Published June 18, 2010 by
Each kernel of popcorn contains a small drop of water stored inside a circle of soft starch which is surrounded by the kernel's hard outer surface. Unlike most other types of grain, this hard outer surface is resistant to moisture. For the corn to be 'popcorn', it needs to contain at 13.5 to 14% moisture inside the hard outer surface.
As the kernel heats up, the water begins to expand turning it into superheated pressurized steam. The pressure builds against the hard hull until it gives way, causing the popcorn to explode. The hull rupture usually occurs at about 135 psi and a temperature of 180 degrees Celsius.
As it explodes, the soft starch inside the popcorn becomes inflated and bursts into an airy foam, turning the kernel inside out. As the foam cools, the starch and protein polymers set yielding the crispy popcorn puff that we all know and love.
Those unpopped kernels of corn that are left at the bottom of popcorn bowls are generally the result of either kernels that don't have the proper moisture content or they have a cracked outer hull which means that the expanding steam can't build enough pressure to cause the kernel to pop.