Question sent by JORGE GIL FERNÁNDEZ (València). FERNANDO SAPIÑA answers:
We can find a lot of videos on YouTube showing different phenomena in slow-motion: a bullet going through a piece of fruit, the neve-rending dance of jelly, the pop of a corn kernel, etc. There is some magic to it and, in some cases, as is the case with the corn kernel, we immediately ask ourselves: why does it explode and turn into popcorn?
American Indians already made popcorn in pre-Columbian times, but despite its long history, a mechanism for their formation was not suggested until 1983. Morphologically, a corn kernel is wrapped in a covering or pericarp and, inside, there is the seed for the future plant and the endosperm, where there is nourishment for the germ in the form of starch with a bit of water. In some species of corn, the covering is thicker and shows a dense array of cellulose fibres. This makes the external cover behave as a watertight container: in other words, it works as a small pressure cooker.
When kernels are heated to a temperature of 66ºC, the starch absorbs humidity contained inside the kernel. If we keep raising the temperature, we will reach water’s boiling point. If the outer wrapping were porous, water steam would escape from the kernel. But the covering in this corn species is not only not porous, but quite resistant, too. So, as kernels are heated to a temperature over 100ºC without significant change, internal pressure keeps increasing. When it reaches a temperature around 175ºC-180ºC, internal pressure in the kernels is nine times higher than the atmosphere, and the covering explodes: the water absorbed in the starch evaporates and quickly expands and the starch turns into dry and crispy foam.
Fernando Sapiña. Professor of Chemistry and Investigator of the Institut de Ciències dels Materials, Universitat de València