Popcorn, or pop-corn, is a variety of corn kernel, which expands and puffs up when heated.
A popcorn kernel's strong hull contains the seed's hard, starchy endosperm with 14-20% moisture, which turns to steam as the kernel is heated. Pressure from the steam continues to build until the hull ruptures, allowing the kernel to forcefully expand from 20 to 50 times its original size — and finally, cool.
Some strains of corn (taxonomized as Zea mays) are cultivated specifically as popping corns. The Zea mays variety everta, a special kind of flint corn, is the most common of these.
The six major types of corn are dent corn, flint corn, pod corn, popcorn, flour corn, and sweet corn.
Each kernel of popcorn contains a certain amount of moisture and oil. Unlike most other grains, the outer hull of the popcorn kernel is both strong and impervious to moisture and the starch inside consists almost entirely of a hard type.
As the oil and the water within the kernel are heated, they turn the moisture in the kernel into pressurized steam. Under these conditions, the starch inside the kernel gelatinizes, softens, and becomes pliable. The internal pressure of the entrapped steam continues to increase until the breaking point of the hull is reached: a pressure of approximately 135 psi (930 kPa) and a temperature of 180 °C (356 °F). The hull thereupon ruptures rapidly and actually explodes, causing a sudden drop in pressure inside the kernel and a corresponding rapid expansion of the steam, which expands the starch and proteins of the endosperm into airy foam. As the foam rapidly cools, the starch and protein polymers set into the familiar crispy puff. Special varieties are grown to give improved popping yield. Though the kernels of some wild types will pop, the cultivated strain is Zea mays everta, which is a special kind of flint corn.
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