An introductory course to the physics of popcorn / Rachel Nauer

Don't forget to bring butter and salt 

Every year, Americans eat more than 17 billion one-liter cups of popcorn. But don't let the simplicity of this snack fool you: science still doesn't understand all the biomechanical details of the process by which popcorn is made from corn. Most of the trivia about popcorn, for example the fact that 96% of corn kernels explode at a temperature of 180 degrees Celsius, originates from research by commercial companies. Two French researchers, a physicist and an aeronautical engineer, decided to change this and undertook a rigorous study of the acoustic and thermodynamic properties of popcorn popping.

The basic details of how hard corn kernels go until they become puffy, soft, edible bits are well understood. The heat causes the water inside the starchy kernels to boil, which leads to pressure building up inside the kernel until the steam bursts out of the shell. But thanks to the fast cameras in their possession, the two French researchers discovered something new: in the first step, a leg-like extension emerges from the heated starch inside the kernels (see illustration). As the two scientists described in the Journal of the Royal Society Interface, after that leg hits a pan or bag, it compresses and then releases like a spring. The result is that the nucleus dissolves like a pole vaulter and flips in the air. "It is quite similar to the way humans jump, because we also contract our muscles and then release them," says one of the authors, Emmanuel Virou, an aeronautical engineer who worked at the École polytechnique while conducting the study.

The researchers also used high-sensitivity microphones to listen to the classic movie theater snack as it was being formed. They found that the characteristic popcorn popping noise is usually produced about 100 milliseconds after the kernel shell has already popped. The conclusion is, of course, that the source of the noise is not in the nuclear fission. The researchers suspect that its origin is actually in the steam trapped in the pockets inside the starch and breaking out of them only after the kernel splits. These palatable results can help physics students understand difficult concepts. Or as the second author of the article, Alexandre Ponomarenko, a physicist now employed by the French National Institute for Agricultural Research, puts it: "Basically, the goal of the study was to provide physics teachers with a fun way to demonstrate all these concepts in the classroom."

The article was published with the permission of Scientific American Israel