Potassium Metabisulphite
What it does
Preservative/antioxidant in wine and dried fruit.
How it's made
Synthesised sulphite salt (E224).
Potassium metabisulphite is a solid source of sulphur dioxide, used as a preservative and antioxidant in winemaking, dried fruit, and other foods, where it releases sulphur dioxide to stop spoilage, browning and oxidation. It is a convenient powder form of "sulphite".
It is made by passing sulphur dioxide gas into a potassium base such as potassium hydroxide or potassium carbonate solution, forming potassium sulphite and then potassium metabisulphite, which crystallises out. The sulphur dioxide itself comes from burning sulphur.
The crystals are dried into a white to pale powder that smells faintly of sulphur. When added to food or dissolved in water, especially in acidic conditions, it releases sulphur dioxide to do the preserving; like all sulphites, it must be declared on labels because it can trigger reactions in sulphite-sensitive people, particularly asthmatics.
Possible reactions
Sulphite — can trigger asthma and allergic reactions.