Whether on the stove or at the movie theater, popcorn kernels display three entertaining reactions when heated: They burst open, jump, and emit a popping sound. According to a fun new study published in Journal of the Royal Society Interface, the “pop!” sound is triggered by the release of water vapor and the jump is similar to a somersault.
When the temperature exceeds 100 degrees Celsius, the water in popcorn boils and reaches a thermodynamic equilibrium at the vapor pressure (like in a pressure cooker). Above a critical vapor pressure, the outer covering (or hull) breaks. Meanwhile, inside, the starch granules expand and form spongy, flake-like petals. Then, the popcorn jumps a few millimeters and you hear that familiar “pop.” Surprisingly, the physical origins of the burst, jump, and “pop!” remain unclear.
So, Emmanuel Virot of École Polytechnique and Alexandre Ponomarenko from Grenoble University turned to thermodynamics and fracture mechanics. First, they tried popping microwaveable popcorn at different temperatures in an oven. Only 34 percent popped at 170 degrees Celsius. Turns out, the critical temperature—and the hull rupture point—is 180 degrees Celsius, where 96 percent of the popcorn popped.
Next, they placed popcorn on a hot plate set at 350 degrees Celsius and recorded them with a high-speed camera at 2,900 frames per second. Within 6.9 milliseconds (ms), the kernel begins to fracture, like the way a tiny bud of a plant begins to open up on a stem. After the fracture of the popcorn hull and the beginning of starch expansion, the duo noticed the formation of a “leg” made of starch compressed on the plate (pictured below at 13.8 ms). When the leg bounces, the popcorn jumps (at 20.7 ms) and basically does a somersault before landing again. Its angle of rotation is about 490 degrees. A human gymnast, by contrast, rotates about 300 degrees.
Click here to see how each of these frames compare to the fracture of a seedpod of a policeman’s helmet plant (Impatiens glandulifera) and a gymnast doing a somersault. Popcorn, they found, is somewhere between two categories of moving systems: explosive plants using fracture mechanisms and jumping animals using muscles.
Finally, the duo synchronized video recordings with the acoustic recordings they made with a microphone set up 30 centimeters from the popcorn on the hot plate. The popcorn opens part of the starch without emitting any sound. After 100 ms, a second fracture starts, followed by the start of a “pop” sound 6 ms later. This lasts about 50 ms, with sharp bursts at 110, 115, and 121 ms. During that time, the starchy leg continues its course towards the hot plate. That “pop” sound, they propose, is triggered by the release of water vapor. The sudden pressure drop excites the cavities inside the popcorn like an acoustic resonator—the same scenario applies to volcanoes and champagne bottle corks. The bursts are successive releases of pressurized water vapor pockets, which cause a series of excitations.
Images: shutterstock.com (top), 2015 The Royal Society (middle)