Physics of popcorn riddled

Why the corn bursts when heated, jumps and makes "pop"

Popcorn - puffed corn © thinkstock
Read out

Surprise in the cinema snack: It bursts, jumps and makes a loud "pop" of it - when corn is popcorn, there is highlife in the pot. But what is physically behind it? This has now been studied French researchers with microphone and high-speed camera. They discovered something surprising - including catapult legs made of starch and a real steam explosion.

Sweet or salty - the puffed corn is very popular. And making the popcorn is incredibly easy: just put the corn kernels in a pot and heat. What happens in the rough is known: The heat evaporates in the corn kernel contained water and the pressure inside increases until the solid grain shell bursts. The popcorn then makes a loud "pop" sound and jumps up in the pot. At the same time, the corn starch expands to form the foamy mass of the finished popcorn.

It starts at 180 ° C

"But the physical causes of these phenomena have not been described to our knowledge, " explain Emmanuel Virot of the École Polytechnique in Palaiseau and Alexandre Ponomarenko of the University of Grenoble. They have now made up for this by "popping" popcorn on a hot plate and filming with a high-speed camera, among other things.

It showed why most of the corn kernels "pop" at the same time: this happens only in a very narrow temperature range of just under 180 ° C - and that very quickly. The reason for this is the behavior of the water, as the researchers found out: Of the 20 milligrams of water that contains an average corn kernel, even before it bursts the largest part is still liquid. This only changes abruptly when the threshold temperature is reached and ensures an enormous vapor pressure - the grain is literally blown up by it. It doubles its volume and reduces its density eightfold.

After 13.8 milliseconds a strength leg is created, which catapults the grain into the air. © Virot and Ponomarenko / Royal Society Interface

The jump: a leg of strength

Why does popcorn jump up when it bursts? So far, it was assumed that this is mainly due to the ausschießenden steam. However, the high-speed footage of Virot and Ponomarenko reveals another mechanism: "After opening up the hull and beginning the strength expansion, we observe the formation of a" leg, "reports Researchers. display

This thickness projection laterally at the bottom of the grain is initially compressed by the weight of the grain. Seconds later, however, the pressure in the leg suddenly abounds and it catapults the entire grain into the sky. "The popcorn, when heated, stores thermal and elastic energy inside it, which is now partially released in the kinetic energy of this 'leg'. After all, this process accelerates the corn kernel to around 200 meters per second, as the measurements showed.

The "Pop": A steam explosion in the mini bar

It was also unclear from where the pop-noise of popcorn is caused. To find out, they recorded the Gerkuluschkulisse when making popcorn with a sensitive special microphone. Based on this soundtrack, they matched the time of the "pop" with the events visible in the high-speed recordings.

The result: The pop noise does not come from the bursting of the shell, as is often assumed, nor is it caused by repelling the grain from the bottom of the pot. Instead, the explosive release of water vapor from the corn kernel triggers this noise: "More precisely, the sudden pressure drop in the grain stimulates the cavities there, just like an acoustic resonator, " says Virot and Ponomarenko, A similar effect could be observed even with volcanic eruptions or the popping of a champagne cork. (Journal of the Royal Society Interface, 2015; doi: 10.1098 / rsif.2014.1247)

(Royal Society, 11.02.2015 - NPO)