Critical water activity

LIG. 37 Stability map for dairy powders containing amorphous lactose. The critical water activity (0.37 aw) corresponds to the water activity of amorphous lactose with Tg of 24 °C (and a moisture content of 6.8 g water/100 g solids) [reproduced with permission from Roos (2003)]. 

LIG. 38 Graph of a modified state diagram for instant dent starch, which includes both DSC rg midpoint data plotted as a function of moisture content (% wb) and sorption isotherm data obtained at 20 °C. DSC Tg midpoint data were fit to the Gordon-Talyor equation, and sorption isotherm data were fit to the GAB equation. The moisture content associated with the Tg at 20 °C is called the critical me and is equal to 21.8% (wb), and the corresponding critical water activity value is 0.92. 

The stability of dairy products decreases sharply above a critical water activity (section 7.8). This decrease in stability is related to the influence of water on the glass transition and the role of water as a plasticizer of amorphous milk constituents (Roos, 1997). 

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... 

The obtained relationships will allow us to know the critical water content (CWC) and the critical water activity (CWA) at which the glass transition in the water-soluble phase occurs at a determined storage temperature of the product. Above these values, this phase in dried pear become sticky and rubber, and the crystallization of the amorphous compounds could take place. At 30°C (temperature at which the isotherms were obtained). 

Tymczyszyn, E. E., Diaz, R., Pataro, A., Sandonato, N., Gomez-Zavaglia, A., Disalvo, E. A. (2008). Critical water activity for the preservation of Lactobacillus bulgaricus by vacuum drying. Interruitional Journal of Food Microbiology, 128(2), 342-347. 

It follows from Equation 5.9 that there is one precise water activity where hydrate and anhydrate are in equilibrium. At lower water activities, the anhydrate is stable and above the hydrate it is the thermodynamically stable form. Knowledge of this critical water activity is of great importance in order to be able to decide which form is the preferred one. If for example, only one hydrate can be formed and the critical water activity for hydrate formation is 0.1 (which corresponds to 10% relative humidity), the hydrate may be a very appropriate form for development, as it will be the thermodynamically stable one at ambient humidities. The critical water activity is also important for the design of a stable crystallization process, since it has to be ensured that the water activity of the solution or suspension stays either below or above this value depending on whether the hydrate or the anhydrate should be produced. 

In Figure 5.7, the free energy of the dihydrate of carbamazepine is plotted as a function of the water activity of the environment. The critical water activity of 0.6 was determined by suspension equilibration experiments, as described in Section 5.5.2. 

The slurry technique can also be used to determine the critical water activity of hydrate formation. If a mixture of hydrate and anhydrate is slurried in a solvent/-water mixture of known water activity, the hydrate has to convert to the anhydrate when the water activity of the solvent is below the critical water activity of hydrate... 

In the pharmaceutical industry, often stability is the overriding factor so, in most cases, the stable polymorph or a hydrate will be chosen for development. As outlined in Section 5.3, the stable form has the normally undesired property of being the least soluble one, but that is often considered a smaller problem than the potential conversion of a metastable form to the stable form during storage. If hydrates exist, the decision becomes more diSicult, since the thermodynamic stability then depends on humidity. Knowledge of the critical water activity, the kinetics of conversion, the conditions of use, and so on are then crucial in order to be able to select the optimal form. 

Thermal/RH stability thermodynamic transition temperature critical water activity transformation kinetics... 

RH stability rate and mechanism of water uptake critical water activity deliquescence... 

Rank order of G Vi. RH or a, critical water activity (flcrit)... 

Eutectic points are not unique to the co-crystal domain. The pHmax for pharmaceutical salts and the critical water activity for hydrates are widely used eutectic points to establish stability conditions in the development of dosage forms. ... 

Figure 11.6 Schematic representation of relationships between water activity, water content, Tg and viscosity for wheat gluten-based films. Calculated values were obtained using the (GAB) equation [176], Couchman and Karasz equation (CK) [171], and Williams Landel and Ferry equation (WLF) [153]. The critical water activity (aw) and Me are indicated when Tg is equal to the ambient temperature...

---