A greenish-gray ring may appear around a hard-cooked egg yolk. It's unattractive, but not harmful. 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.
Those funky, discolored rings in your hard-boiled eggs may make you wonder if they are harmful to eat. Fortunately, the United States Department of Agriculture (USDA) website clearly states the green or gray color in the eggs is “safe to consume.” So, if you happen to see a little ring around your egg, don't worry!
A: The green ring around the yolk of a hard cooked egg happens because hydrogen in the egg white combines with sulfur in the yolk. The cause is most often related to boiling the eggs too hard for too long. The green ring can also be caused by a high amount of iron in the cooking water.
In many cases, it's simply a matter of changing the cooking time or temperature. Rapidly cooling hard-boiled eggs can help to minimize the green appearance on the yolks. Whether your egg yolks are a rich golden hue or slightly yellow in appearance, the color of the yolk shouldn't be a huge concern.
The color of the yolk will change based on the diet of the hen who laid it, so the shade of yellow or orange nothing to do with freshness. Instead, inspect the egg white, or albumen. If it is pink, green, or iridescent, the egg has been contaminated by Pseudomonas bacteria and is not safe to eat.
They should discard eggs with any unusual discoloration inside, such as a pink, iridescent, or greenish egg white or yolk. However, eggs that develop a green ring on the hard-cooked yolk after cooking are safe to eat, according to the USDA.
While it's certainly unsightly, it's not dangerous. However, if the yolk or egg white have dark brown, black or green spots on them, it may be a sign that they have come into contact with some type of bacteria like salmonella. Toss these eggs immediately as they're not safe for consumption.
And if you find your eggs are showing signs of being over-boiled, while Prepared Cooks notes that eating overcooked eggs isn't bad for you, they won't taste very good.
The Fresh Egg Water Test
Fill a bowl with water, and carefully place an egg on top. If the egg sinks like a stone, laying down on its side, it's still very fresh! If it sinks but doesn't lay flat—and instead it kind of stands up, wobbling—your egg is OK and probably just right for hard-boiling.
So, what's up with that icky discoloration? It turns out that hard boiled eggs can take on that greenish gray color around the outside of the yolk because of a chemical reaction between the iron in the yolk and sulfur, which is present in the white.
Hard-boiled eggs, peeled or unpeeled, are safe to eat up to seven days or one full week after they were cooked. To make sure they stay fresh, allow the cooked eggs cool a bit, and then store them in a sealed container in the refrigerator.
As soon as the water begins to boil, turn off the heat and cover the pot. Leave the eggs in the hot water for anywhere from 10-12 minutes, depending on how you like your eggs. The 10-minute eggs will have vibrant, creamy yolks, while the 12-minute yolks will be paler and opaque, with a chalkier texture.
The change in color is all but a result of a chemical reaction caused by too high of a temperature: What happens is the sulfur in the egg white and the iron in the egg yolk combine and react with the heat, creating a compound called ferrous sulfide, which is what can give eggs a grayish color.
Keeping Hard-Boiled Eggs Fresh
The shell will help to protect the egg from bacteria, and can help prevent them from absorbing odors from other foods in your refrigerator. A quick beginner's tip is that hard-boiled eggs can be preserved up to 7 days in the refrigerator.
But if you're hunting for hard-boiled eggs that have been dyed instead of plastic eggs full of the best Easter candies, one question has surely crossed your mind: Can you eat dyed eggs after the hunt is done? The short answer is yes, you can eat hard-boiled eggs that have been dyed.
Hard-cooked eggs can also have black or brown spots on the albumen (egg white) from overcooking or burning. Discoloration due to overcooking does not impact food safety and these eggs are safe to eat.
Simply fill a bowl with cold tap water and place your eggs in it. If they sink to the bottom and lay flat on one side, they are fresh and good to eat. A bad egg will float because of the large air cell that forms at its base. Any floating eggs should be thrown out.
If you boil an egg for five or 10 minutes, it becomes firm and cooked. If you boil it for hours, it becomes rubbery and overcooked.
A bad egg will give off an unpleasant odor—that's a tell-tale sign of a rotten egg. Sure, some eggs release a sulfurous smell once hard-boiled—a chemical reaction that occurs when eggs are over-cooked—but a rotten egg will have a distinctive smell.
If you're wondering how to tell an egg is hard boiled, set it on the counter and give a quick spin. Once it's moving, tap your finger on it to stop the spinning. Eggs that are cooked will spin easily and rapidly and stop quickly.
In a properly hard-boiled egg the white should be fully set but not rubbery, while the yolk should be firm and bright yellow.
What colour egg yolk is healthy? Dark orange, light orange or pale creamy-coloured yolks all contain the same nutritional value. But to really get the best from your egg, you need to think about the white, not just the yolk; the nutritional content is split, so eat it all up.
A chemical reaction turns the yolk grey
So what is that grey, or even black color? It's not a living thing, instead, it's a compound, called ferrous sulfide, also referred to as iron sulfide or FeS. This compound can be formed through a chemical reaction between: iron and hydrogen sulfide.