E339i - Monosodium phosphate

Monosodium phosphate is the single-sodium salt of phosphoric acid — one of the three sodium phosphates that share the number E 339, alongside disodium phosphate (E 339(ii)) and trisodium phosphate (E 339(iii)). It is made by neutralising phosphoric acid with a measured amount of sodium carbonate or hydroxide, then crystallising the salt out.

Of the three, monosodium phosphate is the acidic one. That makes it the form reached for when a recipe needs the buffering and mineral-binding behaviour of a phosphate without raising pH. It appears as an acidity regulator, a raising-agent component, an emulsifying salt and a sequestrant that mops up calcium and iron ions.

Self-raising flour and baking powders, processed and spreadable cheese, evaporated and UHT milk, powdered desserts and instant puddings, some cured meats, and isotonic sports drinks.

Sodium phosphates are authorised in Great Britain as E 339, with monosodium phosphate specified as E 339(i) in the assimilated specifications regulation.

EFSA re-evaluated the whole phosphate group in 2019 and set, for the first time, a group acceptable daily intake of 40 mg per kg of body weight per day, expressed as phosphorus, covering E 338–341, E 343 and E 450–452 together. The Panel added two caveats worth knowing: the ADI does not apply to people with moderate to severe reduced kidney function, and estimated intakes already exceed it for infants, toddlers and children.

It contributes both phosphate and sodium. In products where several phosphate additives are used together, the phosphorus adds up faster than any single ingredient listing suggests.

• FSA Register of Food Additive Authorisations — E 339
• EFSA (2019), Re-evaluation of phosphoric acid–phosphates – di-, tri- and polyphosphates (E 338–341, E 343, E 450–452)
• Assimilated Regulation (EU) No 231/2012 (specifications, E 339(i))