Do you ever peel and cut open an hard-boiled egg and find a very unappetising grey-green ring around the yolk? There’s no need for alarm, as there’s a simple explanation as to why this happens, and how to fix it.

The green ring around the yolk of a hard-boiled egg happens because hydrogen in the egg white combines with sulphur in the yolk. This is usually a result of overcooking – when you leave the eggs to boil for too long or at a temperature that is too high. Likewise, if you find that you still get this greenish yolk ring in a hard-boiled egg even when not overcooking it, it is likely the case that the water you’re boiling the egg in has a relatively high amount of iron in it, resulting in iron sulphide being produced more rapidly than you might otherwise see. The good news is that it is perfectly safe to consume these eggs, but there is also a way to avoid them if you would like to have a pleasing display of your hard-boiled eggs – usually the case when having family or friends over for a meal.

• Keep the eggs in a single layer in the pan.
• Cover the eggs with water, bring the water to a boil over high heat, then turn off the flame.
• Let the eggs sit in the hot water for 15 minutes for large eggs (12 for medium, 18 for extra large).
• Run cold water over the eggs after they’re done so they don’t overcook.

Bonus tip: Adding salt or vinegar to the water you boil eggs in will speed up the denaturing process within the egg.  Why is this helpful in hard-boiling eggs? If the egg’s shell cracks, you might notice some of the egg escaping before it has time to harden.  The vinegar or salt helps reduce the chances of such leakage.

Have you encountered similar problems before, and how did you fix them?