Why boiled eggs turn green

The grey-green ring is iron sulphide, it is harmless, and it means you overcooked the egg. What causes it and how to stop it happening.

The ring is iron sulphide. Heat drives sulphur out of the white and iron out of the yolk, and where the two meet they react. The result is grey-green, harmless, and almost tasteless on its own.

It is a symptom rather than the problem. An egg cooked long enough to go green has usually gone chalky and dry too, and that is what you actually taste.

The two things that cause it

Too long in the pan

The reaction needs time and heat. A large egg pulled at twelve and a half minutes will not have it. The same egg at eighteen minutes will.

This is worse at altitude, because everything takes longer and people compensate by guessing rather than adjusting properly.

Too slow to cool

The shell holds a lot of heat. An egg left to cool in the pan keeps cooking for several minutes after you turn the hob off, and that tail end is often where the ring forms. Draining and leaving them in the sink is not enough.

Iced water stops it immediately. This is the fix that costs nothing and gets skipped most often.

Older eggs go green more easily

As an egg ages its white becomes more alkaline, and the sulphur reaction happens more readily in alkaline conditions. So the same eggs that peel nicely at two weeks old are slightly more prone to the ring.

It is a small effect next to timing, and not a reason to use fresh eggs. Just pull them on time.

Is it safe to eat?

Yes. Iron sulphide in these quantities is harmless and food safety agencies treat green-ringed eggs as perfectly edible. The colour puts people off and the texture is worse, but nothing about it is dangerous.

If you are making egg salad or devilled eggs and the ring is only cosmetic, mashing hides it entirely.

Other boiling times

Egg guides

Common questions