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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