A Model of Mechanical Changes in Biomaterials at and around Their Glass Transition

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Abstract
The transition of biological and food materials from a glassy to a leathery or rubbery state and the accompanying loss of stiffness, hardness, or strength are tranditionally described by a relationahip between the magnitudes of these parameters and the temperature, moisture, or water activity. At the transition region, the curve depicting such a relationship has a sigmoidal shape that can be described by the model, Y(X) = Ys/{1 + exp[X ‐ Xc)/a]}, where X is either temperature, moisture content, or water activity, Y(X) is a mechanical parameter (e. g., modulus), Ys is the latter's magnitude in the glassy state (assumed to be practically constant), Xc is a characteristic temperature, moisture, or aw level of the transition, and a is a constant. This model was tested with the published data of various biomaterials (baker's yeast, casein, coffee creamer, luncheon meat, sodium caseinate, sucrose/glucose glass, wheat grains, and bread). In all cases, it gave a consistent description of the mechanical changes at and around the transition and had a very satisfactory fit.