Water plasticization state diagrams

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

Figure 1-25 Modified State Diagram Showing Relationship Between Glass Transition Temperature (Tg), Water Activity (GAB isotherm), and Water Content for an Extruded Snack Food Model. Crispness is lost as water plasticization depresses Tg to below 24X2. Plasticization is indicated with critical values for water activity and water content. Source Reprinted with permission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 99, 1995, Institute of Food Technologists.

Figure 5.4. A schematic state diagram showing water plasticization at increasing water weight fraction towards glass transition of water at -135°C. Relaxation times decrease rapidly above the glass transition as a result of thermal or water plasticization. Maximally freeze-concentrated solutes show glass transition at Tg and onset of ice melting at TJ. Equilibrium melting is described by the T curve.

Figure 13. A schematic 3-dimensional state diagram for a hypothetical 3-component aqueous system. The two solutes (e.g. polymer + monomer) are toth non-ciystallizing, interacting, and plasticized by water, which is the crystallizing solvent. The diagram illustrates the postulated origin of a sigmoidal curve of Tg vs. w% solute composition. (Reproduced with permission from reference 3. Copyright 1988 Oxbridge University Press.)...

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