Water activity glass transition temperature

Water activity, glass-transition temperature, and equilibrium moisture content for freeze-dried camu-camu pulp ( ) sorption isotherm, adjusted by GAB model, (A) glass-transition temperature adjusted by Equation 62.3. 

Figure 11.6 Schematic representation of relationships between water activity, water content, glass transition temperature and viscosity for wheat gluten based films. Calculated values were obtained by using the GAB equation [151], Couchman and Karasz equation [146] and Williams Landel and Ferry equation [128]. The critical water activity (a ) and moisture content (M ) are indicated when Tg is equal to the ambient temperature...

With continuous development of systems for controlled drug release, new materials are being used whose influence on peptide stability must be carefully examined. Thus, the model hexapeptide Val-Tyr-Pro-Asn-Gly-Ala (Fig. 6.30) embedded in poly (vinyl alcohol) and poly(vinyl pyrrolidone) matrices had rates of deamidation that increased with increasing water content or water activity, and, hence, with decreasing glass transition temperature (Tg). However, the degradation behavior in the two polymers differed so that chemical reactivity could not be predicted from water content, water activity, or T% alone. Furthermore, the hexapeptide was less stable in such hydrated polymeric matrices than in aqueous buffer or lyophilized polymer-free powders [132],... 

The versatility of water-soluble polyphosphazenes is in the variations in the structures that can be prepared. Structures with a low glass-transition temperature backbone can be modified with a variety of versatile side units. These may find use in solid polymeric ionic conductors, as a means to entrap and immobilize enzymes with retention of enzymic activity, and in biological functions as hydrogels with the capability of exhibiting biocompatibility and... 

Figure 1-23 Relationship Between Water Activity aw) and Glass Transition Temperature (Tg) of Some Plant Materials and Biopolymers. Source Reprinted with permission from J. Cherife and M. del Pinar Buera, Water Activity, Water Glass Dynamics and the Control of Microbiological Growth in Foods, Critical Review Food Sci. Nutr., Vol. 36, No. 5, p. 490, 1996. Copyright CRC Press, Boca Raton, Florida.

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.

For example. Figure 7.2 shows the glass transition temperature versus water activity relationship for lactose, sucrose and mixtures of sucrose-fructose and sucrose-Amioca, demonstrating that Tg decreases with increasing water activity (Roos and Karel 199Id). 

Figure 7.7. Effect of molecular weight on glass transition temperature of dextrans at various water activities (Reprinted from Carbohyd. Polym., 62, Icoz, Moraru, and Kokini, Polymer-polymer interactions in dextran systems using thermal analysis, pp. 120-129, Copyright (2005), with permission from Elsevier.)...

Dependence of the maximum force at breakage (MFB) with water activity for a dry starch model. D65 and D105 air dried at 65 and 105°C, respectively FD freeze-dried. Dotted line represents the glass transition temperature. 

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