Science
Sulfur dioxide and sulfites
What SO₂ does in wine, why free and bound fractions are different, why every wine contains some, and what the label is actually telling you.
The short answer
Sulfur dioxide is the principal preservative in winemaking. It works in two ways at once — as an antioxidant and as an antimicrobial — and only the free fraction does that work. Every wine contains some, because yeast produce it during fermentation whether or not any is added.
Overview
No other wine additive attracts as much comment or as much misinformation, largely because it is the only one that appears on the label.
It has been used in winemaking for centuries; burning sulfur candles in barrels is a very old practice.
WineHQ makes no health claims about sulfites in either direction. Sulfite sensitivity is a clinical question and belongs with health authorities.
How it works
The mechanism, at the scope it has actually been established.
Added SO₂ immediately splits into fractions. Some remains free, some binds to acetaldehyde, sugars, anthocyanins and other carbonyl compounds. Only the free fraction is active as a preservative, which is why total sulfite figures on their own tell a winemaker very little.
Within the free fraction, only a portion is present as molecular SO₂, and that portion is strongly pH-dependent. At pH 3.2 a given free SO₂ level provides substantially more antimicrobial protection than the same level at pH 3.6, which is why low-pH wines can be made with less added sulfur.
As an antioxidant it works largely indirectly: it scavenges hydrogen peroxide generated by phenolic oxidation and binds the acetaldehyde produced, rather than reacting with oxygen quickly itself.
As an antimicrobial it inhibits spoilage yeasts and bacteria, including Brettanomyces and acetic acid bacteria — which is the main reason low-sulfur winemaking carries a higher risk of microbial character.
Fermentation itself produces sulfur dioxide as a normal yeast metabolite, typically around ten milligrams per litre and sometimes considerably more.
Why it matters in the glass
It is the single largest technical difference between conventional and low-intervention winemaking, and it explains why low-sulfur wines are more variable bottle to bottle and more sensitive to how they are stored and shipped.
It also explains the labelling threshold: EU and UK rules require a "contains sulphites" declaration above 10 mg/L, which many zero-addition wines exceed on their own fermentation-derived sulfur.
What people get wrong
2 widely repeated claims this page corrects.
Not true
“Natural wine has no sulfites in it.”
All wine contains some sulfites, because yeast produce them during fermentation. Natural wine usually means little or no sulfur dioxide was *added* — which is a different statement.
Not established
“Sulfites in wine are what give you a headache.”
The sulfite–headache link is not established. Sulfite sensitivity is a recognised clinical issue, but it typically presents as a respiratory reaction rather than a headache, and dried fruit contains far more sulfite than wine does.
Compounds named on this page
Where a compound comes from is chemistry; what it smells like is perception. These are kept apart deliberately.
- Sulfur dioxide
- The principal wine preservative — antioxidant, antimicrobial, and produced by yeast whether or not it is added. Caution: Every wine contains some. WineHQ makes no health claim about sulfites in either direction; sensitivity is a clinical question for a health service.
- Acetaldehyde
- The principal oxidation product — a fault in fresh wine, definitional in Fino Sherry. Associated with bruised apple, sherry, nuts. Caution: Bound by free sulfur dioxide, which both removes the aroma and explains why a wine low in free SO₂ becomes vulnerable.
Legal maximum levels differ by jurisdiction and by wine type — sweet wines are permitted more, since residual sugar raises microbiological risk — and organic certification schemes set their own lower ceilings. These are regulatory figures that change, and WineHQ cites them from the regulation rather than from secondary sources.
Sources
Sources consulted
- International Code of Oenological Practices — OIV
- Regulation (EU) No 1308/2013 — Common Organisation of Agricultural Markets — European Union
- The Australian Wine Research Institute — AWRI
- Food Standards Agency — wine standards and food labelling — Food Standards Agency
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