Since I have been eating many of hardboiled eggs lately, it would be an inevitable occurrence for me to leave eggs boiling for too long on the stove top. That happened last week. After turning the flame off and letting the eggs sit for several hours, I later observed that those eggs turned out just fine with only a small yellow spot just below the shell where the bottom of the egg met the hot pot surface. This yellow section formed from overheating. These areas tend to be a bit more hard than normal from the hard boiled egg white (albumen). The rest of the hard boiled egg is fine.
Here is where it gets interesting…
I left one of those hard boiled eggs on top of the stove for three days at room temperature. After the first bite I saw that most of the yolk was green except a small section of the inner core. There was no foul smell or taste. In fact, it tasted quite normal and as expected. So I completed eating the rest of the egg wondering why the yolk was or became dark green.
Apparently, the green color in the egg yolk is the result of overheating eggs (and can also be caused from high iron content in the tap water used to boil the eggs). Heat causes the hydrogen and sulfur inside the yolk to react and form hydrogen sulfide gas (which stinks and is the source of the rotten egg smell). The hydrogen sulfide gas then reacts with iron, also present in egg yolk, to form ferrous sulfide, a gray-green compound on the yolk where it meets the egg white. It didn’t look pretty but was completely edible and safe.
What is odd is that the chemical reaction only seemed to happened with the one egg I left out for several days. So, if this is hydrogen sulfide gas is formed from overheating the water when making hard boiled eggs, then an extended time frame is another requirement for the chemical reaction between the gas and iron in the egg yolk to take place. From my past experience with Easter eggs, this may explain why the hard boiled yolks tended to be dark gray or green.
To eliminate the green ring around the egg yolk, cool the hard boiled eggs as quickly as possible by using cold tap waler or even ice cubes. Then refrigerate the eggs. Refrigerated eggs are good for up to one week.
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