The Physics of Cooking Popcorn, or How to Perfect the Kernal Snack
We are all familiar with popcorn, a type of starch that expands to several times its natural state once heated. We can transform a hard, yellow kernal of corn into a big, white, fluffy mass of starch. But how does this process work, and how can we maximize the results for even bigger (tastier?) popcorn?
Several theories had been advanced to explain the mechanics of popcorn, including attributing the effects of oil, moisture, the endosperm (material that makes up the kernal’s interior), or the pericarp (outer shell) when exposed to heat. In 1993 Brazillian scientists discovered that the pericap of popcorn kernals is 4 times stronger than any of the corn family, which allows it to maintain structural integrity at higher pressures, potentially paying off in a larger piece of popcorn once the pericap fails. They also uncovered another interesting property of the pericarp of popcorn kernals: It is 2 times as effective at transfering heat as other corn’s pericarp. That means that it can be cooked as lower temperatures than regular corn, ensuring that it will not burn, and still have the starches properly prepared. Other studies have also shown that popcorn that is popped is 60% fluffier than regular corn that is popped (25).
That fluff is a result of the endosperm, which along with many factors mentioned above, is the real reason behind the “pop.” It all works by having water inside the kernel heat up until the endosperm is liquefied, then is released once the pressure is too great for the pericarp to contain. Upon leaving the shell, the liquid endosperm solidifies as the white fluff we see as the temperature of the starch decreases rapidly (25).
With all these facts in hand, several people claim to know the ideal mix for maximum fluff and how to achieve it. Over the last 50 years, the size of popcorn has doubled and the number of unpopped kernals as decreased by 75%. Some feel that this push for best results is compromising the quality, namely the taste, of the popcorn. For the popcorn industry, it translates into bigger profits, for popcorn is bought by weight and sold by volume. The bigger the fluff and the less waste, the bigger the income. Where any middle ground may be reached on this remains to be seen (24-5).
Soon, a new technique may result in even larger popcorn. Paul Quinn and his former advisor Daniel Hong looked at how adiabatic expansion, or how pressure and volume differentials result in little to no heat loss, played a part in popcorn cooking. By putting a kernal in an increasingly-vacuumed space, the pressure from the outside began to drop to the point where the internal pressure built up and overcame the pericarp, resulting in a released volume of solidified fluff that is bigger than standard conventions (24). Thus the vacuum popper was born, but it cannot match the output of the big popcorn industries. Yet.
One aspect that gets little attention is why does the popcorn jump in the air? Yes, it is a release of energy but the physics is even deeper. Emmanuel Vitot (Ecole Polytechnique) published a study in the Journal of the Royal Society about how high speed cameras revealed a hidden action. Turns out, once the kernel surface fails, an initial leg forms which hits the bottom of the pan and causes a flipping motion as it acts like a spring. Along with this is a small sound which is emanated over 100 milliseconds after the structural failure of the popcorn. That is way too late for it to be the source, so what is it? Likely water vapor, scientists say (Nuwer 22).
Foer, Joshua. “The Physics of…Popcorn.” Discover: May. 2005. 24-5. Print.
Nuwer,Rachel. "Popcorn Physics 101.) Scientific American May 2015: 22. Print.
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© 2013 Leonard Kelley
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