Crazy But Able

My previous post on the rice steamer has spawned a little discussion in the comments about hard boiled eggs, so I thought I’d share a few geeky facts about cooking eggs.

First, have you ever had a hard boiled egg with a greenish grey yolk? Chemically, there are a few interesting geeky things going on here. When cooked, sulfur is released from the white of the egg, which combines with hydrogen ions to form hydrogen sulfide. And when the hydrogen sulfide gas encounters iron on the border of the yolk of the egg, it forms iron sulfide. That iron sulfide is the icky green-grey gunk on the outside of your yolk. All three of these chemical reactions that help produce iron sulfide are improved in efficiency the hotter and longer you cook your egg.

The second cool geeky thing about cooking eggs is why the white hardens the way it does. Raw egg white is full of tightly coiled proteins. They’re just hanging out there, not doing much. This is why you can see through raw egg white: the proteins aren’t interacting with each other; they’re mostly suspended. (Think of peas suspended in clear jello: if you hold up a block of it you can still (mostly) see around the peas.)

But start to cook egg white, and those proteins unwind into lose strands. The loose strands start interacting with each other (imagine filling a drawer full of loose strings; now mix them up, and then try to get them untangled.) Keep cooking the egg white (mixing the drawer full of strings), and the bonds (knots) get more numerous and tighter. This process squeezes water out from between the proteins and makes the white more rubbery. The excess water diffuses through the shell as steam (another symptom of overcooked eggs is when they crack from the prodigious steam buildup.) Steam is why you can’t make a hard boiled egg in the microwave: the microwaves cook parts of the egg too fast and generate too much steam, too fast. (Think about the tightly coiled proteins quickly unwinding, forming a sponge like structure for a second, then slamming together to squeeze the water out, which quickly becomes steam.) My nephew Gerry learned this the hard way and destroyed my sister’s microwave (blew the door right off its hinges and across the kitchen, after only two minutes of cook time).

So if you just want a hard boiled egg, then you want to time it right if you care about how rubbery your egg is. But if you want to make deviled eggs, then you probably want to cook the eggs longer, to make them less wobbly and keep the devily bits from slipping out.

To get back to the peas (starting a new paragraph to carefully avoid mixing my own metaphors), these mixed up egg proteins are why the white is opaque after its cooked. Imagine for a moment that our jello can get back to a liquid state, and lets dump the whole thing in a blender. *GRRRRNNNNNNNRRRRrrrnn…* Set the jello again, and now its a uniform green, because the peas have “uncoiled” and we can’t see around them anymore.

Incidentally, all this uncoiling and re-tangling of proteins is why you have to be careful making custards. If you heat a custard too hot or too fast, or for too long, then the egg proteins quick fly apart and just as quick fly back together again and squeeze water into the rest of your ingredients. And have you ever seen a cheesecake with a crack (or many little cracks) in the top? There’s those pesky eggs again: overheated eggs shrink when cooled (especially if cooled too rapidly), forming cracks in your cheesecake.

We can use this last fact to our advantage. Don’t you hate hard boiled eggs that are really hard to peel? Plunge your hardboiled eggs into cold water right after cooking. That quickly shrinks the egg white, separating it from the shell. And rapidly cooling the eggs helps put a stop to those iron sulfide generating reactions I mentioned earlier; it’s a win-win situation.

One final geeky egg peeling tip: it also helps to use new eggs. The ph of new eggs is different from that of old eggs, and while I don’t know which one is more acid or alkaline, because of the ph difference it is easier to peel new eggs than old.

This entry was posted on Thursday, January 13th, 2005 at 1:46 pm and is filed under recipes. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.