What is E538?

Complete guide to understanding E538 (Calcium Ferrocyanide) — a water-insoluble anti-caking agent for salt that contains cyanide but poses zero health risk

⚠️ CONTAINS CYANIDE BUT IS COMPLETELY SAFE: E538 (Calcium Ferrocyanide) is an approved anti-caking agent for salt that chemically contains cyanide atoms. However, the cyanide is irreversibly bonded to iron in a water-insoluble crystal structure. It cannot be released in stomach acid, cannot be broken down by digestive enzymes, and cannot decompose at cooking temperatures. The water-insoluble property makes it even more resistant to breakdown than its soluble cousins (E535, E536). It passes through your digestive system unchanged and is excreted. EFSA confirms it poses zero safety concern at approved use levels.

The Quick Answer

E538 (Calcium Ferrocyanide) is a water-insoluble anti-caking agent used in salt and salt substitutes — and it’s completely safe despite containing cyanide.

What makes E538 unique: Unlike E535 (sodium ferrocyanide) and E536 (potassium ferrocyanide) which are soluble in water, E538 is water-insoluble. This makes it particularly resistant to moisture and decomposition. Like its soluble cousins, E538 contains cyanide atoms, but they’re permanently bonded to iron and cannot be released. EFSA’s 2018 re-evaluation confirmed it poses zero safety concern. E538 is an example of how chemical structure determines safety far more than chemical composition.

E538 is safe, approved, and one of the best examples of how “scary-sounding” additives can actually be harmless.

📌 Quick Facts

Chemical Name: Calcium Ferrocyanide; Yellow Prussiate of Lime
Type: Anti-caking agent; food additive
Chemical formula: Ca₂[Fe(CN)₆]
Found in: Salt and salt substitutes (ONLY approved use in EU)
Contains: Cyanide (but irreversibly bonded; cannot be released)
Key property: Water-insoluble (unlike E535 and E536)
Safety Status: Approved and completely safe
Approved by: EFSA, FDA, JECFA; most countries
Permitted level: 20 mg/kg maximum in salt
ADI (EFSA): 0.03 mg/kg body weight/day (group ADI for ferrocyanides)

What Exactly Is It?

E538 is calcium ferrocyanide, also called Yellow Prussiate of Lime — a water-insoluble yellow crystalline compound used to prevent salt from clumping — 100% synthetic, chemically engineered.

Chemical structure: Calcium (Ca²⁺) + Ferrocyanide complex [Fe(CN)₆]⁴⁻

Appearance: Yellowish crystalline powder; water-insoluble; very fine particles

Key properties:

– Anti-caking: prevents moisture absorption; keeps salt free-flowing
– Water-insoluble: Key difference from E535 and E536
– Inert: stable; doesn’t react with salt or food
– Yellow color: from ferrocyanide complex
– Heat stable: survives cooking without decomposition
– Harmless: cyanide irreversibly bonded; cannot be released or absorbed
– Very fine particles: <10 micrometers diameter
– Resistant to breakdown: water-insoluble form resists decomposition

🔬 Understanding E538’s Water-Insoluble Property: Unlike E535 (sodium ferrocyanide) and E536 (potassium ferrocyanide) which dissolve in water, E538 (calcium ferrocyanide) is water-insoluble. This makes it even more resistant to moisture and decomposition. The ferrocyanide complex remains locked in a crystal lattice that water cannot penetrate. This property makes E538 particularly effective for preventing salt from absorbing moisture and clumping. The cyanide atoms are irreversibly bonded to iron within this water-insoluble structure — they cannot escape into water, stomach acid, or digestive system.

Where You’ll Find E538

E538 is used almost exclusively in salt and salt substitutes as an anti-caking agent.

Product	Use	Permitted Level	Frequency
Table salt (refined)	Anti-caking agent; prevents clumping	20 mg/kg maximum	Common (but not all brands)
Salt substitutes	Anti-caking agent (potassium-based products)	20 mg/kg maximum	Some brands
Spice mixtures	Anti-caking (indirect through salt ingredient)	Variable	Rare
Other foods	NOT approved for any other uses (EU)	N/A	N/A

Permitted level in EU: 20 mg/kg (can be used alone or combined with E535 or E536, total 20 mg/kg)

Rarity note: E538 is used but less common than sodium ferrocyanide (E535) or potassium ferrocyanide (E536). Many manufacturers prefer soluble alternatives or use no anti-caking agents.

Is E538 Safe? Absolutely

EFSA 2018 Re-evaluation: Definitive Safety Assessment

The European Food Safety Authority completed comprehensive re-evaluation and concluded:

“Ferrocyanides (E 535–538) are of no safety concern at the current authorised use and use levels.”

Safety Assessment Details

Criterion	Finding	Conclusion
Cyanide release in stomach	Water-insoluble iron-cyanide bonds stable at stomach pH and temperature; cannot be released	Safe
Water solubility risk	Water-insoluble form; ferrocyanide cannot dissolve in water or gastric fluids	Safe
Gastrointestinal absorption	Very low (0.25-0.42% in humans); insoluble form absorbed even less; mostly passes through unchanged	Safe
Bioaccumulation	No accumulation in tissues; excreted unchanged in feces	Safe
Acute toxicity	Very low (LD₅₀ >2000 mg/kg in rats)	Safe
Chronic toxicity	No adverse effects at permitted levels; target organ at high doses: kidney (with NOAEL identified)	Safe
Genotoxicity	Negative Ames test; negative chromosome aberration tests; negative micronucleus tests	Safe
Carcinogenicity	No evidence of cancer in long-term rat studies	Safe
ADI (Acceptable Daily Intake)	0.03 mg/kg body weight/day (expressed as ferrocyanide ion); group ADI for E535-E538	Safe at all realistic consumption levels

Actual Consumption vs. Safety Limit

Real exposure calculation:

Maximum permitted level: 20 mg/kg in salt

Typical salt consumption: 5-10 g/day

E538 intake at max level: ~100-200 μg/day (0.0001-0.0002 mg/day)

EFSA ADI for 70 kg adult: 2.1 mg/day

Safety margin: 10,000x between actual consumption and ADI limit

✅ Safety Reassurance: E538 is completely safe. EFSA’s 2018 re-evaluation is definitive. The cyanide cannot be released at physiological conditions (stomach acid, body temperature, normal cooking) or into water. The water-insoluble form makes it even more resistant to breakdown. The compound passes through your system unchanged. You have a 10,000x safety margin between actual consumption and the safety limit. There is no legitimate health concern.

E538 vs. E535 vs. E536: The Ferrocyanide Family

Property	E535 (Sodium Ferrocyanide)	E536 (Potassium Ferrocyanide)	E538 (Calcium Ferrocyanide)
Chemical formula	Na₄[Fe(CN)₆]·3H₂O	K₄[Fe(CN)₆]·3H₂O	Ca₂[Fe(CN)₆]
Water solubility	Highly soluble	Soluble	Insoluble
Appearance	Yellow crystals (deliquescent)	Yellow crystals (bitter salty)	Yellow powder
Approved uses (EU)	Salt; salt substitutes	Salt; salt substitutes	Salt; salt substitutes
Permitted level	20 mg/kg maximum	20 mg/kg maximum	20 mg/kg maximum
Safety (ADI)	0.03 mg/kg bw/day (group)	0.03 mg/kg bw/day (group)	0.03 mg/kg bw/day (group)
Cyanide safety	Safe; bonded to iron	Safe; bonded to iron	Safest; water-insoluble

Key distinction: All three ferrocyanides share the same ADI and safety profile. E538’s water-insoluble property makes it particularly resistant to decomposition, making it the safest from a moisture-absorption perspective.

The Bottom Line

E538 (Calcium Ferrocyanide) is a completely safe anti-caking agent for salt, despite containing cyanide atoms.

What you should know:

It’s safe: EFSA definitively confirmed; zero safety concern
It contains cyanide: But cyanide is permanently bonded; cannot be released
It’s water-insoluble: Even more resistant to decomposition than soluble E535/E536
It’s approved: EU, FDA, JECFA, most countries
It’s used in salt only: Salt and salt substitutes (EU approved use)
You have huge safety margin: 10,000x between consumption and ADI limit
It passes through unchanged: Not absorbed; excreted in feces
No alternatives needed: E538 is genuinely safe; choosing E538-free salt won’t reduce risk

✅ Bottom Line: E538 is one of the best examples of how “scary chemistry” can actually be safe. Despite containing cyanide, it poses zero health risk because the cyanide is permanently bonded to iron in a water-insoluble crystal lattice and cannot be released, absorbed, or cause harm. Its water-insoluble property makes it even safer than its soluble cousins. Choosing salt with E538 is just as safe as choosing salt without it.

Final Thoughts: Water-Insolubility as a Safety Feature

E538 is instructive because it demonstrates how water-insolubility can be a safety advantage.

E538’s water-insoluble property might seem problematic at first — “won’t it just sit in my stomach?” — but actually, it’s a safety feature. Because ferrocyanide cannot dissolve in water, it cannot dissolve in stomach acid or intestinal fluids either. The entire ferrocyanide complex passes through your digestive system as an inert particle, completely unchanged.

Comparison to soluble ferrocyanides:

– E535 (sodium ferrocyanide): Soluble in water; dissolves but ferrocyanide ion is stable
– E536 (potassium ferrocyanide): Soluble in water; dissolves but ferrocyanide ion is stable
– E538 (calcium ferrocyanide): Water-insoluble; doesn’t dissolve; passes through unchanged

All three are safe, but E538’s water-insolubility is an advantage: the ferrocyanide complex remains locked in place and cannot release cyanide even theoretically.

Why manufacturers use all three:

1. Cost considerations: Different prices for different salts
2. Formulation preferences: Some prefer soluble, others insoluble
3. Specific applications: E538 particularly good for humid conditions (won’t absorb moisture into solution)
4. Regulatory approval: All three approved; manufacturers choose based on practical factors

Why E538 is rarer than E535/E536: Soluble salts are often easier to formulate and handle industrially. E538’s insolubility requires different handling procedures. Most manufacturers prefer the more familiar E535 or E536.

Consumer takeaway: If you see E538 on a salt label, you’ve found a high-quality anti-caking agent. It’s safe, approved, and poses zero health risk. Its water-insoluble property is actually a safety advantage — the ferrocyanide cannot possibly dissolve and release cyanide because it cannot dissolve in water at all.

The bigger picture: E538 exemplifies how modern food safety science works — rigorous evaluation of actual safety mechanisms (bond strength, solubility, absorption), not reflexive fear of chemical names. The water-insolubility of E538 was evaluated by EFSA and confirmed as safe. This gives confidence that modern regulatory systems understand chemistry deeply.

What is E538? – Complete guide to understanding Calcium Ferrocyanide