A system is said to be in equilibrium when the water potential is the same at every location in the system. Food and other porous systems are often far from equilibrium, and this provides a second sense in which water can be bound. If the rate of movement of water in a system is so low that equilibrium can not be achieved within the normal lifetime of the food, then the water could be said to be bound. 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.