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Originally Posted by P.Aitken
scott123 wrote:
Eggs begin with a very tiny air pocket that grows over time. They crack
because this air pocket expands when heated. The fresher the egg, the
smaller the air pocket, the less tendency toward cracking. At the same
time, though, the fresher the egg, the harder they are to peel.
This is not completely accurate. Egg shells are permeable to air, so as
the air exmands it passes thru the shell - this is why you see bubbles
rising from eggs in the hot water. The main reason eggs crack while
being boiled is that they are already cracked - small hairline cracks
that are not visible initially. Knocking around in too-rapidly boiling
water is another reason. Temperature shock is not involved. I remember
seeing Julia Child take eggs from an ice water bath directly to boiling
water with no cracking.
Peter
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The bubbles you see rising from boiling eggs are NOT from the air inside the shell. It's from the air trapped in the tiny crevices on the outside of the shell. Egg shells are permeable to air, but the permeability isn't instantaneous. We aren't talking about an air conditioner filter here. The air exchanged from the outside to the inside of the shell occurs over days, not seconds.
It's basic physics. The torque required to break a shell from the inside is very small. Air expands when heated. Hairline cracks/structural integrity/rough handling certainly play a role, but the age of the egg and the size of the air pocket are critical in the egg breaking equation.
As far as Julia Child's incontrovertable proof... most refrigerators are in the 35-40 degree realm. Ice water can't be colder than 32 (unless it's salted ice water). Those few degrees difference isn't going to increase the thermal shock that dramatically. Besides, she was probably just using fresh eggs.
Age isn't the best measurement for determining the size of the air pocket. Depending on the storage conditions and the composition of the shell, evaporation can vary greatly. The most foolproof method is to shine a light through them (called candling). Leave some eggs in the fridge for a few months and candle them to make sure the air pocket is substantial. If the pocket is sizable, I promise you, they will all crack when they hit boiling water, regardless of the presence of hairline cracks or whether they knock around or not.
Every one of them.