The greyish-greenish ring you sometimes see around the yolk of hard-boiled eggs is a result of a chemical reaction. When eggs are boiled at 70°C, the sulfur in the white converts to hydrogen sulfide. This gas smells like rotten eggs; however, in a boiled egg, it reacts with iron to form a compound called ferrous sulfide, which is greyish green. The proteins in the egg also entangle with one another at temperatures over 77°C.
Now, why does this happen more at times than others? The key is the cooking time and temperature. If eggs are boiled for too long or at too high a temperature, or if they are cooled slowly, there’s a higher chance of this reaction occurring.
To avoid the grey ring:
1. Avoid over-boiling. It’s usually enough to boil eggs for about 9-12 minutes.
2. Once they’re done boiling, transfer the eggs immediately to a bowl of cold water. This rapid cooling stops the cooking process and reduces the chance of that grey ring forming.
Remember: The grey ring is harmless! It might not look as appealing, but the egg is still perfectly safe to eat.
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