The green (or gray-green) ring around a hard-boiled egg yolk is completely harmless — and it’s caused by a simple chemical reaction.
🧪 What Actually Happens
When eggs are overcooked or cooled too slowly:
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The egg white releases hydrogen sulfide gas (a sulfur compound).
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The yolk contains iron.
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The sulfur reacts with the iron at the surface of the yolk.
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This forms iron sulfide, which looks greenish-gray.
That ring forms where the yolk and white meet.
🥚 Why It Happens
You’re more likely to see it when:
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Eggs are boiled too long
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Eggs sit in hot water after cooking
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Eggs aren’t cooled quickly
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Older eggs are used (they contain more reactive sulfur)
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