The Physics of Popcorn

A lot of us see popcorn as a healthy snack that’s quick and easy to prepare at home. We know what temperature to pop it at and how long to heat it for, and the grain itself has been bred over the years to expand more impressively. But, until recently, nobody knew how or why the kernels pop. Two scientists from France have now filled that gap in scientific knowledge by experimenting with popcorn they bought from their local supermarket.

Having established the critical temperature at which popcorn pops (180 °C, in case you wondered!), the scientists recorded a piece of popcorn on a hot plate with a high speed camera; possibly the first time popcorn has been the subject of a movie instead of just the accompaniment. They saw that a ‘leg’ of starch forms between the kernel and the ground, causing the popcorn to jump.

The rotation achieved by a popcorn jump somersault outperforms that of a running gymnast. However, a flea accelerates under muscle power five times faster, and the explosive sandbox tree catapults its seeds with an acceleration 20,000 times greater than popcorn. So it’s not an outright winner in every category.

But what about the most characteristic aspect of popping corn; its sound effect? The sound could be caused by either the formation of a hull fracture, the rebound on the ground, or the release of pressurised water vapour. To investigate, the scientists returned to their trusty high-speed camera, synchronised with a microphone. They proved, conclusively, that a pressure drop produced by vapour release excites cavities inside the popcorn to produce an acoustic resonator. This is the same reason for the similar ‘popping’ sound of a champagne bottle cork.

So this Friday night when you prepare some popcorn and sit down to a film, take a moment to think about the scientific research that has been done into your tasty, inexpensive snack.

But scientists can’t answer the most important popcorn question of all: sweet or salted?

Photo credits: Journal of the Royal Society Interface

Author - Katie Renouf