Why Popcorn Pops

by | Feb 9, 2018


Popcorn has been consumed by humans for hundreds, and possibly thousands, of years. Until recently, however, the exact mechanism of how popcorn pops has not been known. A 2015 paper published in the Journal of the Royal Society by two French researchers reveals details of what happens when popcorn pops. What they found is after the video below, first, some basics that we’ve known for a long time:

  • Popcorn pops because it has moisture inside the kernel. Optimally, around 14%. As the kernel heats the water vapor expands until it bursts – creating popcorn.
  • Specifically, when heated above 100 degrees Celsius, the water inside the kernel turns to steam and water vapor forces its way into the “endosperm” inside the kernel, which is a matrix of protein. The hot water vapor and endosperm combine to create a molten mass similar to a thick batter or dough. Pressure continues to build up until the hull can no longer hold it in. Then the kernel explodes. The starch cools as soon as it bursts, solidifying into a spongy white flake of popcorn. The popped kernel is over twice as large and eight times less dense as un-popped.
  • Not all corn pops. Popcorn is a special type of corn kernel with a thicker hull that is not porous. Other types of corn (as well as most other grain kernels) have shells that are porous that allows the moisture to escape gradually from the kernel as it is heated.
  • Plant breeders have reduced the rate of un-popped kernels by 75% since the 1950s. One important feature of popcorn that increases its popping propensity is that it is more spherical than other types of corn.
  • Other grains will pop, but not nearly as spectacularly as popcorn. Notably, amaranth, quinoa, barley and millet can pop.

Here’s video of popcorn popping in slow motion:

What the French researchers found in their 2015 study on popcorn is interesting. The study authors explained that “the phenomenon of popcorn popping contains interesting physics from different fields: thermodynamics, biomechanics and acoustics.” Here’s what they found:

  • As we all know, popcorn jumps when it pops. They figured out why it jumps. Using a camera that took 2,900 pictures a second they found that when the hull of the kernel begins to fracture as the starch inside expands that a “leg” of molten starch kicks out of the kernel and causes it to jump as it explodes. Like a gymnast doing a somersault. This happens in 1/15th of a second. Steam has no role in powering the jump – it’s just the heated starch leg that causes the jump.
  • They examined what causes the popping sound and using a very sensitive microphone synchronized to the high-speed photos they found that the popcorn first opens without emitting a sound. Through process of elimination they determined that release of water vapor creates the popping sound. The mechanism was similar to the one that makes a champagne cork pop.
  • Popcorn pops best at 177-187 degrees Celsius. At 180 degrees 98% of popcorn kernels are popped. Higher or lower temperature than that and there are more un-popped kernels. Only 34% of kernels popped at 170 degrees.

Link to paper: Royal Society Journal – Popcorn


1 Comment

  1. Thanks for trying to take our minds off of the free fall in the market.


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