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.
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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?