|MadSci Network: Biochemistry|
Popcorn pops because each kernel contains a small amout of moisture within the starchy center, specifically the cotyledon which is there to nourish the embryonic corn plant as it germinates. If the popcorn kernel is heated to above the boiling point of water (100°C, 212°F), the moisture in the center of the kernel will convert to steam.
According to Chemistry, a standard mass of water (1 mole, which weighs 18g, about 5/8 of an ounce) would fill 18 ml of volume, or just over 3 1/2 teaspoons. If this water were converted to steam, it would weigh the same (same mass), but it would fill 22.4 liters of volume, or just over 6 gallons. That's over 1200 times more space for the same amount of matter!
So, when the temperature of the inside of the popcorn kernel gets above the boiling point of water, the moisture will try to expand the kernel to over 1200 times its normal size. Because the outer covering, or hull, of the kernel is so tough, the kernel cannot expand. So when the pressure inside is high enough, the hull breaks and the center expands explosively, releasing the trapped steam. About 1-2% of kernels become too dry during storage, and lose their internal moisture. These dry kernels account for the unpopped kernels, or "old maids," at the bottom of the popper after the rest have popped.
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