Sodium Metabisulphite
What it does
Preservative/antioxidant, dough conditioner.
How it's made
Synthesised sulphite salt.
Sodium metabisulphite is a widely used solid source of sulphur dioxide, employed as a preservative and antioxidant in dried fruit, some wines and beers, pickles and other products, and also as a dough conditioner. It is one of the most common "sulphite" additives.
It is produced by passing sulphur dioxide gas (made by burning sulphur) into a sodium carbonate or sodium hydroxide solution, which first forms sodium bisulphite; concentrating this causes sodium metabisulphite to crystallise out.
The crystals are dried into a white to pale-yellow powder with a sulphurous smell. When dissolved in food, particularly under acidic conditions, it releases sulphur dioxide to prevent oxidation, browning and microbial growth. As a sulphite it must always be labelled, because it can provoke reactions in sensitive individuals, especially people with asthma.
Possible reactions
Can provoke asthma and allergic reactions in sensitive people.