E-numbers / Acidity regulators / E366
also: Potassium salt of fumaric acid
The potassium salt of fumaric acid, used as an acidity regulator. Assigned an E number but not actively authorised for use in UK or EU food.
Scan a barcode to check a food for potassium fumarate. NutraSafe Pro · £3.99/month · iOS
This is not on the UK list of approved food additives, so you will not normally see this E number on a UK label.
Why it’s worth knowing. This is not on the UK list of approved food additives, so you will not normally see this E number on a UK label.
Potassium fumarate is the potassium salt of fumaric acid (E297), a naturally occurring organic acid found in many plants including fumitory herb and some fruits. It is a white crystalline powder. The compound exists in two forms: monopotassium fumarate and dipotassium fumarate. Fumaric acid itself is a normal intermediate in human cell metabolism (the citric acid cycle).
As an acidity regulator, it would buffer or adjust pH in food products, helping stabilise taste and texture. Fumarate salts release fumaric acid and a cation (in this case potassium) in solution, which moderates acidity. The potassium content also contributes to the buffering effect. In practice, E366 has essentially no current approved food applications in UK or EU markets.
E366 is not currently authorised for use in food products in the UK or EU, so it does not appear on ingredient labels of legally sold UK or EU food. If encountered on a label it would read as 'potassium fumarate' or 'E366'.
Fumarate is a normal intermediate in the citric acid cycle, the core energy-producing pathway in human cells. The fumarate released from potassium fumarate entering the body is metabolised by the same pathway as endogenous fumarate. No specific toxicological signal has been identified for potassium fumarate itself at food-additive exposure levels.
Fumaric acid and its salts are metabolised via the citric acid cycle and do not accumulate; the body handles them the same way it handles fumarate produced in normal cell metabolism.
The potassium released from potassium fumarate contributes to dietary potassium intake. For the general population this is not a concern, but people with impaired kidney function who must limit potassium may need to be aware of potassium-containing additives. This is a general consideration for all potassium salts, not specific to the fumarate form.
Individuals with chronic kidney disease are advised to limit dietary potassium as impaired kidneys cannot efficiently excrete it; all potassium-bearing food additives contribute to total intake.
People with chronic kidney disease who follow a potassium-restricted diet should be aware that potassium-bearing additives contribute to total potassium intake. Look for 'potassium fumarate' or 'E366' on labels. In practice E366 is not currently authorised for food use in the UK or EU, so encountering it on a UK food label would be unusual.
Potassium fumarate is a simple salt of a compound (fumaric acid) that the body produces and uses naturally. The science on the fumarate anion itself raises no concern at food-use levels. The notable fact is regulatory rather than toxicological: E366 exists as an assigned E number but does not have active approval for food use in the UK or EU. It is rarely if ever encountered in UK food products.
E366 is not listed on the UK FSA approved-additives list as a currently authorised food additive. It is not the same as being formally banned, but without a positive authorisation it cannot legally be added to food sold in the UK. Encountering it on a UK food label would be unusual.
No, but they are closely related. Fumaric acid is E297; potassium fumarate (E366) is the potassium salt formed when potassium reacts with fumaric acid. Both release fumarate in solution. Fumaric acid (E297) is an approved additive in the UK and EU; potassium fumarate (E366) lacks active food-use authorisation in the same regulations.
E366 is not currently authorised for food use in the UK or EU, so it should not appear in UK food products. Fumaric acid (E297) and some related fumarate salts have specific food uses, but E366 specifically does not have established food-category permissions under UK or EU rules.
Yes. Potassium fumarate is a mineral salt with no animal-derived ingredients.
Last reviewed: 20 June 2026
It's the only app that does thisUKJaneK
You Need This AppGmac2005
Best AppGK getting skinny UK App Store reviews
NutraSafe reads the label and puts every additive into plain English, with the source, right in the app.
NutraSafe Pro · £3.99/month · iOS
