They're the foundation for burritos, tacos, sandwich wraps and other popular foods, but wheat tortillas are most tasty and functional when they're soft, tender and less than a couple of weeks old.

Hoping to give tortillas a longer shelf life to satisfy frugal consumers, Agricultural Research Service Chemist George Lookhart is studying the causes of staling in aging tortillas. With that insight, he'd like to someday stall the everyday occurrence of staling, which scientists have been trying to explain for more than 150 years.

According to Lookhart, the perfect tortilla is about 2 millimeters thick and evenly opaque, with ample diameter and at least a 3-week shelf life. He recently developed a test that can determine whether a particular wheat flour will yield this ideal tortilla.

His research and the new testing method could eventually help the tortilla industry and breeders develop wheat varieties ideally suited for tortilla production.

Lookhart, who works at the ARS Grain Marketing and Production Research Center in Manhattan, Kan., bases a tortilla's quality partly on how strips of the flat breads perform when being stretched across a special plate. He then measures the forces involved in this stress test and determines the level at which they cause the tortilla strips to break.

A wheat tortilla's tenacity is largely dependent on the kinds of proteins that are found in its flour. Known as gluten, the proteins can impart strength to a tortilla, allowing it to endure the stress of being rolled up without cracking.

But when flour from a particular wheat variety has excessively strong gluten, says Lookhart, the tortilla made from it will have too much "spring" — causing it to shrink in and lose valuable surface area when the dough is released from the tortilla press. Finding the right balance of proteins is an important part of the wheat-breeding research.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.