You’re standing in your kitchen, patiently waiting for your hard boiled eggs to finish cooking… only to slice into one and find a greenish-gray ring around the yolk. Aside from looking funky, the off-putting color can make you ask a lot of questions — like, um, why the heck do hard boiled eggs turn green? And are they safe to eat?
The off-putting ring of color has to do with how long you waited to take the eggs out of the hot water. Maybe you forgot to set a timer or walked away for longer than you intended, but even just a few minutes can make a big difference.
The University of Nebraska-Lincoln explains, “The ring is caused by a chemical reaction involving sulfur (from the egg white) and iron (from the egg yolk), which naturally react to form ferrous sulfide at the surface of the yolk.” This happens when you overcook your eggs or if there’s a high amount of iron in the water you boil.
Either way, the university claims you can avoid the problem by cooling the eggs immediately after taking them out of the water (or oven) by running cold water over them or putting them in a bowl of ice water. This should help even if you accidentally cook them a bit too long.
The culinary pros at Food52 confirmed this after experimenting with several batches of hard boiled eggs. They found that cooking them for 13 minutes resulted in the grayish ring — but only if they neglected to put them in cold water afterwards. Those that they cooked for the same length of time and allowed to chill out immediately did have a chalky yolk texture, but no discoloration.
As for whether or not those gray-green yolks are OK to eat, the University of Nebraska-Lincoln explains they aren’t harmful… but Food52 warns they won’t wow you with flavor (or taste of much at all), either.
Moral of the story: Always cool your hard boiled eggs as soon as they’re done cooking!