Sodium ferrocyanide (decahydrate) also known as yellow prussiate of soda with the chemical formula Na4Fe(CN)6·10H2O. It is a cyanide salt used as an anticaking agent in table salts to keep them free flowing and with the European food additive number E535.
Given that it is a cyanide salt, you may be surprised why this ingredient can be in salt and is it really safe? Let’s move on.
It is a synthetic compound that can be commercially produced by the reaction between sodium cyanide and ferrous sulfate in aqueous medium with heat condition as the following chemical equation indicates:
6NaCN + FeSO4 + Heat → Na4Fe(CN)6 + Na2SO4
Sodium ferrocyanide decahydrate can be then obtained after the process of crystallization.
It can also be made from the reaction among calcium cyanide, iron(II) sulfate, and sodium carbonate.
- CaCN2 + FeSO4 → Ca2Fe(CN)6 + CaSO4
- Ca2Fe(CN)6 + 2Na2CO3 → Na4Fe(CN)6 + 2CaCO3
Yellow crystals or crystalline powder, the yellow color comes from ferrocyanide anion ([Fe(CN)6]4-), iron is in the state of Fe2+.
|Other names||Sodium hexacyanoferrate, Tetrasodium hexacyanoferrate|
|Chemical formula||Na4Fe(CN)6 · 10H2O|
|Solubility||Soluble in water, insoluble in ethanol|
Sodium ferrocyanide is the cyanide complex and its major purposes in food is as an anticaking agent in salts (table salt, sea salt, iodized and non-iodized salt) for human consumption. It is vegan and gluten free.
The surface of crystals of table salt is easy to dissolve after absorbing moisture in air with high humidity, and then recrystallization occurs after water evaporation. Finally, caking, lumping, or aggregation occurs.
Some kinds of salt added with small amounts of sodium ferrocyanide to prevent such caking (or clumping) and by reducing the amount of recrystallization through decreasing the solubility of sodium chloride in water.
First of all, please be assured that there are almost no side effects of sodium ferrocyanide when used as a food additive. Its safety has been approved by the U.S. Food and Drug Administration (FDA) and European Food Safety Authority (EFSA), as well as the Joint FAO/WHO Expert Committee on Food Additives (JECFA). So please don’t worry about it.
The safety concern is generally caused by its chemical name containing cyanide, a known toxin.
As you can imagine, fears may present in many consumers if they see ferrocyanide (a cyanide complex) listed in the salt label, and that’s why most salt manufacturers prefer to label it as “yellow prussiate of soda”.
Many salts of cyanide are dangerous, but the cyanide in food grade ferrocyanides is non-toxic as it is tightly bound to an iron atom and so does not tend to release free cyanide.
In acidic environments, the bonds between sodium and ferrocyanide will break, and the toxic gas – hydrogen cyanide gas will be released (1), but the stomach acid is a weak acid that cannot promote this reaction in our body.
Yellow prussiate of soda functions as an anticaking agent in salt And as an adjuvant in the production of dendritic crystals with the maximum addition of 0.0013% expressed as sodium ferrocyanide anhydrous. (2)
It can also be safely used as an anticlumping agent in salt for animal feed. (3)
Sodium ferrocyanide (E535) is listed in Commission Regulation (EU) No 231/2012 as an authorised food additive and categorized as “additives other than colours and sweeteners”.
Ferrocyanides (potassium ferrocyanide, sodium ferrocyanide and calcium ferrocyanide) are only approved to be used in salt and salt substitutes with the maximum level 20 mg/kg individually or in combination (calculated as potassium ferrocyanide anhydrous). (4)
Safety re-evaluation in 2018
EFSA concluded no safety concern at the current authorised use and use levels of sodium ferrocyanide in its safety re-evaluation report in 2018.
The following are some key points (5):
- Its absorption in human is low and no accumulation.
- No genotoxicity and no carcinogenicity.
- Considering the only toxicity of ferrocyanide ion to the kidney, an acceptable daily intake (ADI) of 0.03 mg/kg bw per day (expressed as ferrocyanide ion) was established.
Function class: food additive, anticaking agent. (6)
Acceptable daily intake: ADI “0-0.025 mg/kg bw” set in 1974. (7)
Now you may have an understanding of the food additive – Sodium ferrocyanide or Yellow prussiate of soda (E535), from its production; function in table salts; the approved safety; where the health issues come from and the safety of Cyanide in it.
What do you think of this anticaking agent? Let me know in the comments.