Sulphur Dioxide (SO₂)
A preservative and antioxidant widely used in winemaking to prevent oxidation, inhibit unwanted bacteria and wild yeasts, and stabilise wine during production and storage.
In depth
SO₂ (sulphur dioxide) has been used in winemaking since ancient Roman times. It performs two key functions: as an antioxidant (it scavenges free oxygen that would otherwise oxidise the wine), and as an antimicrobial agent (it inhibits spoilage bacteria and unwanted wild yeasts).
SO₂ is added at multiple stages: at crush (to suppress wild yeast and bacteria), during winemaking (to prevent oxidation), and at bottling (to protect the finished wine). The "free SO₂" is the active protective fraction; "bound SO₂" has reacted with wine components and is no longer protective.
At high levels, SO₂ produces a sharp, pungent, struck-match smell on the nose and a prickling sensation at the back of the throat. This is not a fault per se, but indicates over-sulphuring or poor winemaking practice. With aeration, excess SO₂ usually disperses.
Natural wine producers minimise or eliminate SO₂ addition, arguing for "naked" wine — but wines with no or very low SO₂ are vulnerable to oxidation and spoilage, requiring meticulous cellar hygiene. All wine labels in most countries must declare "contains sulphites" when SO₂ exceeds 10 mg/L.
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Frequently asked questions
- Is sulphur dioxide in wine harmful?
- For most people, no — the amounts used in wine production are very small and well within safe limits. A small minority of people (particularly those with asthma) are sensitive to sulphites and may experience reactions. All wines naturally contain some sulphites from fermentation; added SO₂ increases this. "Sulphite-free" wines still contain naturally produced sulphites — the label means no additional SO₂ was added.
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Practise questions on this topic
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