So why is it that whenever you make popcorn there are always unpopped kernels left at the bottom of the bowl or the bag at the movies, the ones that stick in your throat, plug up your teeth and pop your fillings?

"It's a matter of scientific curiosity, and we are in the food business, " said Purdue University's Rengaswami Chandrasekaran. "So we decided to find out."

In research scheduled to appear in the July ññ edition of the journal Biomacromolecules, scientists led by Purdue food chemist Bruce Hamaker analyzed ñ4 varieties of microwaveable popcorn to determine what causes the BB-hard leftovers known as "old maids."

Popcorn makers for years have used trial-and-error breeding to make popcorn pop more thoroughly, but the team found the results were mixed. In analyzing the ñ4 varieties, the amount of unpopped kernels ranged from 4 percent to 47 percent.

The key popability factor, it turned out, is the kernel's cellulose hull, known as the pericarp. Chandrasekaran, a crystallographer, found that the pericarp in the better performers had a stronger crystalline structure than the pericarp in the losers. "With a better-organized crystal structure, the kernel retains moisture better," Chandrasekaran said in a telephone interview. The kernel swells as it heats up, until it finally explodes, creating popcorn. If the moisture leaks out prematurely, or the pericarp collapses, the pressure will not build, and the frustrated consumer is left with old maids.