Improved understanding of biscuit checking using speckle interferometry and finite-element modelling techniques

Abstract

For almost a century, manufacturers have tried to understand and predict the appearance of small hairline cracks in biscuits and crackers, an effect known as ‘checking’. Results are presented here from a combined experimental and numerical study of the phenomenon. Experimentally determined material properties of semi-sweet biscuits were used as inputs to an axisymmetric finite-element model, which predicts that, at low relative humidities (RHs), the rim of the biscuit expands and the centre contracts. The expansion is caused by uptake of moisture at the rim, which is initially dry after baking, and the contraction is attributable to loss of moisture from the more moist centre. The finite-element predictions were validated by strain measurements of freshly baked biscuits using speckle interferometry. These measurements are the most sensitive and the first whole-field measurements yet made of strains in biscuits. Experiments using a range of storage conditions enable us to confirm that it is the presence of moisture gradients, in combination with a low environmental RH, which increases the chance of checking. These results will aid biscuit manufacturers in the development of more effective strategies to avoid the damaging phenomenon of checking, potentially enabling consumers to benefit from fewer broken biscuits.