Why does popcorn pop?
Popcorn is the most amazing food! It all starts with a kernel only several millimeters in diameter which explodes into a 40-50 times bigger fluffy, tasty, white wonder. The kernel is made of three parts: the pericarp, the endosperm and the germ. The pericarp is the outer shell, which is air-tight and extremely tough. The endosperm is mostly carbohydrate in the form of starch, with smaller amounts of protein, fat, minerals, and water. The germ is the part that sprouts and is not important in the process of popping.
When you heat a popcorn kernel, water inside (about 13-14% by mass) begins to expand. When the temperature reaches 100 deg C (212 deg F), the water tries to evaporate but the pericarp is so strong that it can't. Instead, pressure begins building inside the kernel just like in a pressure cooker. The pericarp is so strong and air-tight to preserve the water inside the kernel for the germ when it begins sprouting. Some 4,000 year old popcorn kernels discovered in Bat Cave, NM still pop, which means that their pericarp has managed to maintain this water inside for all this time.
As the temperature continues rising, so does the pressure. At approximately 175 deg C (347 deg F) the pressure is as high as 9 atmospheres, and the kernel explodes. If the pericarp has even a tiniest hole in it, the pressure inside the kernel will not be able to build up and it will not pop. The water content is also very important; if the kernel has been dried up (it was left out in the sun or heat for a long time), it will not pop. The expanding water and steam drive the endosperm out. The endosperm starch forms jelly-like bubbles, which quickly dry and solidify into a three-dimensional network - which is the white stuff we like to eat. Mmm … I am getting hungry now. How about you? Let's pop a bag.
