I had the wrong picture of popcorn. A kernel bursts, steam rushes out, and the little white flake rockets off the pan. That seemed sufficient. Then I read about the leg.
In 2015, Emmanuel Virot and Alexandre Ponomarenko studied popcorn with a high-speed camera and a microphone. Deborah Netburnβs account for the Los Angeles Times describes what they found. The jump and the popping sound have different causes. Escaping steam does not propel the jump.
A piece of expanding starch pushes against the hot surface. For a moment, the snack has something to stand on.
Before takeoff
The hull holds the kernel together while heat changes what is inside. Water becomes vapor, pressure builds, and the starchy interior softens. Eventually the hull cannot contain it. The starch expands out through the opening and cools into the white foam we eat.
The researchers also tested batches in an oven, raising the temperature in steps. At 170 degrees Celsius, 34 percent of their kernels popped. At 180 degrees, 96 percent did.
Those are results from their experiment, not a promise for every bag or a suggested setting for your stove. What interests me is how much changed across that small temperature interval. A kernel can look thoroughly uneventful while getting close to failure.
A very short gymnastics career
On the hot plate, the first starch emerging from a broken hull forms what the researchers called a leg. It presses against the surface, compresses, and releases. The reaction launches the popcorn into a somersault.
The distinction is worth keeping. An object can release gas and jump at almost the same instant without the gas acting as its rocket exhaust. Here, the contact with the ground matters. The brief shape of the starch matters too.
I especially like the awkwardness of calling it a leg. There is no joint, muscle, or intention. It is a temporary bit of food that happens to push in a useful direction. By the time you pick up the finished flake, its athletic career is over.
The sound arrives separately
To investigate the pop, the researchers synchronized the microphone with the camera. They compared the sound with the hull breaking and the popcornβs motion. The initial opening was silent. Their conclusion was that the audible pop came from the release of pressurized water vapor, rather than the hull cracking or the flake bouncing.
A pan gives us the noise and movement together, too quickly to separate by eye. The recording pulls them apart. One part of the event pushes against the pan. Another makes the sound that sends someone into the kitchen asking whether the popcorn is ready.
I came for an explanation of an explosion. The detail I will keep is a soft little leg, briefly strong enough to throw its entire world into the air.
Moo for now,
Maude π