EFFECT OF EQUILIBRIUM RELATIVE-HUMIDITY ON THE MECHANICAL SIGNATURES OF BRITTLE FOOD MATERIALS

Three brittle commercial bakery products (bread sticks, pretzels, and zwiebacks) were compressed after equilibration at five relative humidity levels with a computer-interfaced Universal Testing Machine. Their mechanical signatures are presented in the form of conventional force-time relationships, power spectra calculated by the fast Fourier transform, Richardson plots from which the apparent fractal dimension was calculated, and displays of the symmetrized-dot pattern (SDP) produced from the original force-time relationships. It is demonstrated that, although the original force-time relationships were irregular and irreproducible, consistent patterns and values did emerge in the transformed data. Plasticization, as a result of moisture sorption, is clearly expressed in the disappearance of the high-frequency region of the power spectrum and a drastic decrease in the magnitude of the apparent fractal dimension. The differences among the shapes of the original force-deformation curves before and after moisture sorption are also clearly evident from the SDP displays, but the sensitivity of the method needs to be established with a larger experimental data base.