Why does an egg explode at all?
If you stick a potato in the microwave without piercing its skin first, steam pressure can build up under the skin and cause the potato to detonate. That's a simple mechanism for an explosion, the researchers wrote, similar to a grenade going off and shattering the device's outer shell.
But a hard-boiled egg doesn't have a skin with the high tensile strength of a potato's, and an eggshell — designed for a baby bird to peck through — isn't strong enough to contain high internal steam pressure. There is a membrane between the white of an egg and its shell that might allow pressure to build up, but that comes off when you shell an egg and shelled eggs still pop.
The researchers suggested an alternative explanation.
The yolk of an egg, they discovered with their meat thermometer, heats up much faster than the surrounding water. Perhaps, they reasoned, tiny pockets of water are getting trapped inside the proteins and getting superheated.
At normal air pressure, those pockets would have room to expand and turn into steam. But inside an egg, pressure from surrounding, hardening proteins might be forcing the pockets to remain liquid even as their temperatures climb.
But disturb one of those pockets, let it expand, and the water molecules would rush to fill the void — expanding, disrupting the surrounding tissue, and allowing any other pockets to flash through a phase change at the same time. The resulting collective bubble-bursting would tear the egg to bits, flinging the pieces far and wide in a way that might resemble a more typical explosion under pressure.
"To an observer, the egg appears to have exploded," the researchers wrote in the paper, but, "it is probably more accurate to describe the phenomenon as a rapid boiling of superheated water.