Why Do Boiled Eggs Have Green Yolks? When a hard-boiled egg sometimes reveals a greenish, grayish or even blackish ring surrounding the yolk, it’s a sign that the egg wasn’t cooked properly.
So, what’s up with that icky discoloration? The discoloration is evidence that the egg was overcooked. Overcooked meat dries out, rice gets burned, and hard-boiled eggs become discolored.
Unlike the other foods, the eggs are still perfectly edible — and taste good — they just look, well, kind of gross. Despite the less-than-appetizing color that results, the reaction between iron and sulfur that occurs when you boil your eggs for too long does not make them unsafe for consumption.
In conclusion, now we know why do hard-boiled eggs sometimes have green yolks. To avoid the dreaded gray ring in the future, I would suggest following one simple, yet foolproof, technique: Set a timer.
Now for the scientific part of this explanation: what does that coloring mean? The green ring forms when you overheat the egg, causing hydrogen and sulfur in the egg white to react and form hydrogen sulfide gas.
The hydrogen sulfide reacts with iron in the egg yolk to form a grayish-green compound (ferrous sulfide or iron sulfide) where the white and yolk meet.
While the color isn’t particularly appetizing, it’s fine to eat. You can keep the yolk from turning green by only cooking the eggs long enough to harden them and then chilling the eggs as soon as they have finished cooking. One way to do this is by running cold water over the hot eggs as soon as the cooking time has elapsed.
In order to get the just-firm, perfectly golden yolks of hard-boiled eggs, this is the method I’ve found to be most reliable:
*Bring a pot of water to a boil first (yes, before your eggs go in).
*Gently place the eggs into the pot of boiling water using a slotted spoon.
*Cover the pot and turn off the heat.
*Let stand for 8 minutes (set a timer!).
*Using the slotted spoon, remove your eggs and immediately plunge them into an ice bath until cool (I’ve also found that my egg shells generally slip off easily using this method).
*Remove shells and enjoy glorious hard-boiled eggs.
Content for this question contributed by Lionel Brockman Richie, Jr., resident of Tuskegee, Alabama, USA