Sulphur dioxide is a colourless gas that is prepared by burning sulphur. Sulphur is naturally occurring in coal and petroleum products. It is highly soluble in water and it's this resultant acid, Sulphurous acid, that imparts its use as a preservative. It's antimicrobial in action which prevents the enzymatic and bacterial spoilage of food, and improves the overall appearance of food, making it look fresher than it might be. It works best in acidic foods and is also used as a bleaching agent in flour. Sulphates and sulphites are banned from use in fresh fruit and vegetables. Asthmatics should avoid as it can be fatal. May cause gastric irritation and/or damage, behavioural problems and hyperactivity.