Definition: Sulphur dioxide

Sulphur dioxide, formerly also known as sulphurous anhydride, is a chemical compound with the formula SO2. It is a colourless, dense and toxic gas, which is highly irritating when inhaled. It is released into the Earth's atmosphere by volcanoes and numerous industrial processes, as well as by the combustion of certain types of coal, oil and natural gas that have not been desulphurised.

Sulphur dioxide is used as a disinfectant, antiseptic, antibacterial agent, refrigerant gas, bleaching agent and food preservative, particularly for dried fruit, in the production of alcoholic beverages and especially in wine making.

This antiseptic has always been used by winemakers.

When used carefully, it exerts its many antioxidant and antibacterial properties, ensuring the proper preservation of bottled wines.

Sulphur dioxide*: contains sulphur dioxide or sulphites. Since 2005, it has been possible to read this warning on wine labels. This means that the wine contains sulphur dioxide (S02) or, more simply, sulphur. A European directive requires producers to mention the presence of sulphites in wine above a content of 10 mg/l. Due to its antioxidant and antimicrobial properties, sulphurous gas, or sulphur dioxide (SO2), is the oldest and most widely used oenological product today, both in winemaking and preservation. It is used in winemaking because it inhibits or stops the development of yeasts and bacteria. It thus stops fermentation at the desired moment and ensures that the wine is sterilised. During winemaking, the introduction of sulphur dioxide allows yeasts to be selected. Those necessary for winemaking are more resistant to sulphur dioxide than other yeasts present. However, this use is increasingly being questioned. It should be noted that for winemakers, sulphur dioxide means safety. As we have seen above, sulphur dioxide can prevent premature fermentation. It ensures that the wine remains in good condition. It also helps to preserve and stabilise the wine without altering its aromas and taste (in small doses). Some wines may contain up to 350 mg/l, which could be harmful to health, particularly for people with allergies.

Only free SO2 is considered an antiseptic or antioxidant.

Its total absence can cause the wine to taste stale or chewy. When sulphur dioxide is added to wine (in gaseous form, liquid solution, potassium bisulphite or potassium metabisulphite), what happens to it is quite complex! Very little of it evaporates. In fact, much of it combines with sugars, colouring agents, vitamins (including B1, which it destroys), acids, etc. However, only the uncombined part of SO2 (the free part) is considered to be antiseptic or antioxidant. A good winemaker who uses SO2 is recognisable because in their wines, the combined SO2 does not exceed twice the free SO2.

*Modern nomenclature would have us replace sulphur dioxide with sulphur dioxide (SO2).

Given its toxic effects**, the maximum total SO2 content in wines ready for consumption is set by EU regulations.

Maximum total SO2 permitted in mg/l for AOC wines in France:

Red: 160

White and rosé: 210

Red (sugars > 5 g/l): 210

White and rosé (sugars > 5 g/l): 260

Sweet wines: 300 to 400

Sparkling: 185

Natural sweet wine, fortified wine (sugar content > 5 g/l): 200

Red wine (local wine): 120

White or rosé wine (local wine): 150

**In winemaking, quantities that are harmless to the human body are added. However, the toxicological effects of sulphur can lead to headaches, respiratory and cardiac problems. Some people are allergic to it.

Source: IFV (French Institute of Vine and Wine).

See also "sulphur in wine".