A green or grey ring around the yolk of a hard-boiled egg can look alarming, but it usually does not mean the egg has gone bad. In most cases, the discolouration is harmless — the result of a natural chemical reaction that happens during cooking, not a sign of spoilage.
The ring forms when sulfur compounds released from the egg white meet the iron naturally present in the yolk. Together they create iron sulfide, which appears as that familiar greenish or greyish band around the yolk. It shows up most often when eggs are cooked for too long or exposed to too much heat. Though the eggs may look less appetising, they are generally safe to eat, provided they have been handled and stored properly.
Preventing the ring is fairly simple. Cook the eggs gently for around nine to twelve minutes, depending on their size and how firm you like the yolks, and avoid letting them boil for too long, since excess heat encourages the reaction. As soon as they are done, transfer them straight into ice water or very cold water — this stops the cooking process, keeps residual heat from dulling the yolks, and also makes the shells easier to peel.
It is still worth knowing the real warning signs: a cooked egg with an unpleasant smell, a slimy texture or any other suspicious change should be thrown away rather than eaten. And storage matters too — refrigerate hard-boiled eggs within two hours of cooking, keep them chilled, and they will generally keep for up to a week. With the right cooking time, quick cooling and proper storage, you can enjoy eggs with firm whites and beautifully yellow yolks — no green ring in sight.