Why Does a Popcorn Kernel Suddenly Turn Inside Out?

A popcorn kernel already contains a small amount of water. As it heats, that water produces hot vapour and the internal pressure rises, while water and heat soften the starch in the endosperm. The decisive feature is not moisture alone but a comparatively intact, tough outer hull. Other kinds of maize have different structures and moisture conditions, so they generally do not expand in the same way.

Experiments have found that most kernels pop at around 180°C. At that point the hull fails at a weak spot. The sudden pressure drop allows superheated water to flash into steam, forcing the softened starch through the rupture and expanding it. The kernel jumps and turns as material that was inside is carried outwards. It is not simply a shell inflating like a balloon; it is closer to a sticky foam being blown open by steam.

Away from the heat, vapour escapes and the starch foam cools rapidly into the familiar crisp, white structure. The pop comes mainly from rupture and pressure release, not from the kernel hitting the pan. Unpopped “old maids” may have damaged hulls, too little moisture or uneven heating, preventing pressure from building at the right time. Popcorn requires three conditions to work together: a sealing hull, enough water and starch that can soften and then set.

https://pmc.ncbi.nlm.nih.gov/articles/PMC4345489/
https://pubs.acs.org/doi/10.1021/bm049220l