Popcorn is the funniest corn to cook, because it jumps and makes a ‘pop’ sound in our pans. It is extremely amusing, that much we’ve established. But exactly how does
it work? French physicists Emmanuel Virot and Alexandre Ponomarenko from
the École Polytechnique decided to find out.
The only one capable of becoming popcorn available is everta variety of corn (Zea mays) and that because of its far more compact shell amongst of the available corn varieties. So the team put some microwavable Zea mays everta together on the hot plate, and turned the heat up in gradual increments
while a high-speed camera is recording the whole thing.
After observing the film recorded at 2,900 frames per second on camera, the scientists feel
just 34 percent of the kernels would pop at 170 degrees Celsius, but
once the temperature reaches 180 degrees, they all went crazy - 96 percent
of them forced their fluffy white guts out of their golden hulls like
good popcorns.
Virot told the AFP news agency, "We found that the critical temperature is about 180
degrees Celsius, regardless of the size or shape of the grain."
Why the popcorn behaves like this?
Virot and Ponomarenko describe in a paper published in the Journal of the Royal Society Interface:
"When the popcorn temperature exceeds 100°C, its water content (moisture) boils and reaches a thermodynamic equilibrium at the vapour pressure, as in a pressure cooker. Above a critical vapour pressure, the hull breaks.
At the same time in the popcorn endosperm, the starch granules expand adiabatically [which means heat is reduced through a change in air pressure caused by volume expansion] and form a spongy flake of various shapes. Then, the popcorn jumps a few millimetres high to several centimetres high and a characteristic 'pop' sound is emitted. To the best of our knowledge, the physical origin of these observations remains elusive in the literature."
They observe each kernel of corn contains a
combination of water and starch - the best kernels have between 13 and
14.5 percent moisture. When the kernel is heated up, the water is
converted to super-hot steam, which liquifies the starch. So a fraction
of a second before you get popcorn, you have a gooey starch mess, which
means the process of making corn is strangely similar to that of
caterpillar-to-butterfly metamorphosis.
Also once the pressure
has been built to a critical point, the liquid starch bursts through the
hull and cools down hyper-fast into a solid. That’s why no popcorn
shape is the same, and when you look at the surface through a
microscope, you’ll find it’s
made up of lots of tiny, solidified starch bubbles. But herein lies the
other strange thing about popcorn - that ‘popping’ sound doesn’t
actually occur when the kernel explodes open to release the starch.
"By looking at the footage and using a microphone to capture the pops, the team found that the pop sounds didn't happen when the hull first split. Instead, the pop seemed to occur when water vapour inside the kernel was released, creating empty cavities inside the popcorn kernel that resonated once the pressure dropped. Similar phenomena occur when volcanoes or champagne corks pop.”, Tia Ghose explains at LiveScience.
So the liquid starch has broken through the hull and changed into a
solid - all in the course of a few hundredths of a second. And here’s
where the weirdness continues. The scientists say the newly formed
popcorn uses a ‘starch leg’ to propel itself from the ground, just like a
human gymnast. "As a result, popcorn is midway between two categories
of moving systems: explosive plants using fracture mechanisms and
jumping animals using muscles." The comparison is shown below:
Source: LiveScience
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