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Why microwave eggs explode

Hot splatter on the tongue, loud ringing in the ears: these are the consequences of biting an egg heated in the microwave without thinking.

Hard-boiled eggs do not react well (or, depending on the perspective, they react extremely well)) to microwaves. Heat one in a microwave and, assuming it does not burst while the timer is still running, there is a good chance it will trigger with a burst and a shower of hot sand as soon as it is disturbed.

But how strong is that pop? That is the subject of a lawsuit, and a related scientific discovery, which depends on the acoustics of the exploding eggs. [The Mysterious Physics of 7 Everyday Things]

Somewhere in America, at some point in the past (the details are still unclear) someone entered a restaurant and bit an egg. That egg had been reheated in a microwave and exploded when the poor patron bit his benign skin. The employer, severely burned, sued the restaurant and claimed to have suffered a hearing loss from the outbreak in addition to the most obvious injuries, according to a press release.

Charles M. Salter Associates, a San Francisco-based firm specializing in acoustics, was hired to provide expert testimony in the litigation. Specifically, they were hired to answer this question: Could an egg explode create a pressure wave powerful enough to cause hearing damage?

In an unpublished document presented today (December 6) at the 174th Meeting of the Acoustic Society of America and provided to Live Science, researchers Anthony Nash, vice president of Charles M. Salter Associates, and Lauren von Blohn, consultant Company's acoustics, they described the results of their study on the explosion of microwaved eggs.

Nash and von Blohn individually microwaved about 100 cooked and hard boiled eggs for study. Because the eggs sometimes burst while cooking in the microwave, the researchers placed them inside thin socks before dropping them into glasses of water to heat them.

Then, carefully, they took the eggs out of the flask in the microwave and placed them on the floor. With a precise microphone only a foot away, they pierced the eggs with fast-acting meat thermometers, causing some to explode. [Watch a Video of Popcorn Exploding]

The pops they recorded were quite noisy, with pressure waves that reached a maximum of between 86 and 133 decibels. That was similar to the sound of a typical motorcycle running 30 feet (9.1 meters) away from the sound of a jet airplane at 100 feet (30 m) away, respectively. An explosive egg no doubt generates more noise than you would like to undergo for an extended period of time, but has a "low probability" of being strong enough to damage your hearing in a single burst, the researchers wrote.

If you put a potato in the microwave without piercing its skin first, the vapor pressure can accumulate under the skin and cause the potato to detonate. That's a simple mechanism for an explosion, the researchers wrote, similar to a grenade that explodes and destroys the outer shell of the device.

But a hard-boiled egg does not have a skin with the high tensile strength of a potato, and an egg shell, designed for a bird to pass through, is not strong enough to contain a high internal vapor pressure . There is a membrane between the white of an egg and its shell that could allow the pressure to accumulate, but that is released when an egg is laid and the eggs with the shell still explode. [10 Weird and Terrifying Medical Instruments from the Past]

The researchers suggested an alternative explanation.

The yolk of an egg, discovered with its meat thermometer, heats up much faster than the surrounding water. Maybe, they reasoned, small pockets of water get trapped inside the proteins and overheat.

At normal air pressure, those pockets would have room to expand and become vapor. But inside an egg, the pressure of the surrounding and hardening proteins can force the pockets to remain liquid even as their temperatures rise.

But disturb one of those pockets, let it expand and the water molecules would rush to fill the void – expand, interrupt the surrounding tissue and allow any other pocket to flicker through a phase change at the same time. The resulting collective bubble burst would tear the egg to pieces, throwing the pieces far and wide in a way that might resemble a more typical explosion under pressure.

"For an observer, the egg seems to have exploded," the researchers wrote in the document, but, "it is probably more accurate to describe the phenomenon as a rapid boiling of superheated water."

Originally published in Live Science.

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