Potassium ferrocyanide
An anti-caking agent added to table salt and salt substitutes to stop grains clumping together in damp conditions.
What is it?
Potassium ferrocyanide is an inorganic salt in which iron is tightly bound to six cyanide groups in a stable complex. Despite containing cyanide, the iron-cyanide bond is extremely stable at food use levels and the compound behaves very differently from free cyanide. It appears as pale yellow crystals and is water-soluble.
What does it do?
It acts as an anti-caking agent by coating salt crystals and preventing them from absorbing moisture and binding together. The additive works by forming a physical barrier between crystals rather than by a chemical reaction with the salt itself.
Where you will see it
Found almost exclusively in table salt, rock salt, sea salt and salt substitutes sold for cooking or as condiments. Because only a small proportion of salt products on the market use ferrocyanide anti-caking agents, most shoppers will not encounter it regularly. On a UK food label it appears as 'potassium ferrocyanide' or 'E536' in the ingredients list.
What the science says
Kidney effects in animal studies
The kidneys are the primary target organ in animal research. In a two-year rat study, the highest doses produced measurable changes in urinary cell excretion, which is an early marker of kidney stress. These effects were used to set the acceptable daily intake. The doses that caused this effect are far above what a person would realistically consume from salt.
A 2-year chronic rat study identified the kidney as the target organ; a NOAEL of 4.4 mg sodium ferrocyanide per kg bodyweight per day was derived from urinary cell excretion changes in male rats.
Cyanide release at food use levels
The cyanide groups in ferrocyanide are bound tightly to iron and do not release freely under normal digestion. Animal absorption studies showed that after ferrocyanide was given orally, less than 0.06 mg of free cyanide per kg bodyweight was detected. EFSA concluded that at the level used in food, the amount of cyanide potentially released is not of concern.
Absorption studies showed less than 0.06 mg free cyanide per kg bodyweight released after oral ferrocyanide dosing; at the group ADI of 0.03 mg ferrocyanide ion per kg bodyweight per day, the potential cyanide released was not considered a safety issue.
Group ADI and exposure margin
EFSA set a group acceptable daily intake of 0.03 mg ferrocyanide ion per kg bodyweight per day, covering sodium, potassium and calcium ferrocyanide together. Estimated real-world dietary exposure from salt was around 0.003 mg per kg bodyweight per day for children, roughly one tenth of the ADI. The panel noted that only about 13% of salt products on the market are labelled as containing ferrocyanides, so actual exposure for most people is lower still.
EFSA established a group ADI of 0.03 mg ferrocyanide ion per kg bodyweight per day; refined dietary exposure estimates for children were approximately 0.003 mg per kg bodyweight per day, giving a roughly 10-fold margin below the ADI.
Data gaps noted by regulators
The EFSA review flagged that no reproductive toxicity studies are available and that genotoxicity data are limited to a single in vitro study, which showed effects only at very high concentrations. These gaps mean the data set supporting the ADI is narrower than for some other food additives. EFSA judged the existing evidence sufficient for the authorised levels but recorded the gaps formally.
EFSA noted the absence of reproductive toxicity studies and limited genotoxicity data (one in vitro study with effects only at high concentrations) as uncertainties in the re-evaluation.
Where it stands with the regulators
Who should be careful
There are no specific population groups identified by regulators as needing to avoid E536. People who are monitoring their salt intake should be aware that this additive appears in table salt and salt substitutes, but is present in only a minority of products on the UK market. Look for 'potassium ferrocyanide' or 'E536' on the label if you want to identify which salt products contain it.
The honest read
The name 'ferrocyanide' alarmed consumers for years because 'cyanide' is in it, but ferrocyanide and free cyanide are chemically distinct compounds with very different behaviour in the body. The iron-cyanide bond is stable and only a tiny fraction releases as free cyanide during digestion. The regulator-estimated exposure from food is about one tenth of the ADI. The picture is unusually straightforward for a food additive: a narrow set of permitted uses, a specific numerical ADI, and a 2018 EFSA review that found no concern at authorised levels. The one honest caveat is that reproductive toxicity data do not exist, and the genotoxicity database is thin. EFSA recorded those gaps formally rather than filling them. For anyone eating ordinary amounts of salted food, the exposure margin is wide.
Related additives
Common questions
Is E536 banned in the UK?
No. E536 is an approved food additive in the UK under the assimilated EU Regulation 1333/2008. It is permitted in table salt and salt substitutes at a maximum of 20 mg/kg.
Does potassium ferrocyanide contain cyanide that is harmful to eat?
Potassium ferrocyanide contains cyanide groups, but they are locked in a stable chemical bond with iron and only a very small fraction releases as free cyanide during digestion. EFSA's 2018 review concluded that at the levels permitted in salt, the amount of free cyanide potentially released is not a safety concern. The compound behaves very differently from free cyanide or hydrogen cyanide.
What foods contain E536?
E536 is only permitted in table salt and salt substitutes. It is not authorised in other food categories. Market surveys suggest roughly 13% of salt products use ferrocyanide anti-caking agents, so the majority of salt products do not contain it. Check the ingredients list for 'E536' or 'potassium ferrocyanide'.
Is E536 vegan?
Yes. Potassium ferrocyanide is a synthetic inorganic compound and contains no animal-derived ingredients.
Sources
- EFSA ANS Panel: Re-evaluation of sodium ferrocyanide (E 535), potassium ferrocyanide (E 536) and calcium ferrocyanide (E 538) as food additives, EFSA Journal 2018
- UK FSA: Approved additives and E numbers
- UK FSA Authorised Regulated Products register: E536 Potassium ferrocyanide (authorised GB, last updated 1 April 2025)
- EU Regulation 1333/2008 on food additives (Annex II), as assimilated into UK law
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