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Americans eat more than 17 billion quarts of popcorn every year, yet some of the biomechanics of this deceptively simple treat still escape science. Most existing popcorn trivia—that 96 percent of kernels pop at 356 degrees Fahrenheit, for example—originated from commercial research, so a French physicist and aeronautical engineer took it on themselves to meticulously investigate the thermodynamic and acoustic properties of popcorn popping.
The basic journey of hard kernel to fluffy morsel is straightforward. Heat causes water inside the kernel's starchy interior to boil, building up pressure until the vapor bursts through the shell. But with high-speed cameras in hand, this team discovered something new: starch inside heated kernels first forms a leglike appendage. As the scientists described in the Journal of the Royal Society Interface, after the leg comes into contact with the bottom of a pan or bag, it compresses and releases like a spring, vaulting the kernel into the air and causing it to somersault. “It's quite similar to when we humans jump because we also compress our muscles and release them,” says co-author Emmanuel Virot, an aeronautical engineer then at École Polytechnique.
The researchers also monitored the motions of the classic movie snack with supersensitive microphones and found that the characteristic “pop” sound typically occurs about 100 milliseconds after the kernel cracks open. Thus, the kernel's splitting is not the source of the sound. The researchers suspect instead that the pockets of heated water vapor within cause the noise as they burst through their starchy cages. These delicious results could help clarify tough physics ideas for students. As co-author Alexandre Ponomarenko, a physicist now at the French National Institute for Agricultural Research, puts it: “The aim of this research was really to provide physics teachers with a fun way to show all of these concepts in the classroom.”