Why the cookie crumbles

A PhD student from Loughborough University in the UK has discovered why biscuits sometimes break-up after being baked.

A PhD student from Loughborough University has discovered why biscuits sometimes break-up after being baked.

The discovery, published in the Institute of Physics journal Measurement Science and Technology, should help manufacturers work out how to make the perfect biscuit and also avoid the costly exercise of having to discard biscuits that don’t meet the high demands of their customers.

Biscuits such as the ‘Rich Tea’ type sometimes develop cracks spontaneously up to a few hours after baking, making the biscuit liable to break under the application of small loads such as being packaged or transported to supermarkets. Consumers often misinterpret this as due to mishandling. Qasim Saleem and his colleagues set out to understand exactly why these cracks occur in order to help biscuit manufacturers avoid this costly phenomenon.

They 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 this beam produces, and is sensitive enough to detect the very small deformations that evolve as a biscuit cools.

They 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. 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 and so biscuits with these minor cracks often end up in packets of biscuits that reach the customer.

Qasim Saleem said, ‘We now have a greater understanding of why biscuits develop cracks shortly after being baked. This 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 and hence producing the perfect biscuit’.