That's the way the cookie crumbles!

Manufacturers on the quest for the perfect biscuit will find comfort in a discovery coming out of the UK this week that explains exactly why biscuits crumble.

Doctorate student Qasim Saleem at Loughborough university set out to understand why biscuits sometimes develop cracks spontaneously up to a few hours after baking, leaving them liable to break when transported or packaged.

For the project - funded by Loughborough university and Campden & Chorleywood Food Research Association - Saleem and colleagues used an optical technique called 'digital speckle pattern interferometry' to look at the surface of a biscuit as it cools to room temperature after baking.

This technique involves illuminating the surface of an object with a laser beam, studying the scattered light the beam produces, and is sensitive enough to detect the very small deformations that evolve as a biscuit cools.

The scientists found that as a biscuit cools down after coming out of the oven, it picks up moisture around the rim which causes the biscuit to expand while at the same time loss of moisture at the centre of the biscuit causes it to contract.

This difference results in the build-up of strain and associated forces which act to pull the biscuit apart, and which ultimately can be released by developing cracks or final break-up. Ultimately, these cracks make the biscuit weaker than it ought to be and so very easy to break apart when handled, moved or packaged.

The scientists point out that although manufacturers currently tackle the problem by removing the offending products before they reach the customers, no quality control system is perfect and so biscuits 'with these minor cracks often end up in packets of biscuits that reach the customer'.

"Our greater understanding will help biscuit manufacturers adjust the humidity or temperature of their factory production lines to change the cooling process in such a way that the biscuits won't break up due to normal handling,"​ said Saleem. "Hence producing the perfect biscuit,"​ he added.

The paper 'A novel application of speckle interferometry for the measurement of strain distributions in semi-sweet biscuits' by Q Saleem, R D Wildman, J M Huntley and M B Whitworth is published in Measurement Science and Technology​​(volume 14 issue 12 pp 2027-2033).