Water Activity

Foods can contain different kinds of water, depending on the water content. Water content is a poor predictor of food stability. The stability of foos will be determined by the extent to which water is available as a solvent, as reactant or to support microbial development. Despite some foods have a low moisture content, the availability of water can be high because of poor interaction with other food matrix components. Hence it can act as free water.

Water activity is a way to describe in a quantitative manner the interaction between food and water, giving an idea of how strongly the water is bound to the food, thus indicating the availability of water in food.

It is the chemichal potential of the water in foods over the chemical potential of free water at the same temperature. It is practically equal to the partial vapour pressure just above the food matrix in a very small layer of air which is in full equilibrium with the food. (formula 4 de water activity)

Its value can vary between 0 and 1 (1 represents free water and 0 constitutional water).

Due to the fact that temperature has an impact on the mobility of the molecules, water becomes more available at higher temperatures because.

From the temperature dependence of water activity at constant water content, the heat of sorption can be calculated.

In order to measure stability in foods we need to identify the relationship between water content and water activity ( Eg. If water activity and water content are low, it means the water is bound, if water is bound there is high stability) this relationship is temperature dependent; therefore it can only be described under isotherm conditions.

Factors affecting aw

·         moisture content

·         temperature

·         ice/water

·         kind of compounds

·         history of the food

·         sorption isotherm