Eat your way to the bottom of almost any bag of popcorn and there they are: the rock-hard, jaw-rattling unpopped kernels.
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.
It turns out there is an optimal hull structure that allows kernels to explode, and leaky hulls prevent the moisture pressure buildup needed for kernels to pop.
"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."
He and his associates compared the microwave popping performance of 14 popcorn varieties grown in the state of Indiana and examined the crystalline structure of the translucent hulls of both the popped kernels and the duds.
In the varieties popped, the percentage of unpopped kernels ranged from 4 percent in premium brands to 47 percent in the cheaper ones.
Hamaker said two popcorn manufacturers have already expressed interest in Purdue's findings.
The research, funded by Purdue's Whistler Center for Carbohydrate Research, which Hamaker directs, has been published online and will appear in the July 11 edition of the journal BioMacromolecules.
RICK CALLAHAN, Associated Press Writer