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, dense type.[4]
As the oil and the water are heated past the boiling point, they turn the moisture in the kernel, which has a moisture-proof hull, into a superheated pressurized steam. Under these conditions, the starch inside the kernel gelatinizes, softens and becomes pliable. The pressure continues to increase until the breaking point of the hull is reached: a pressure of about 135 psi (930 kPa)[4] and a temperature of 180 °C (356 °F). The hull ruptures rapidly, 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.[4]
Special varieties are grown to give improved popping yield. Some wild types will pop, but the cultivated strain is Zea mays averta, which is a special kind of flint corn.
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