Cereal Chem. 73 (2):225-231  |  VIEW ARTICLE

Rheology

Patterns of Textural Changes in Brittle Cellular Cereal Foods Caused by Moisture Sorption (1).

Meg Harris (2) and Micha Peleg (2,3). (1) Contribution of Massachusetts Agricultural Experiment Station, University of Massachusetts, Amherst. Supported by the USDA-NRI program under grant 920954. (2) Department of Food Science, University of Massachusetts, Amherst, MA 01003. (3) Corresponding author. Fax: 413/545-1262. E-mail: <micha.peleg@foodsci.umass.edu> Accepted October 30, 1995. Copyright 1996 by the American Association of Cereal Chemists, Inc. 

The irregular compressive force-deformation curves of two puffed snacks and two types of croutons at various water activities (a(w)) were fitted with a polynomial equation. The values of the fitted force at 20 and 30% deformation were measures of these materials' stiffness. The degree of jaggedness of the normalized mechanical signatures, created as a plot of the residuals divided by the corresponding fitted force values, was quantified in terms of an apparent fractal dimension and the mean magnitude of the Fourier power spectrum parameters whose value appears to be associated with brittleness and crunchiness. Plots of the magnitude of the two jaggedness parameters versus a(w) had a typical sigmoid shape that could be described by the original or shifted Fermi equation with similar characteristic constants. The relationship between the stiffness parameters and a(w) either had the typical Fermian sigmoid shape or had a peak at an intermediate a(w). This suggests that partial plasticization that reduced brittleness could also reduce fragility. In all four products, the characteristic a(w) level for jaggedness loss differed from that of the stiffness loss. This observation, which is in agreement with previous reports, indicates that different textural attributes need not change in unison as a result of moisture sorption, and therefore, that a transition from a glassy to a rubbery state can have a different manifestation in different mechanical properties.