The Physics of Popcorn
What are the forces that make a round corn kernel into a fluffy piece of popcorn? It is a perfect example of how physics can apply to all things—even a snack for your favorite movie. I also have special interest in this due to my experience in concessions where popcorn is a big seller.
So far, food engineers have determined that popcorn kernels pop most efficiently and consistently when water is 13.5-14% of the kernel’s weight and when the kernel is close to the shape of a sphere (due to a more even distribution of force). Using these statistics, food engineers have been able to reduce the rate of unpopped popcorn by 75% since 1950. A kernel of popcorn is made up of a hull that surrounds the seed, a protein matrix called the endosperm, and the germ.
When heated, the water inside the kernel turns into vapor, which forces its way into the endosperm to make a hot doughy mixture. The continuation of heat causes the pressure to build up, which causes the hull to burst, and the endosperm/steam mixture to burst. This mixture cools once outside of the kernel. The pop is not caused by the breaking of the hull, but by the release of the steam.
Gas laws that state that volume is proportional to temperature divided by pressure can explain this process.
As the temperature increases, the volume increases, and thus the pressure increases. The gas pressure builds up to give a force that will break the hull. Gas pressure is defined by NASA to be a scalar quantity that is the measure of the linear momentum of the gas molecules. In other words, the gas molecules that are bouncing off of the walls of the hull are imparting a momentum on the surface. This means that gases produce a force that is perpendicular to the surface (a normal force). This force allows the water vapor to mix itself with the endosperm that produces the popcorn.
When the popcorn pops, it is launched into the air by the emergence of the endosperm “leg”. The angle of this launching causes the popcorn to move in a circular motion in the air, being brought down by the force of gravity. The leg provides an initial velocity, and the popcorn has acceleration throughout the entire rotation because it is changing direction.