Literature on the crispy/crunchy behavior of cellular solid foods with a crust is discussed. The emphasis is on products with a dry crispy or crunchy crust as bread and various snacks and especially on mesoscopic and macroscopic aspects. Successively, the sensory sensations involved, the mechanical and fracture behavior of crispy/crunchy products, morphological aspects, and the relation between crispy and crunchy behavior and mobility of the macromolecules and plasticizer (primarily water) involved, are discussed. Finally, some ideas for an integrated approach of crispy/crunchy behavior of cellular solid foods with a dry crust will be discussed.

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This article describes sound properties that are special for crispy foods and relates them to general knowledge on the acoustic emission and mechanical properties of fracturing materials. From properties like the occurrence of sound, the sound energy, the duration of single sound waves, the time interval between the sound pulses and from the force drops involved, it was possible to calculate the morphological constraints for dry, crispy cellular foods. We present data for the minimum and maximum sizes of the pores and of the sizes of the solid material element surrounding them. The loss of crispness in foods caused by aging under deteriorating circumstances was found not to be caused by changes in the type of elements that fracture.

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FRe usage, including eating, of foods and intermediate products normally involves large deformations. Fracture and/or yielding then become the salient features. Several methods to study the behaviour food materials at large deformations are discussed. It is shown that it is possible to measure real material properties, independent of the method and the size and shape of the test-piece used. Results for Gouda cheese and potato starch gels are given.JournaI of Texture Studies 23 (1992) 245-266. A11 Rights Reserved.

The fracture properties of foods are relevant to texture but can be dificult to measure because of limitations of size or shape. Many established engineering tests for the measurement of crack growth and unstable propagation of cracks require specific test geometries, sizes and compliances of the specimen. With food materials this is rarely possible. The wedge penetration technique can usefully be adapted to foods to determine fracture parameters of brittle and semibrittle foods such as moderately hard cheeses and raw and cooked fruits and vegetables.Journal of Texture Studies 22 (1991) 45-57. All Rights Reserved.

A detailed study was performed to simultaneously measure the mechanical and acoustic properties of crispy cellular solid foods. Different critical aspects are discussed in order to assess optimal test conditions. These are primarily data sampling rate, microphone positioning, frequency spectrum of interest, sound/noise ratio and selection of measuring probe. A data sampling rate of more than 50 kHz was shown to be sufficient to register fracture event and acoustic event, and the frequencies audible by human ear (at least 40 kHz needed). The optimum positioning of the microphone with respect to the test piece should be a compromise between a distance that the microphone registers a good sound over the whole human audible frequency spectrum and a good sound/noise ratio. It is shown that test method selection has to depend on whether the goal is determining material fracture behavior or correlation of data to consumer perception. The best method from a fracture mechanics point of view does not have to be the best choice for a combined fracture and acoustic measurement. PRACTICAL APPLICATIONS The method described will especially be useful for the study of materials that fracture in a brittle way whereby during the fracture process, an audible sound is emitted (as is the case for crispy and crunchy food products). Although the data reported are for dry products, the method will be relevant for the study of all kinds of crispy/crunchy products. This work also shows that for a study directed on elucidating the mechanisms determining crispy/crunchy behavior of foods, a much higher data sampling rate is required than used in most studies published in literature. Moreover, guidelines are given for the positioning of the microphone and the selection of the measuring probe for measuring simultaneously the mechanical and acoustic fracture properties of crispy/crunchy cellular solid foods.

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