Here's what people need to know about the gross green ring around the yolk of hard boiled eggs: it's not the egg's fault -- it's yours. And, it doesn't have to happen.
When a hard boiled egg is cut open to reveal a greenish, blackish ring surrounding the yolk of an egg, it's a sign that the egg wasn't cooked properly. So, essentially, it's your fault. (Or whoever made the hard boiled egg for you.) The discoloration is evidence that the egg was cooked for too long. Meat dries out, rice gets burned, and hard boiled eggs become discolored. Unlike the other aforementioned foods, the eggs are still perfectly edible -- and taste good -- they just look, well, kind of gross.
Now for the scientific part of this explanation: what does that coloring mean? The greenish-gray color is visual evidence of the formation of iron sulfide where the yolk and white parts of the egg meet. It happens because the iron from the yolk reacts with hydrogen sulfide from the white when it's been overcooked. Totally harmless, but it doesn't have to happen.
Here's how to avoid it: learn to boil an egg for the right amount of time, because this reaction only happens when a boiled egg is overcooked. There's more than one way to boil an egg perfectly, but the method and timing outlined below has never failed us.
- First, cook your eggs in a saucepan that's large enough to hold the eggs in a single layer.
- Second, cover the eggs with water, bring to a boil, then turn off the heat. Put a lid on the saucepan.
- Let the eggs sit in the boiled water for 15 minutes.
- Run cold water on them to stop them from cooking any further.
- That's it.
Now go enjoy beautiful yellow yolks forever more.