On Food and Cooking

reaches and passes the boiling point, the
protein matrix and starch granules soften, and
moisture in the granules turns into steam. The
steam softens the starch even more, and the
many thousands of little steam pockets exert a
growing pressure against the hull. The
softening of starch and protein continues until
the internal pressure approaches seven times
the external pressure of the atmosphere, at
which point the hull breaks open. The sudden
pressure drop inside the kernel causes the
pockets of steam to expand, and with them
expands the soft protein-starch mixture, which
puffs up and then stiffens as it cools off,
becoming light and crisp. (If the popping is
done in a tightly covered pan that offers no
escape for the water vapor, the endosperm
will retain it and be tough and chewy; the pan
lid should be left slightly ajar.)
Popcorn pops best at a temperature around
380ºF/190ºC, and can be popped in hot oil, in
hot air poppers, and in the microwave oven.