Why does a green ring appear around hard-boiled eggs?

You have probably seen it happen yourself. You are making a fresh batch of hard-boiled eggs to put in a salad, as a snack, or for deviled eggs, and when you cut one open, there it is: a dull green ring around the yolk like a bruise on a piece of meat.

Although such an effect might appear to be an indication of something having gone wrong, the reality is not so bad and even easier to correct than you might imagine.

This is a natural, but ugly, chemical reaction that forms this greenish-grey ring. The yolk of the egg contains iron, and when the eggs are overcooked, or cooked too hot, the iron combines with the sulphur of the white.

The heat makes them react to form iron sulphide – a non-toxic compound that forms a discoloured layer around the yolk. It is perfectly safe to consume; however, the truth is that it does not make the eggs any more attractive.

The positive thing is you do not have to be a gourmet chef to prevent this. You can make perfect hard-boiled eggs with only a couple of minor adjustments to your technique.

Begin with a cold strategy. Put your eggs in a saucepan one layer at a time. Do not overcrowd them; give some breathing space to the eggs. Cover them with cold water such that there is approximately 3 to 5 centimetres of water over the eggs.

Heat the water to a light boil on medium heat. As soon as you observe a rolling boil, switch off the heat. Turn it off, yes, really.

Then, put the lid on the pot and allow the eggs to stand in the hot water precisely 9 to 12 minutes, according to the size of your eggs and how firm you want them. After nine minutes, you will have a slightly soft centre, and twelve minutes will provide a fully cooked, dense yolk.

The moment the timer rings, it is important to halt the cooking process at that point. Make a bowl of ice water, or at least very cold water, and put the eggs into it. This quick temperature change does stop any further cooking and makes peeling them much easier.

Your eggs should be cool enough to handle after five minutes of the ice bath. Shell the eggs and peel them under running cold water. The shell will peel away far more readily when the eggs are well cooled.

In case you are planning in advance, remember a couple more tricks. Eggs that are a few days old, say five to seven days, are easier to peel than very fresh eggs. When you are afraid that the shells will crack during cooking, you can add a pinch of salt or a dash of vinegar into the water to stabilise the shells.

Eggs that are hard-boiled may be kept in the refrigerator in their shells for up to a week. When they have been peeled, they should be consumed within two or three days to ensure they are fresh and tasty.

Could you please share the key to achieving hard-boiled eggs with golden-yellow yolks and no green ring? The key factors are time and temperature. Do not overcook, and chill them rapidly. That’s it. This easy routine yields healthy, tasty, and beautiful eggs.