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Before you start cooking popcorn, the pressure inside and outside the kernel is the same.

As the kernel heats, the moisture turns to steam, and the internal pressure of the kernel rises. When the temperature inside the kernel climbs above 100 ºC, you might expect that all the water would turn to steam.

In fact, only a small amount vaporizes because the tough pericarp acts like a pressure cooker. The high-pressure steam penetrates the starch granules and transforms them into hot, gelatinized globules. Finally at about 175 ºC, when the pressure inside the kernel is about 9 atm [~132lbs/sq inch], the pericarp ruptures.

The steam and superheated water, now surrounded by normal-pressure air, become the driving force that expands the kernel.

The gelatinized starch granules do not explode, but expand into thin, jellylike bubbles. Neighboring bubbles fuse together and solidify, forming a three-dimensional network much like a sink full of soapsuds. This is the white fluffy solid we eat.

The moisture content of the kernel is now about 1-2% by mass, and the popcorn is transformed into a tender, fluffy morsel.

Popular Kettle Corn

Popcorn Information:

Figure Source:

The World of Corn 2003, p.3 National Corn Growers Association (NCGA)

Did you ever wonder how a hard little yellow popcorn kernel becomes a soft, white, snack food? Whether you “nuke” your popcorn in the microwave, pop it in oil, or use a hot air popper the hard kernel is transformed in a similar manner. The key to popping corn is water. When the popcorn kernels are heated either via hot air, hot oil, or the microwave oven, some of the small amount of water contained inside becomes steam. This steam is then able to pop the corn by breaking the outer hull and blowing-up the starch inside (see the more detailed description from the American Chemical Society below).