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Every corn grain has three parts: the embryo, where the genetic material is, the endocarp and the pericarp, composed mainly of starch and water. The difference of popcorn corn is that it has less water (about 14.5%) than green corn and its pericarp has a shell four times more resistant than the corn we use to eat and make hominy.
When popping the popcorn in a pan or in the microwave, the heat causes the water inside the grain to turn into steam, which tries to get out and pushes the pericarp shell. At the same time, the starch, which used to be solid, begins to turn into a kind of gelatin, increasing in size. Added together, the pressure of water vapor and starch reach 10 kg/cm2, five times more than that of a car tire!
The pressure is so much that the shell bursts! In contact with the air, the gelatinized starch solidifies and turns into the white foam that we eat. When the pericarp has cracks or is not very hard, water vapor escapes, the popcorn does not set and piruá appears. Another reason popcorn doesn’t pop is when the grain has more or less water in the composition 😉