Sulphur dioxide (SO₂) is a compound used in winemaking as an antioxidant and antimicrobial agent. It helps protect grape must and wine from oxidation and unwanted microbial activity during processing, fermentation, maturation and bottling. Small amounts can also be produced naturally by yeast during fermentation. Sulphur dioxide is closely related to the sulphites declared on wine labels.
Sulphur dioxide exists in wine in different forms. Free SO₂ is the portion available to provide protective action, while bound SO₂ has reacted with other wine compounds. The relationship between free SO₂, total SO₂ and molecular SO₂ depends on wine pH and other factors. Winemakers measure and manage these forms rather than relying on one fixed addition for every wine.
Yeast can produce small amounts of SO₂ during fermentation, so “no added sulphites” does not necessarily mean that a wine contains no sulphites. In Canada and other markets, sulphite declarations are required when total levels exceed the applicable labelling threshold. Requirements vary by jurisdiction.
Example: a winemaker may make a measured SO₂ addition after fermentation to protect a wine during maturation and before bottling.
Chef’s note: SO₂ is a winemaking tool, not a quality grade. Sulphite sensitivity affects some individuals, so follow ingredient disclosure and allergen communication requirements.
Key distinction: Sulphur dioxide is the chemical compound used in winemaking; “sulphites” is the broader label term for sulphur dioxide and related sulphite forms.
This term is one of hundreds covered in depth across OCS courses, learning paths, and 1-on-1 coaching with Chef Eric.
