Sulphur

Sulphur is shorthand for sulphur dioxide (often written as SO2), an additive that winemakers have used for centuries to keep wine stable and fresh. The substance has two important functions: it works as an antioxidant, so the wine does not oxidise and turn dull, brownish and tired, and it keeps unwanted microorganisms in check, so the wine does not start to ferment or spoil again in the bottle. Some of the sulphur quickly binds to other substances in the wine (this is called bound sulphur), while the rest stays free and active. It is the free sulphur that actually protects the wine.

For you as a drinker, sulphur means the wine tastes the way the producer intended when it was bottled, rather than being oxidised or faulty. Typically a little is added when the grapes are pressed and again before bottling. In white wines, sulphur can also slow the early browning of phenols and delay unwanted re-fermentation in sweet wines, where the sugar might otherwise tempt the yeasts back.

A widespread misunderstanding is that sulphur causes headaches, or that wine without sulphur is always better. Sulphur is found in the vast majority of wines, and even naturally made wine forms small amounts during fermentation. The quantity is usually small and carefully measured, and skilful winemaking is about using as little as possible, but enough for the wine to keep.