Web Extra

Popcorn Provides Edible Physics Lesson

By Natalie van Hoose - Published November 6, 2015

a box of popcorn 

Popcorn is an ancient food, dating back thousands of years. But only recently have scientists unlocked the physics behind what we observe when popcorn pops: fluffy starch bursting from a tough hull, a leap into the air and a muffled "pop" sound—all in hundredths of a second.

Earlier this year, scientists Emmanuel Virot of École Polytechnique and Alexandre Ponomarenko of Grenoble University studied the science behind popcorn's pop and described their findings in the journal Interface.

Each kernel of popcorn contains a tiny amount of trapped moisture. As the kernel heats up, some of the moisture turns into steam and expands, increasing pressure inside the hull. At about 356 degrees Fahrenheit—the sweet spot at which almost all popcorn pops—the mounting thermal stress exceeds the hull's ability to contain it. The hull fractures, the steam escapes, and the tightly packed starch inside the hull bursts into a large flake.

Virot and Ponomarenko used a high-speed camera to capture the details of the pop that happens too quickly for the human eye to discern. They showed that when the starch explodes from the cracked hull, part of it forms a kind of foot that pushes off the surface—in their study, a hot plate—and launches the popcorn into the air where it somersaults gracefully several times. At about 490 degrees, the rotation of the whirling flake is "slightly better than the somersault of a running gymnast," the researchers write. Though a flake of popcorn "is thousand times smaller than a gymnast … they have rather the same performance."

The researchers also found that the popping sound is caused by the release of water vapor, not the explosion of starch or the cracking of the hull. They believe the sudden drop in pressure as the hull cracks creates a cavity inside the popcorn that acts as an acoustic resonator, similar to the way in which a champagne bottle amplifies the pop of its cork.

Virot and Ponomarenko conclude that from a physics standpoint, the behavior of popping popcorn lies somewhere between that of certain plant species that use explosive fracture mechanisms to disperse their seeds and animals that rely on their muscles to jump.

To watch popcorn's aerial acrobatics, see this video from The New York Times.