Sulphites, or sulphiting agents, are defined as sulphur dioxide and several inorganic sulphite salts with E numbers E220-E224 and E226-E228, respectively. They are added to foods as a preservative to prevent the growth of bacteria and moulds. Sulphites are also added to foods to prevent browning. They can occur naturally in low levels in onions and cabbage and as a consequence of fermentation of wine and beer.
Sulphites may liberate sulphur dioxide, which is volatile and does so during storage and heating. Sulphite sensitivity appears among steroid dependent asthmatics but in the general population the condition is rare. The risk of reacting to sulphite is higher with higher level in a food product.
Labeling
The presence of sulphur dioxide and sulphite in food products at concentrations of more than 10 mg/kg or 10 mg/liter, expressed as SO2, must always be declared, see further in LIVSFS 2004:27 Livsmedelsverkets föreskrifter om märkning och presentation av livsmedel [in Swedish] (see link to the right).
Examples of methods of analysis
Sulphite can be analyzed with two different methods. In the standard EN 1988-1 sulphite is determined with an optimized Monier-Williams method and in the standard EN 1988-2 with an enzymatic method. The latter is identical to NMKL method Nr 135, 1988. The limit of quantification of the methods is 5-10 mg sulphite/kg, i.e. 5-10 ppm sulphite.
Commercial assays for enzymatic determination of sulphite are available.
Providers of test kit often have a validation protocol to be submitted with the test upon request. Laboratories using commercial test kits must establish in house control of limit of detection and limit of quantification in actual matrices even if the test has been validated by the provider.
Allergic reactions / doses
Threshold doses for adverse reactions to sulphite in sulphite sensitive individuals have not been systematically assessed and the smallest concentration of sulphite able to provoke a reaction is unknown. According to one report most sulphite sensitive individuals will react to ingested metabisulphite in quantities ranging from 20 to 50 mg. Others recommend that sulphite sensitive individuals should avoid food containing more than 100 mg/kg.
Acid food and drinks might cause a greater problem for sulphite sensitive individuals since SO2 is liberated easier in an acid environment.