It is hypothesized that when foods enter the glassy state the movement of water is so slow that it is effectively bound. Colligative properties of solutions. Water activity is a direct measure of the energy state of the water in food. The following table shows water activity, and associated free energy values in various units. Freezing point depression.
Most people are familiar with the colligative properties of solutions.
One mole of an ideal solute in water lowers the freezing point of the water by 1.86 degrees C, raises the boiling point 0.5 degrees C and increases the osmotic pressure 22.4 atmospheres. People are often not aware, however, that these properties also apply to the adsorption of water in porous materials. These adsorptive forces are generally much larger than solute effects in intermediate moisture foods and other moderately dry porous media. Freezing point depression. An interesting result of this reduction in freezing point from the binding of water is that the water in food and other porous media does not all freeze at a single temperature like pure free water does.