Why are some hard boiled eggs hard to peel?

29 November 2009

Question

Why is it that sometimes when you hard boil an egg, the shell membrane will peel off easily, whereas at other times, it’s really hard to peel the egg? You can’t actually separate the shell away from the inside of the egg.

Answer

Chris - Well, I have one theory on this which is that, when you hard boil the egg, what you do is to create a potential vacuum because the proteins in the egg white denature and they form a solid in close apposition to the inside of the shell, squeezing out any air that was in there. And so what you basically get is the egg-equivalent of the thing that holds your tax disc to your car windscreen. - You've got a potential space there which if you have the egg shell's not separated, is being held squeezed on to the egg on both sides by atmosphere. And so you've basically got to create a bit of a vacuum before you can separate the two, and I think that's probably why it's so hard, in some cases, to peel it.Dave - It's also possible that, especially if you've got an older egg, the white tends to shrink a bit. So you get a nice big air sac at the top when you put that into hot water, and that air is going to expand, and it might squeeze its way around in between the membrane and the shell and that might be making it easier to peel the two apart.

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