The below reference to hard boiled eggs spurred reader Tim to ask why it is that over-boiled eggs have that green film on the yolk. He said he’s looked it up at various points but has never gotten a satisfactory answer. Which is just the kind of explanatory challenge that a spaz like me lives for. So Tim, pour yourself a nice beverage and settle back in your chair.
Those green/grey films are a result of couple of different iron sulfides (ferric sulfide and/or ferrous sulfide) that form when eggs are heated over a certain point. Egg whites, you see, are rich in protein. Two of those proteins in particular, methionine and cysteine, contain sulphur. When methionine and cysteine break down they release their sulphur in the form of hydrogen sulfide, which is what gives old eggs that characteristic smell.
But age isn’t the only thing that causes methionine and cysteine molecules to break into pieces. Heat does it too, and the higher the heat, the more breakdown you get, up until about the boiling point, where hydrogen sulfide is being produced at about 200 times the rate that it is when an egg simply sits out on the counter.
And hydrogen sulfide is reactive. When it comes into contact with iron, it produces the green/grey ferric sulfide and/or ferrous sulfide I mentioned up above. And where is the most iron found inside an egg? Why the yolk of course. Which is why you get that greenish film around the surface of the yolk. It’s the point at which the reaction happens: where the hydrogen sulfide-rich white and iron-containing yolk meet.
This reaction will happen almost anywhere you find over-heated eggs. You may have seen evidence of iron sulfides in a pan of scrambled eggs that got a little too hot — and/or were prepared in a cast iron pan. I usually notice those greenish tones in large chafing dishes of scrambled eggs, the kind you routinely find at breakfast buffets. The heat it takes to prepare massive batches of scrambled eggs can be extreme.
Obviously there’s nothing in all this that’s dangerous to a human. The worst all this overheating will do is over-coagulate the remaining intact proteins, causing rubbery texture. The abundance of hydrogen sulfide can also give the eggs more/stronger sulfurous flavor notes than they would have otherwise had. It’s yet another reason to obey the time-tested wisdom: where eggs are concerned, always cook them as gently, gently.