Eat your way to the bottom of almost any bag of popcorn and there they are: the rock-hard, jaw-rattling unpopped kernels known as old maids.



The nuisance kernels have kept many a dentist busy, but their days could be numbered: Scientists say they now know why some popcorn kernels resist popping into puffy white globes.



It’s long been known that popcorn kernels must have a precise moisture level in their starchy center — about 15 percent — to explode. But Purdue University researchers found the key to a kernel’s explosive success lies in the composition of its hull.



Unpopped kernels, it turns out, have leaky hulls that prevent the moisture pressure buildup needed for them to pop and lack the optimal hull structure that allows most kernels to explode.



“They’re sort of like little pressure vessels that explode when the pressure reaches a certain point,” said Bruce Hamaker, a Purdue professor of food chemistry. “But if too much moisture escapes, it loses its ability to pop and just sits there.”



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