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Boiling down the specifics of a hard-cooked egg

If you really want to know how to cook a hard-boiled egg, it helps to know something about what's going on inside it.

April 07, 2012
  • Kirk McKoy / Los Angeles Times
Kirk McKoy / Los Angeles Times (m1px21pd20120405102234/600 )

The reason this hard-boiling technique works so well is really pretty simple, but it helps to know a little bit about how eggs work.

Eggs consist largely of protein — mixed with mostly water in the white, or albumen, and mixed with fat and water in the yolk. When the eggs are raw, the proteins are like strands of yarn curled up in little balls. As the eggs are heated, these strands relax and unfold. As they unfold, they bump into each other and link up. (Fun fact: This is why egg "whites," which are clear when raw, are white when cooked — the unfurled, linked-up proteins block light from passing through.)

The higher the temperature, the tighter the linkages, at first getting firmer but gradually getting tight enough that they squeeze out the moisture. That's bad. Even worse, as the egg white warms, it gives off hydrogen sulfide gas and when that gas meets the iron particles in the yolks, it forms iron sulfide, which is what that green ring is made from.

The perfect temperature for a hard-cooked egg is right around 160 degrees (actually a little lower for the white, which doesn't have any fat to lubricate it, and a little higher for the yolk, but we'll settle for an average).

This cooking technique is so spot-on because by the time the temperature of the inside of the egg warms to that point, the water has already started cooling, preventing any overcooking. (In my experiments, I found that between 12 and 13 minutes after I removed the pan from the heat, the temperature of the water had already dropped below 160 degrees — essentially stopping the cooking process.)

Furthermore, because the eggs are heated gradually (as opposed to dropping cold eggs into boiling water), you won't have any problems with shells cracking during cooking. That cracking is caused by the sudden heat causing the air pockets inside the eggs to expand rapidly. When the eggs are heated gradually, the air can leak gently out through the porous shells (indeed, you can see the bubbles emerging).

And one final hint: The eggs will be much easier to shell if after cooking you give them a gentle crack and then put them in an ice water bath. The cold shrinks the egg just enough to pull it away from the shell. This will be even easier if you use older eggs — the chemistry of the egg changes as it ages, making the white less "sticky."

— Russ Parsons

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