Why does a green ring form on the yellow of the hard-boiled egg? (And how to get rid of it?)

perfect hardcore egg every time:

    Start with cold water.

Put egg with a saucepan, pour into the cold – 2.5-5 cm above the level of eggs.

    Heat it slowly.

Bring the water to a boil over medium heat. Turn off the stove as soon as it boils.

 

    Leave it

    Standing the eggs in the hot water.

Cover the dish with a lid and leave the eggs to stand in the hot water.

9 minutes to achieve a slightly creamy yolk consistency