Sodium Metabisulfite — Frequently Asked Questions

In theory 1 kg of pure sodium metabisulfite yields about 0.674 kg of sulphur dioxide. Commercial food grades are typically specified at 65 % SO2 or more, and the exact figure should be taken from the certificate of analysis for your batch. Legal limits and product targets are expressed as SO2, so convert your addition rate on that basis and remember that some SO2 is lost or bound during processing and storage.

Both release sulphur dioxide in acidic foods and share the same regulatory group, E220 to E228. Sodium metabisulfite (E223) is the more concentrated source of SO2 per kilogram and gives a slightly acidic solution. Anhydrous sodium sulphite (E221) is alkaline and supplies less SO2 per kilogram. The choice depends on the pH of the process, the SO2 required and how the additive is dosed.

Sulphur dioxide and sulphites can trigger reactions, including asthma symptoms, in sensitive people. EU and UK labelling law therefore lists them among the 14 allergens that must be emphasised when present above 10 mg/kg or 10 mg per litre, expressed as SO2, in the food as sold. The threshold covers sulphites from all ingredients, including those carried over from dried fruit or wine.

Air and moisture slowly oxidise metabisulfite to sulphate, which has no preservative or antioxidant effect, and some SO2 escapes as gas. Losses are faster in warm, humid stores and in opened bags. Keep packs tightly sealed, store them cool and dry, use them in rotation and recheck the SO2 content of older stock before using it in a product with a tight specification.

After capture, an enzyme in crustacean shells turns tyrosine into dark melanin, giving black spot on the shell and head. Sulphites inhibit that enzyme. Treatment is usually a dip or a dusting on board or at landing, controlled so that the residual SO2 in the edible part stays within the legal limit for the category. Sulphite residues trigger allergen labelling, so many processors monitor residual SO2 on every lot.

It is harmful if swallowed and can cause serious eye damage, and it releases toxic sulphur dioxide gas on contact with acids or when solutions are heated. Handle it with eye protection, gloves and adequate ventilation or extraction, keep it well away from acids and oxidisers, and prepare solutions in a ventilated area. Follow the safety data sheet for storage, spill and first-aid measures.