LONDON, Oct. 3 -- Researchers in England have used laser beams to determine why "biscuits," i.e., cookies, sometimes break after they're baked. They say the discovery will help manufacturers work out how to make the perfect cookie and avoid discarding cookies that don't meet customers' quality-control standards. Their work was published in the Institute of Physics' journal, Measurement Science and Technology, this week. AS THE COOKIE COOLS: A telltale crack in a tea biscuit.Qasim Saleem, A PhD student at Loughborough University, in Leicestershire, and his colleagues set out to understand exactly why cookies, such as the "tea-type biscuits" shown in the photo, sometimes spontaneously develop cracks up to a few hours after baking. The cracks make them vulnerable during packaging and en route to supermarkets, and consumers often misinterpret this as due to mishandling, Saleem said. The bottom line: Cracked cookies are costly. The team used an optical technique -- digital speckle pattern interferometry -- to look at the surface of a biscuit as it cools to room temperature after baking. This involves illuminating the surface of an object with a laser beam and studying the scattered light the beam produces, and it is sensitive enough to detect the very small deformations that evolve as a cookie cools. They found that as a biscuit cools after it comes out of the oven, moisture develops around its rim, causing it to expand; at the same time, lost moisture at the center of the biscuit causes it to contract. "This difference results in the buildup of strain and associated forces that act to pull the biscuit apart, and which ultimately can be released by developing cracks or final breakup," Saleem said. "These cracks make the biscuit weaker than it ought to be, and so very easy to break apart when handled, moved or packaged. Manufacturers currently tackle this by removing the offending products before they reach the customers, but no quality control system is perfect, so biscuits with these minor cracks often end up in packets of biscuits that reach the customer." The research will help biscuit manufacturers produce the "perfect biscuit," Saleem said, by enabling them to adjust the humidity or temperature of their factory production lines to change the cooling system, ensuring no cookies are harmed in the process. The work was funded through collaboration between the Loughborough University School of Mechanical and Manufacturing Engineering and Campden & Chorleywood Food Research Association. For more information, visit: www.iop.org/journals/mst