The proper popper: corn kernel's chemistry is key to its ka-pop.

For the hungry snacker, unpopped kernels at the bottom of a bag of microwavable popcorn are missed opportunities. In a constant quest to lessen such disappointments, food manufacturers relentlessly breed corn with their sights set on varieties that pop best. Now, a group of chemists claims to have found that the secret to better popping is the crystalline structure of the kernel's hull.

Stored in every kernel are starch and a bit of moisture. As popcorn heats up, that moisture vaporizes and pressure builds inside the kernel. When the pressure reaches about 9 atmospheres, the kernel bursts. Within milliseconds, the softened starch noisily expands into a white foam.

Cereal chemist Bruce Hamaker and his colleagues at Purdue University in West Lafayette, Ind., analyzed 14 varieties of Indiana-grown popcorn. To assess popping, the researchers sealed kernels and some shortening inside microwavable bags and heated them in a microwave oven for 2 minutes.

The percentage of unpopped kernels ranged from 4 percent for the best variety to 47 percent for the worst. Not surprisingly, kernels that retained the most moisture during microwave heating produced the fewest unpopped kernels.

To determine what underlies this moisture retention, Hamaker's group examined the chemical structure of a kernel's peri-carp. This hard casing is made primarily of the polymers cellulose and hemieellulose. X-ray-diffraction analysis showed that heating a kernel causes the long cellulose molecules to align and assume a highly ordered crystalline structure.

With heat, the periearp becomes a better moisture barrier, Hamaker says. Those popcorn varieties with the highest degree of crystallinity yielded the fewest unpopped kernels, the Purdue team reports in a forthcoming Biomacromolecules.

David Jackson, a food-chemist at the University of Nebraska-Lincoln, says that scientists have known for years that the kernel's pericarp is important to popping. What's novel here, he says, is finding the specific molecular changes during the heating process that are critical.

Popcorn breeders at major food companies including ConAgra, which is headquartered in Omaha, Neb., and owns the Orville Redenbacher's brand, have already contacted Hamaker about his findings.

Measures such as pericarp thickness have been used in the past to assess popcorn quality. However, "those are not 100 percent reliable," says Jackson. He suggests that popcorn breeders might look instead for varieties that produce the most-crystalline pericarps in response to heat.

Genetically engineering popcorn or chemically modifying the pericarp to generate the best crystalline structure might also offer routes to better popping, adds Hamaker.