Sodium metabisulphite

Sodium metabisulphite (E223, sodium pyrosulphite) is an inorganic sulphite salt with the formula Na₂S₂O₅, produced by passing sulphur dioxide through a sodium hydroxide or sodium carbonate solution and then crystallising. In the presence of moisture it releases SO₂ and bisulphite ions, making it functionally equivalent to sulphur dioxide as a preservative and antioxidant. It is used to prevent oxidative browning in fresh-cut fruits and vegetables, to sanitise wine and brewing equipment, and as a preservative in dried foods, seafood, and pharmaceutical preparations.

JECFA evaluates sodium metabisulphite under the same group ADI established for sulphur dioxide and sulphite salts: 0.7 mg/kg body weight per day expressed as SO₂ equivalents (evaluated 1999). One gram of sodium metabisulphite releases approximately 0.67 g of SO₂, so use levels are calibrated accordingly. The European Food Safety Authority's 2016 comprehensive re-evaluation of the E220–E228 sulphite group confirmed this ADI and noted that combined dietary exposure from all sulphite-containing foods should be assessed cumulatively, not per-additive, because all share identical metabolic pathways and health endpoints.

Sodium metabisulphite poses the same sensitisation risk as all sulphite compounds: sulphite-sensitive individuals, particularly asthmatics, can experience bronchospasm from foods and beverages containing it. EU Regulation (EC) No 1333/2008 mandates "contains sulphites" labelling above 10 mg/kg or 10 mg/L SO₂ equivalents. In the United States, sodium metabisulphite is prohibited from use on fresh fruits and vegetables intended to be served raw or presented as fresh (following a 1986 FDA ban triggered by severe asthmatic reactions), though it remains approved for other food categories. Consumers with sulphite sensitivity should check ingredient labels on dried fruits, shrimp, wine, and processed potato products.