“What would happen if you let a hard-boiled egg just keep boiling?”
— Drew H., Oakland, Calif.
If you boil an egg for five or 10 minutes, it becomes firm and cooked. If you boil it for hours, it becomes rubbery and overcooked. Beyond that, things get a little mysterious.
Eggs are full of coiled-up protein molecules. Heating the proteins makes them uncoil and link up with one another to form a three-dimensional lattice, transforming a runny raw egg into a firm, rubbery cooked egg. This scaffolding helps give baked goods their structure.
Keep boiling the egg and the proteins continue to form cross-links, making the egg even more firm and rubbery. The egg white will also release hydrogen sulfide, which is why overcooked eggs also have a green film on the yolk; the sulfur in the white reacts with the iron in the yolk to form iron sulfide at the boundary between the two regions. The white of the egg will also turn a tan color as the glucose in the egg undergoes a Maillard reaction, the same process that makes cooked meat and caramelized onion turn brown.
Most hard-boiled egg recipes call for heating the egg for a matter of minutes, or, in the case of slow-cooked eggs, a few hours. If you kept going, you would enter somewhat uncharted territory.
Shelly McKee, a food scientist at Deb-El Foods and an expert on egg chemistry, said that boiling an egg for a long time would cause it to expel moisture. As the proteins in the egg white clump together ever more tightly, they would squeeze out the water contained within the egg. These little puffs of expelled water are what cause small holes to appear in baked goods if they are overcooked.
Eggs are robust enough to survive for quite a while in boiling water, but problably not forever. The interior of an egg is protected by the shell and several membranes, but if the egg spent enough time bobbing in turbulent, bubbling water, the physical stress on the egg — the shell, the white and the yolk — may eventually cause it to crumble and fragment.
Dr. McKee thought that disintegration was likely, but couldn’t say for sure. “It would probably break apart over time, and turn into bad soupy consistency,” she said. “But it depends on shell and water quality, so I really don’t know.”
There it is: If you boil an egg long enough, it will probably turn into bad soup. But the transformation could take months, years or perhaps much longer. There are better and faster ways to make soup.
Which is probably for the best, because even simple egg-speriments can be risky. “I accidentally exploded an egg in the microwave one time,” Dr. McKee said, “and would not suggest that!”