INDIANAPOLIS — 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 dentists busy, but their days could be numbered: Scientists say they now know why some popcorn kernels, too pooped to pop, resist exploding into puffy white globes.
It long has been known that popcorn kernels must have a precise moisture level in their starchy center — about 15 percent — to burst. But Purdue University researchers found that the key to a kernel’s explosive success lies in the composition of its hull.
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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.”
The findings may help popcorn breeders select the best varieties — or create others — with superior hulls that yield few, if any, unpopped kernels. But for now, there’s no way to screen out potential old maids before they end up in bags of popcorn.
Mr. Hamaker and his associates compared the microwave-popping performance of 14 Indiana-grown popcorn varieties 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 four in premium brands to 47 in the cheaper ones.
The findings could have a big impact on people who savor the snack and those who grow the 17 billion quarts of popcorn sold each year in the United States.
Wendy Boersema-Rappel, a spokeswoman for the Chicago-based Popcorn Board, said popcorn processors are always looking for ways to improve their product, including reducing the number of old maids.
“It’s one of life’s annoyances — it’s not rocking anyone’s world, but our members always like to improve their product,” she said.
Mr. Hamaker said two popcorn manufacturers already have expressed interest in Purdue’s findings.
The research, funded by Purdue’s Whistler Center for Carbohydrate Research, which Mr. Hamaker directs, has been published online and will appear in the July 11 edition of the journal BioMacromolecules.
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