Have you ever noticed the green film that sometimes forms around the yolk in a hard-boiled egg? It doesn't look very yummy - what is that stuff? Eggs don't start out with anything green in them! The green film is iron sulfide (chemical formula, FeS), and it forms during cooking when iron in the yolk combines with some of the egg's proteins. You can't taste it, and it's not dangerous to eat.
How can you avoid forming this green film? You need to cool the eggs quickly after they are done cooking. As soon as the eggs are finished, pour off the hot water and cover them with cold water. The reaction happens at high temperature, and by cooling the eggs, you stop it from occurring!
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