Thickness. Starch. Mechanics. And why popcorn kernels pop and hop when heated.
If a baby popcorn kernel came up to you one day and asked if it could grow up to be an acrobat, what might you say? If it were me, I would say yes, of course! Here’s why.
Popcorn grains (or kernels), like other grains such as wheat and rice, have an endosperm, aleurone layer (outer layer of endosperm), bran and germ. These are all contained in a husk (also called a hull). The grains also hold a small amount of water.
Because of this water, when we heat the popcorn, the starch granules in the endosperm start to swell. With further heating, they break down, releasing a sticky, starchy goo inside the kernels. You can think of this goo as a 3D fishnet that traps water and starch (read about the science of this fishnet in the 1-Minute NomNom “Are you straight?“).
At 100°C/212°F, the water turns into steam. It expands and builds up pressure in the kernels until it breaks through the husk between 170-180°C/338-356°F. According to scientists at École Polytechnique and Grenoble University, this release of steam and pressure — which you can see in the high-speed video above — causes the “pop” sound and is very much like the “pop” sound when a bottle of champagne is opened.
… and the Hop
The expanding steam also inflates the starchy 3D fishnet in the kernels. When the kernels pop, this 3D fishnet is first pushed outward by the expanding and escaping steam. It wraps around and is cooled down immediately by the much colder air outside. What results is the fluffy form we are so familiar with.
It is how this fluffy form jumps that makes the popcorn kernel such a great acrobat. The steam pressure creates a fracture in the husk when it breaks through. The fracture allows the inflating starch to “poke” through and stick out a “leg”.
This “leg” of starch compresses itself against whatever the kernel was resting on, pushes off, and propels the puffy popcorn into the air. Take a look at the high-speed video above — the mechanics are just like how acrobats would use their legs to spring off the ground into a somersault!
Popcorn kernels have this unique pop and hop because they have just the right husk thickness and right amount of water (about 13-14% of the grain). When the water vaporizes and the steam pressure builds up to 135 psi, the husk thickness is just right to allow the steam to break through.
The Daily Mail has some pretty awesome still photos of this pop and hop. If you are not corn-tented with the corn-tent here and want to read more, pop and hop over to check them out.
Popcorn-fess you like me and you will discover more! All you need is a minute a day to explore the world’s marvels through the phenomenon of food!