Boiling eggs

Why

During long boiling, the proteins of the white release hydrogen sulphide, which travels towards the centre of the egg and combines with iron from the yolk on its surface to form grey-green iron sulphide. The ring is harmless, but it gives away overcooking – the yolk turns dry and crumbly and the egg smells more sulphurous; cooling quickly in cold water limits it, because the egg stops cooking and the hydrogen sulphide heads for the cold surface rather than the yolk. Hard peeling is mainly down to freshness: the white of a very fresh egg is less alkaline and clings firmly to the membrane under the shell, whereas in eggs about a week old it lets go more easily. Lowering eggs straight into boiling water also makes peeling easier, because the white next to the membrane sets before it has time to stick to it. Young children, pregnant women, older people and anyone with a weakened immune system should only eat eggs with a firm yolk.

How to do it

• Lower the eggs into the boiling water gently with a spoon so they don't crack on the bottom.
• Cook at a gentle boil; a rolling boil knocks the eggs against each other.
• Cook medium (M, 53–63 g) eggs straight from the fridge for roughly 5–6 minutes for soft-boiled and 10–11 minutes for hard-boiled, timed from when they go into boiling water.
• Move the cooked eggs straight into cold water, ideally with ice, and let them cool completely.
• For hard-boiled eggs you plan to peel, use eggs about a week old rather than the freshest ones.
• Peel from the blunt end where the air pocket is, ideally under running water.

If it goes wrong

The green ring won't go away, but the egg is edible and harmless – in a spread or salad nobody will notice. Tap a stubborn egg all over, leave it in cold water for a while and peel it under running water; next time reach for eggs that are about a week old.