Why does boiling an egg make it solid!!?

In theory, shouldn’t something that you heat up give energy to the individual atoms in the solid, thereby enabling them to separate? And thus the particles should turn from Solid–>Liquid—>Gas?

So why then does the yolk of a boiled egg turn from mushy liquid inside (the yellow egg yolk - I think that it’s fat if I recall) to a solidified substance upon a long time of boiling in hot water (say for 25 mins+)…?

Anyone with any answers?

Because in an egg, the heat causes chemical reactions to occur, not merely a state change. IANAC but I think the heat causes the proteins to bond into longer chains or something.

Bend, not bond, IIRC. The proteins curl up, thus locking into one another like velcro. This is also why they turn opaque (seen as white) - light is no longer able to pass through this tangled mess.

This is what lots of proteins do (e.g. meat), which is why you get discolorations when you cook things. Plus, the curled up proteins squeeze water out of their cells, which is why you notice shrinkage too (that Big Mac is a 1/4 pounder of pre-cooked weight).

A good book on all of this is: “On Food and Cooking” Great stuff on the chemistry and history of preparing food. I loaned my copy to a friend, so I can’t verify what I said 100%, but I’ll check in a day or two.


Incidentally, boiling an egg for 10-15 minutes makes a pretty rubbery egg, since the white is being overcooked. Also you’ll get that nasty-looking (if harmless) green substance around the yolk (caused by sulphur compunds, IIRC). To make the perfect hard-cooked egg, place the eggs in a pot with enough water to cover them completely, then add a teaspoon of salt and bring it just to a rolling boil. Remove from heat, cover the pot, and let sit for 15-20 minutes. Then dump out all the hot water and fill the pot with cold water, just covering the eggs and let it sit for another 10 minutes. All the ones I’ve done like this have come out perfectly, with no trace of green stuff on the yolk. And I’m told that fresh eggs are harder to peel than ones that have sat in the refrigerator for 2 weeks, but I’ve not tested that.

And on the same topic, if you think about it, you could ask the same question about all manner of foods that you cook. Breads, cakes, all sorts of things made of liquids and batters undergo chemical changes that essentially turn them from liquid to solid. The energy that you’re concerned about goes into moving the molecules about and forging the new bonds and configurations.

I can verify this is true. I have heard of another method to make eggs easier to peel, When they are almost done cooking swish the pan around a bit so they knock together and crack slightly. I haven’t tried this method though.

Very interesting.

So does the fact that when you peel a hard-boiled egg, you don’t get a splash of water, mean that there was no (or very little) water in the egg to begin with? That it’s all protein, basically? Or is the water being pushed out through the shell somehow?

Egg albumen is mostly water. Take a look at this site. It gives a pretty good explanation in layman’s terms about what is happening to the insides of an egg when it’s cooked.

Just on the topic of proteins, I thought heating caused the opposite - the chains uncurl and eventually break apart. I though this is why its so important to maintain our particular body heat - too high and the proteins start “melting”.

Isn’t the term “denaturized”? Or am I getting this confused with a different process of proteins.

Excellent, thank you. However (from the link):

So… that big juicy steak gets firmer when it’s cooked because now it’s basically got “scab-ness” woven throughout?

Yes! Tasty, eh? :smiley:

[homer j simpson]
Mmmmmmmm…steaaaaaaak! Graaarggglllllaaahhh!
[/homer j simpson]

That’s mostly correct. Unless they are stabilized by internal disulfide bonds, most proteins will denature (lose their natural shape) in boiling water. However, it takes much harsher conditions, such as boiling concentrated acid, to actually break the bonds between the amino acids which make up the protein chain. When that happens, it’s called hydrolysis, not denaturation. Anyway, when a globular, i.e. round shaped, protein like ovalbumin denatures, its’ peptide chain goes from a neatly wrapped compact configuration, like a ball of string, to a random, spread out shape, like a ball of string string after the cat has got to it. This spread out tangle of linked amino acids gets tangled up with all the other similar proteins and you end up with a great big solidified mess.
There’s actually not much classical chemistry involved here, just molecules changing shape and getting caught up with each other. There are few covalent or ionic bond changes involved in denaturation, and the subsequent gel formation, just a lot of elaborate physical chemistry.

Yes, same thing with Alzheimers plaques, irreversible protein denaturation. Figuring out to reverse* that* is a big deal these days.

Let me second that opinion! It’s a rather dry book, written around 1985, but it has all the essential chemistry/physics and history of our everyday foodstuff. (My only cmplaint is that it’s rather US-centric, but that’s maybe to be expected from a US based author.)

The full title is On Food and Cooking - the science and lore of the kitchen, written by Harold McGee. Therein he also explains why boiled eggs kept warm develop a green film around the youlk. It’s hydrogen sulfide that migrates from the albumen towards the iron-rich yolk, where they form a greenish compound. The best way to avoid that is to cool the eggs quickly after boiling.

The most likely source of the water splashing out when you peel an egg? The stuff you boiled it in. :slight_smile:

Notwithstanding Squink’s accurate answer, I take exception to the “scab” part. Steak contains extremely little blood, so the firmness is not due to actual blood scab-ness. (The red juice is other stuff from the muscle fiber, but it’s not blood.)

Scab-ness was a little extreme.:wink:
With regard to the water content of cooked eggs; consider what happens to the water when you make Jello[sup]®[/sup]. All the water gets trapped in the matrix of denatured gelatin. It’s the same way, only more so, with the water/albumin mix inside a cooked egg.

I can’t get my book back! Aargh!

It seems I was wrong, but I could swear that the book showed pictures of straight-ish protein chains that curled, which explained why the albumen turned opaque.

OK, I’ll make up for it: How can you easily tell if an egg is hard boiled or raw? Spin it on the counter. Hard boiled eggs will spin like a top because they are solid, whereas raw eggs will wobble and roll and stop quicker because the liquid inside is slushing around. You can also use a bowl of water to test, but that is a pain.