Water activity is a dimensionless quantity used to represent the energy status of the water in a system. It is defined as the vapor pressure of water above a sample divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one. It is widely used in food science as a simple, straightforward measure of the dryness of food; foods typically have an optimum water activity at which they have the longest shelf life.

It is also used in the pharmaceutical industry and chemicals for moisture sensitive products. There are several factors that determine the water activity of a substance. Colligative effects of dissolved species (e.g. salt or sugar) interact with water through dipole-dipole attraction, ionic bonds and hydrogen bonds.

Capillary effects also influence water activity, because the vapor pressure of water above a curved liquid meniscus is less than that of pure water. Surface interactions, in which water interacts directly with chemical groups on undissolved ingredients (e.g. starches and proteins), can also affect water activity. In this case the interaction can be through ionic bonds, van der Waals forces, hydrophobic interaction and hydrogen bonds.

These factors combine to increase the energy required to cause the water to evaporate, and therefore reduce the humidity above the sample at equilibrium. These factors can be grouped under two broad categories: osmotic and matrix effects. Due to varying degrees of osmotic and matrix interactions, water activity describes the continuum of energy states of the water in a system.

The water appears “bound” by forces to varying degrees. This is a continuum of energy states rather than a static “boundness”. Water activity is sometimes defined as “free”, “bound”, or “available water” in a system.

Although these terms are easier to conceptualize, they fail to adequately define all aspects of the concept of water activity. Water activity is very temperature dependent. Temperature changes water activity due to changes in water binding, dissociation of water, solubility of solutes in water, or the state of the matrix.

Although solubility of solutes can be a controlling factor, control is usually from the state of the matrix. Since the state of the matrix (e.g. glassy vs. rubbery state) is dependent on temperature, one should not be surprised that temperature affects the water activity of the food.

The temperature dependence of water activity varies between substances. Some substances have increased water activity with increasing temperature, while others show a decrease with increasing temperature. Most high moisture foods have negligible change with temperature.

One can therefore not predict even the direction of the change of water activity with temperature, since it depends on how temperature affects the factors that control water activity in the substance.

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