Microwaved Exploding Eggs Explained Using an Acoustic Experiment
An acoustics expert wanted to see if an exploding egg could produce a big enough bang to damage an eardrum, and why it detonates in the first place.
It’s just the kind of dangerous domestic stunt that’s bound to go viral, but we strongly recommend you don’t try it at home. According to scientific tests, an exploding egg is not only a serious burn risk, but the percussive pop can reach 133 decibels.
Anthony Nash is an acoustical consultant with Charles M. Salter Associates in San Francisco and the de facto expert on exploding egg acoustics. His firm was hired as an expert witness in a case involving an unfortunate restaurant customer who bit into a reheated hard-boiled egg only to have it blow up in his face. The man suffered burns, but also, he alleged, hearing loss.
It was Nash’s job to figure out whether or not an exploding egg could indeed produce a big enough bang to damage an eardrum, and why it detonates at all.
Maybe you’ve seen this on YouTube. A guy sticks a peeled hard-boiled egg in the microwave for a minute, pokes it with a fork, and — kablooey! The egg explodes like a high-protein grenade, sending a violent shower of crumbly yolk and quivering whites across the kitchen.
Eggs and potatoes are infamous to microwave oven manufacturers as explosion risks. The cause of potato blowouts is fairly simple. The tuber’s skin is strong enough to trap the steam as it tries to escape the increasingly hot potato. Internal pressure builds and builds until the fluffy innards finally explode through a weak spot in the skin.
But is that what was happening with reheated hard-boiled eggs? Nash had his doubts.
So he ran some tests. He and his colleague Lauren von Blohn bought 100 cooked and peeled hard-boiled eggs from Costco. They placed each egg inside a thin spandex tube sock (to trap exploding debris) and submerged the stockinged ovum in a wide glass beaker filled with tap water. Then they put the entire water bath inside a standard 1200-watt microwave oven.
They cooked each egg on full power for 180 seconds (three minutes) with a pause at 90 seconds to monitor the water temperature. When the time was up, Nash and von Blohn gently removed the sopping hot sock and placed the egg on the floor beneath a precision sound level meter. Using the metal tip of a meat thermometer, they gingerly punctured the hard-boiled yolk.
The results weren’t as dramatic as the YouTube videos. Two-thirds of the eggs either ruptured while still inside the microwave or failed to explode at all when poked with the meat thermometer. But the remaining 30 produced audible pops ranging from 86 to 133 decibels, with a median value of 108 dB.
A gunshot at 100 feet registers at 140 dB, but that’s significantly louder than 133 dB, which is more like the roar of a stock car in a NASCAR race. If your neighbor runs his leaf blower all autumn, then you know what 108 dB sounds like. At those decibel levels, an exploding egg could certainly be painful, Nash and von Blohn concluded, but wasn’t likely to cause permanent hearing damage.
Which left the question: What the heck was happening inside a microwaved hard-boiled egg to make it explode?
The pressure theory didn’t hold, because the whites of a hard-boiled egg lack the tensile strength of an eggshell or even potato skin. The pressure required to produce such a violent bang would burst through the soft whites long before reaching the tipping point.
Nash had another theory — and thankfully, it had its own viral YouTube following. The theory was based on the principle of superheated water. As various intrepid YouTubers have shown, if you microwave a cup of water for way too long — like 10 minutes on high — the water can reach a temperature greater than 100℃ without actually boiling.
Boiling is the process of liquid water molecules expanding into gas molecules. As they expand, they gather in bubbles of steam that rise to the surface. But for bubbles to form, there need to be tiny specks of dust or other impurities in the water. If bubbles don’t form, the excess heat has no way to escape. The surface tension of the water acts like a lid holding in the potential energy.
All it takes is the slightest disturbance of the water surface — a few grains of sugar, or the corner of a teabag — and the water bursts into a violent, explosive boil, releasing all of the pent-up energy at once.
At the Acoustic Society of America Conference in New Orleans this week, Nash said that the same superheating process was at work inside the yolk of an egg. When eggs are hard-boiled, tiny bubbles of water are trapped within strands of tightening proteins in the yolk. When the egg is reheated, those pockets of water get superheated, reaching temperatures above 100℃, but with nowhere to boil.
All it takes is one poke with a fork to set off a chain reaction that sets all of the tiny water pockets boiling at once.
“Once one goes, it takes the others with it,” said Nash at the ASA conference. The bang “is a rapid expansion of steam coming out of the yolk.”
Did we mention to not try this at home?