E538 - Calcium ferrocyanide

At a glance

• Role: Anti-caking agent that keeps table salt free-flowing
• Where found: Table salt and some salt substitutes
• Also known as: E538, calcium hexacyanoferrate(II), yellow prussiate of lime
• Status in the EU: Authorized with strict purity rules and use limits
• Safety: Evaluated by European food safety authorities, with a conservative acceptable daily intake (ADI)

Why is Calcium ferrocyanide added to food?

It prevents moisture from making salt clump. Even a tiny amount helps salt grains slide past each other, so the product pours and measures consistently. This keeps salt easy to use at home and in food manufacturing.¹

What foods contain Calcium ferrocyanide?

In the EU, calcium ferrocyanide is permitted as an anti-caking agent in salt and salt substitutes. Its use is typically limited to low levels (commonly up to 20 mg per kilogram of salt).¹ The Codex international food standards also allow ferrocyanides in salt within similar limits.²

Labels may list it as “calcium ferrocyanide,” “E538,” or “anti-caking agent.”

What can replace Calcium ferrocyanide?

Other anti-caking agents often used in salt include:

• Potassium ferrocyanide
• Silicon dioxide
• Calcium silicate
• Sodium aluminosilicate
• Calcium carbonate

The best choice depends on the product, local rules, and desired label wording.

How is Calcium ferrocyanide made?

Chemically, it is the calcium salt of hexacyanoferrate(II), sometimes written as calcium hexacyanoferrate(II). EU law sets identity and purity criteria for E538, including how it should meet specifications for composition and contaminants before it can be used in food.³ In reference databases it is listed under names such as “calcium ferrocyanide” and “calcium hexacyanoferrate(II).”⁴

Is Calcium ferrocyanide safe to eat?

The European Food Safety Authority (EFSA) re-evaluated ferrocyanides (E 535–E 538) and set an acceptable daily intake (ADI) of 0.03 mg per kilogram of body weight per day, expressed as the ferrocyanide ion.¹ Based on reported use levels in salt, EFSA concluded exposure is generally below the ADI and that ferrocyanides are not a safety concern at authorized uses and levels.¹

Toxicologically, the cyanide groups are strongly bound to iron in the ferrocyanide complex, which limits the release of free cyanide under normal food and stomach conditions.¹

Does Calcium ferrocyanide have any benefits?

Its benefit is functional: it keeps salt free-flowing and easy to dose. That helps maintain product quality and consistent seasoning during cooking and food production.¹

Who should avoid Calcium ferrocyanide?

Most people do not need to avoid it when used as allowed in foods. People who are advised to limit salt for medical reasons should focus on reducing overall salt intake rather than the anti-caking agent itself. If you have specific medical concerns, ask a healthcare professional.

Myths & facts

• Myth: “It contains cyanide, so it must be highly poisonous.”
  • Fact: The cyanide is tightly bound in the ferrocyanide complex and is not readily released during normal digestion at allowed use levels.¹
• Myth: “It is added to many foods.”
  • Fact: In the EU it is mainly authorized for use in salt and salt substitutes, and at very low levels.¹

Calcium ferrocyanide in branded foods

You will most often see it in the ingredient list of table salt or seasoned salts. Look near the end of the list for “anti-caking agent (E538)” or “calcium ferrocyanide.” Rules may differ by country, so some salts may use alternatives like silicon dioxide or calcium silicate instead.

References

Footnotes

1. Re-evaluation of ferrocyanides (E 535–538) as food additives — European Food Safety Authority (EFSA). https://doi.org/10.2903/j.efsa.2018.5374 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

2. Codex GSFA, INS 535–538 (Ferrocyanides) permitted in Food Category 12.1 (Salt) with specified maximum levels — FAO/WHO Codex Alimentarius. https://www.fao.org/gsfaonline ↩

3. Commission Regulation (EU) No 231/2012 laying down specifications for food additives listed in Annexes II and III to Regulation (EC) No 1333/2008 — European Union. https://eur-lex.europa.eu/eli/reg/2012/231/oj ↩

4. Calcium hexacyanoferrate(II) — PubChem (NIH). https://pubchem.ncbi.nlm.nih.gov/compound/Calcium-hexacyanoferrate-II ↩