Sulphur Dioxide
The short version
Free sulphur dioxide — the active species in all sulfite preservation and a major label allergen.
A preservative and antioxidant (sulphur dioxide). It works well, but sulfites can trigger breathing problems in people with asthma and reactions in other sensitive people, so labels must declare them. That sensitivity risk is why we flag it.
Where it comes from
Used since Roman antiquity as a fumigant for wine barrels. Industrially produced by burning elemental sulfur or by roasting sulfide ores. The active species in all sulfite-class preservatives is free SO2, in equilibrium with bisulfite and sulfite ions in solution.
How it's used
Applied as a gas to dried fruits and to wine in fermentation. Sulfite salts (E222–E228) are convenient solid forms that release SO2 in solution. Acts as antimicrobial, antioxidant, and enzyme inhibitor. Typical residual in dried fruit: 500–2000 mg/kg.
Safety profile
ADI of 0.7 mg/kg body weight (combined sulfites, expressed as SO2). A documented allergen — triggers severe asthma in an estimated 5–10% of asthmatics and rare anaphylaxis. Mandatory allergen labelling above 10 mg/kg in the EU, US, and most jurisdictions. Destroys dietary thiamine (vitamin B1).
Physical & chemical properties
Colorless pungent gas, very soluble in water. Generates the bisulfite-sulfite equilibrium responsible for the reducing and antimicrobial activity of all sulfite-class additives.
Regulation worldwide
Permitted globally with strict maximum levels per category. Mandatory allergen declaration above 10 mg/kg as SO2.
Educational information only. Not medical or dietary advice.