Sodium metabisulfite (also known as sodium pyrosulfite) is a food additive classified as E223. It serves multiple technological functions: as a preservative that inhibits microbial growth, as a flour improver that relaxes gluten by releasing free sulfur during thermal decomposition, and as a leavening aid in certain applications.
Physical and chemical properties
Chemical formula: Na₂S₂O₅ (or Na₂(SO₂·SO₃)). CAS number: 7681-57-4. Appearance: white or light yellow crystalline powder. Melting point: 150 °C. Water solubility: 540 g/L at 20 °C — highly soluble, making it easy to incorporate into liquid-based food systems.
Applications and dosage
Sodium metabisulfite is used across a wide range of food products. Typical permitted dosages include:
- Grape juice — up to 6 mg/L
- Fruit juices for beverages — up to 100 mg/L
- Wines (grape, semi-sweet, fruit) — 200–400 mg/L
- Cider — up to 150 mg/L
- Marmalade, pastilles, marshmallow, jams — up to 20 mg/kg
- Peeled potato (anti-browning treatment) — 20 mg/kg raw, 2 mg/kg cooked
- Dried potato and cabbage — up to 400 mg/kg
- Dried fruits (subject to heat treatment) — up to 1,000 mg/kg
- Berry semi-finished products — 1,500–3,000 mg/kg
In addition to these dosage limits, most products are subject to a residual SO₂ limit in the finished product, which must comply with national food safety regulations.
Safety considerations
Sodium metabisulfite can cause allergic reactions in sensitive individuals, particularly asthmatics. Workers handling the pure substance should avoid inhalation of dust and contact with skin and eyes. The sulfur dioxide released during processing has a characteristic sharp odor and can irritate the respiratory tract at high concentrations.
Find more information about food additives and preservatives in the articles below.