Influence of water activity on acoustic emission of flat extruded bread

Flat extruded wheat bread and rye bread were subjected to three-point breaking test with simultaneous registration of generated vibrations. Piezoelectric sensor was used to detect vibrations and amplitude-time records were analyzed. It was found that breaking of flat extruded bread generated vibrations in whole audible spectrum. Acoustic emission signal energy expressed in arbitrary units was more dependent on water activity in low frequencies region than those in high frequencies. Transformation of amplitude-time records from time domain to frequency domain showed that regions with high level of power spectrum occurred. Dominating power spectra were in 1-3 and 7-15 kHz. It was proposed to use a ratio between those power spectra as a measure of acoustic activity of the investigated material. This descriptor was called partition power spectrum slope-beta. The slope was doubled in the water activity range 0-0.5 and at higher water activities it increased sharply with increasing wetness of the material. Special software was developed to analyze amplitude-time records and detect single acoustic emission events. Majority of acoustic emission events lasted 68.1 mu s, and their energy statistically was not dependent on water activity. However, number of acoustic emission events depended strongly on water activity and decreased almost 20 fold in the a(w) range from 0.03 to 0.75. Analysis of acoustic activity of extruded breads in both time and frequency domains suggested that it originated from simultaneous disintegration and structure microdislocations. Breaking of air cell walls caused large displacements and generated vibrations with low frequencies. Deformation of substructures, splitting of fibers, vibrations and dislocations of macromolecules in the network of the solid matrix could be responsible for generation of high frequency vibrations. It was also noted that both breads were alike when acoustic emission was analyzed in frequency domain, but they were different in the time domain. Combining both frequency and time domains showed that the breads, from the acoustic point of view, are different. (c) 2006 Published by Elsevier Ltd.