Sulphites in wine: what they are and what they do

Almost every bottle of wine on the market carries that small label note: contains sulphites. For many consumers, those two words have become a kind of warning signal, something to approach with caution. And yet behind that mandatory declaration lies a story of chemistry, preservation and winemaking decisions that deserves a clear-eyed look — without prejudice, and without oversimplification.

Sulphites are sulphur-based compounds — the most common form in winemaking being sulphur dioxide, often written as SO₂ — that work on two fronts simultaneously. On one side, they counter oxidation: they slow down the chemical process that, without protection, would degrade a wine’s colour, aromas and freshness over time. On the other, they act as antimicrobials, inhibiting the growth of unwanted bacteria and yeasts that could compromise the wine during production or bottle ageing. This is not a modern invention: the use of sulphur to protect wine dates back centuries, and its widespread adoption fundamentally changed what winemakers could achieve in terms of preservation and long-term development.

A presence that begins before the winemaker intervenes

One of the most overlooked points in the public debate on sulphites is this: a portion of sulphur dioxide forms naturally during alcoholic fermentation, as a by-product of the yeasts converting grape sugars into alcohol. It does not come from outside, it is not a winemaker’s decision — it is a physiological consequence of the fermentation process itself. Which means that a wine completely free of sulphites, in an absolute sense, simply does not exist.

When a label reads no added sulphites, the claim is accurate and meaningful, but it does not mean the wine contains none at all. It means the producer chose not to supplement them during winemaking, relying solely on those generated during fermentation — which remain present, albeit in very small amounts. European regulations set a threshold below which the declaration can be omitted from the label; above that level, disclosure is mandatory, following the same principle that applies to other food allergens such as gluten or tree nuts.

Why reds need less, and whites more

Not all wines require the same amount of added sulphites, and the difference between reds and whites is one of the clearest examples of how a wine’s natural chemical composition shapes decisions in the cellar. Red wines contain tannins and anthocyanins — polyphenolic compounds present in the skins of dark-skinned grapes — which already provide a degree of antioxidant protection. This natural shield reduces the need for added SO₂. White wines, produced without extended skin contact, are more exposed to the effects of oxygen and bacterial contamination: they therefore tend to require a higher level of sulphites to maintain stability and freshness over time.

The same logic applies to sweet wines and dried-grape wines, where residual sugars create a more hospitable environment for unwanted microorganisms, making stronger protection necessary. The maximum limits set by European regulations vary accordingly by wine type: dry reds conventionally fall around 150 mg/l, dry whites and rosés around 200 mg/l, while passito-style wines are permitted considerably higher levels. Certified organic wines follow stricter thresholds than conventional ones, for both reds and whites.

Headaches and the sulphite myth

It is hard to discuss sulphites without addressing the most persistent association: the headache after drinking wine. It is a connection deeply embedded in popular belief, often cited as evidence of these compounds’ supposed harmfulness. The evidence, however, tells a different story. Adverse reactions to sulphites do exist, but they affect a limited proportion of people — primarily those with asthma or documented hypersensitivity. The typical symptoms in these cases are mainly respiratory, not headaches.

Post-wine headaches have far more likely causes: the alcohol itself, biogenic amines such as histamine, tannins in particularly sensitive individuals, and more generally the amount consumed. Automatically blaming sulphites means ignoring these variables and reinforcing a distorted reading of what is actually in the glass. The fact that dried fruit, packaged juices or certain cured meats contain sulphites in amounts comparable to or greater than those found in wine — without triggering the same alarm — is a detail worth keeping in perspective.

This does not mean sulphites are irrelevant from the standpoint of individual sensitivity: for those with a genuine hypersensitivity, reading labels and choosing wines with minimal levels is a concrete and legitimate precaution. But for the vast majority of consumers, the presence of sulphites in a wine produced with care and within regulatory limits does not represent a concern.

What remains, in the end, is a matter of informed reading: understanding the difference between no added sulphites and sulphite-free as an absolute claim, knowing that the amounts vary depending on the wine type and the producer’s choices, and recognising that SO₂ serves real and well-documented purposes. Not to defend every winemaking practice uncritically, but to pick up a bottle with the same clarity of thought one would bring to any other food product.