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.

No abstract

Very little is known on the rate dependency of the fracture behavior of crispy products as a function of water activity (Aw). Therefore, the effect of deformation speed on instrumental and sensory crispness was studied as a function of Aw. Deformation speed clearly affects the transition Aw range from crispy to non‐crispy. At low deformation speed, the critical Aw was 0.40 ± 0.02 and at high deformation speed between 0.5 and 0.6. The transition was completed at an Aw = 0.56 ± 0.10 and 0.74 ± 0.00 for the low and high speed, respectively. The transition of sensory crispness at regular biting speed started at Aw = 0.48 and was completed at Aw = 0.75. The deformation speed effect is assumed to be caused by the viscoelastic nature of the material in the transition region causing it to behave locally relatively more viscous at low deformation speed and relatively more elastic at high deformation speed. This behavior depends on the plasticizer content, thus, Aw. For the product used in this study, the instrumental deformation speed that best correlate to sensory data is 10 to 40 mm/s. Sensory tests show that crispy attributes are perceived more intense when biting at higher speed. PRACTICAL APPLICATIONS A good characterization of the water activity (Aw) range over which crispness of a food product is lost is of great practical importance. The present study describes the effect of deformation speed and Aw on crispness characteristics as determined by instrumental and sensory measurements. Crispness characteristics were found to depend not only on Aw but also on deformation speed. The Aw at which the crispy material started to lose it crispness started at lower values of Aw for the low deformation speed. Instrumental data should follow a similar trend as sensory crispness as function of Aw. At high deformation speed, instrumental sound data were more closely related to loss of crispness than force data, mainly caused by problems in the force registration. To get a good correlation of sensory and instrumental data, a correct deformation speed should be selected for this purpose. Sensory tests at different biting/chewing speed were done. Crispness attributes were scored lower when Aw was higher, as well as when decreasing the biting/chewing speed.

Toasted rusk rolls with a coarser structure are sensed crispier than those with a fine structure, at the same water activity (aw). The present paper shows that this difference in crispness perception is related to differences in fracture behavior and accompanying acoustic emission. Both sensory and instrumentally determined crispness decreased gradually with increasing aw in roughly the same manner for both coarse and fine products. Nevertheless, the coarse rusk roll was perceived as being crispier than the fine one. Typically, in the coarse structure the measured “Number of Force Drops” of a relatively large size and the “Number of Sound Events per cross section area” of relatively large intensity were more numerous than in the fine one. Our data show that relatively large force drops and sound events are related to the more intense crispness perception and stronger sound sensation for the coarse structure toasted rusk roll. We propose the “Total Sound Energy per cross section area” and the “Mean Sound Event Intensity” to be primarily responsible for the higher crispness perception of the coarse rusk over the whole aw range. PRACTICAL APPLICATIONS Coarse structured toasted rusk rolls are sensed crispier than fine structured ones (same aw). Difference in crispness is related to fracture behavior and accompanying acoustic emission. Crispness decreased gradually with increasing aw similarly for coarse and fine products. Sound energy cq. Intensity seems primarily responsible for the higher crispness perception.