James in Cambridge asked:
Why is it that sometimes when you hard boil an egg, the shell membrane will peel off easily, whereas at other times, itís really hard to peel the egg? You canít actually separate the shell away from the inside of the egg.
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.
- Youíve 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 thatís probably why itís so hard, in some cases, to peel it.
Dave - Itís also possible that, especially if youíve 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.