Physics of popcorn

13 February 2015
Posted by Chris Smith.

Scientists have solved the mystery of what makes popcorn pop.

Armed with a packet of popcorn, a high speed camera and a hotplate, PopcornEmmanuel Virot and Alexandre Ponomarenko, who have published their analysis in the Royal Society's Interface journal, began by measuring precisely the temperature at which popcorn pops, which occurs over a very narrow thermal range.

At 170 degrees celsius only about one third of the kernels popped, but by 180 degrees almost all had.

Based on moisture measurements and the popping temperature the duo calculated that the pressure inside the corn before it pops reaches about 10 atmospheres.

The tough outer hull or pericarp of the corn works like a pressure cooker and prevents the water inside from from boiling. Instead it is forced into the starchy material producing a molten paste that issues explosively later after the popcorn pops.

Next the researchers filmed individual corn kernels popping, capturing 3000 images per second to unpick the process in detail.

At the critical popping temperature, they found, the hull of the corn first extends a "leg" which flips the corn up into the air accelerating it at about 200 metres per second every second.

The momentum imparted to the popping corn, the researchers observed, causes it to describe a 490 degree rotation, which is better than a somersaulting gymnast which, they point out, usually manages about a 300 degree rotation!

But what of the sound itself. To probe the origins of the noise, Virot and Ponomarenko synchronised sound recordings of their corn popping with the iamges from their high speed camera.

This shows that, surprisingly, the sound does not coincide with the hull of the corn rupturing but instead with the release of the 20 milligrams or so of water vapour locked up inside.

Exiting at high speed, this leaves a low-pressure void inside the corn kernel, causing air to rush back in and setting up an echoing resonance that we here as the pop.

The same science underpins the popping of a champagne cork, or even how the how voicebox works. So next time you chew on a tasty morsel of popcorn, just think of the physics that has gone into it!

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