/L total). Dry whites sit a bit higher, around 100 mg/L. Sweet and dessert wines need the most, sometimes 250–350 mg/L, because residual sugar binds the SO₂ and demands higher doses to keep the wine protected.

Are They Safe?

For the vast majority of people, yes. The FDA estimates that fewer than 1% of the population has any sensitivity to sulphites at all.

That small group is disproportionately made up of people with asthma. Around 3–10% of asthmatics may be affected, and for them, sulphite exposure can trigger respiratory symptoms: wheezing, coughing, chest tightness. If that applies to you, it's worth knowing about and discussing with your doctor. But for most people, "Contains sulphites" is about as alarming as "Contains grapes."

One thing worth clearing up: a sulphite sensitivity has nothing whatsoever to do with being allergic to sulfa drugs. They're completely different chemical classes that happen to share an element (sulfur). No cross-reactivity has ever been demonstrated.

What About the Headache?

"Red wine gives me a headache" is probably the most common complaint in the wine world, and sulphites have taken the blame for decades. The science doesn't support it.

If sulphites caused headaches, you'd expect sweet white wines, which contain two to five times more sulphites than dry reds, to be the worst offenders. They're not. Dry red wine has the lowest sulphite levels of any wine type, yet it's the one people complain about. The culprit is clearly something else.

In 2023, researchers at UC Davis and UCSF published a study pointing to a much more plausible explanation. The suspect is quercetin, a natural flavonoid that grapes produce in their skins as protection against sunlight. Red wines are fermented on their skins, so they contain roughly ten times more quercetin than whites.

When you drink red wine, your body metabolises the quercetin into a compound that blocks an enzyme called ALDH2. That enzyme is responsible for clearing acetaldehyde, a toxic byproduct of alcohol metabolism. With ALDH2 blocked, acetaldehyde builds up, causing flushing, nausea, and a throbbing headache. These are the same symptoms experienced by the 35–40% of East Asians who carry a genetic variant of this enzyme.

The theory is still being tested in clinical trials, but it's the first explanation that actually fits the pattern: why red wine, why not white, and why some people but not others.

So, Stop Worrying

Sulphites are an ancient, natural, and (for the overwhelming majority of us) entirely harmless part of winemaking. They're the reason your wine arrives tasting fresh and the way it was meant to. The label is a regulatory requirement, not a health warning.

If you have asthma or a diagnosed sensitivity, it's sensible to be aware of which wines contain higher levels and to talk to your doctor. For everyone else, you can stop worrying about the back label and get on with enjoying what's in the glass.

THE WICKLOW WINE Co.