Effect of water activity on fracture and acoustic characteristics of a crust model

A crust model is described that is suited to study crispness of bread crusts as a function of steady state water activity. The study of crispness of this type of products as a function of water activity is complicated since the way a bread crust fractures does not depend on the crust only but also on the properties of the crumb, the curvature of the crust and on the gradient of water activity/content inside the crust. To circumvent these problems a baked crust model was developed. The crust model has similar properties as bread crust in terms of morphology and water adsorption behaviour. The suitability of using the crust model to investigate crispness was validated by studying the effect of water activity on its fracture behaviour and accompanying acoustic emission by using a wedge fracture test. Crispness of the crust model was characterized by a low work of fracture and a high number of sound and force events. When the water activity increased to 0.65 a significant increase of the force required to break the sample was observed, due to moisture toughening. Simultaneously a decrease in the number of force and sound events was found, pointing to a decrease of crispness of the crust model due to water plasticization.