What is the importance of water activity in food?

Water is an important constituent of all foods. In the middle of the twentieth century scientists began to realize that the active water could be much more important to the stability of a food than the total amount of water present.

Qualitatively, water activity or Aw is a measure of unbound, free water in a system available to support biological and chemical reactions.

Water activity, a thermodynamic property, also can be defined in still other terms in accordance with Raoult’s law. Thus, Aw of a solution is quantitatively equal to the vapor pressure of the solution divided by the vapor pressure of pure water.

Water activity affects the textural properties of food. Foods with high Aw have a texture that describe as moist, juicy, tender and chewy.

When the Aw of these products is lowered undesirable textural attributes such as hardness, dryness, staleness and toughness are observed.

Water activity is widely used to predict the stability of food with respect to the potential for growth of micro-organisms and also some for the physical, chemical and enzymatic changes that lead to deterioration.

Critical water activities are found where the product becomes unacceptable from a sensory standpoint.

In packaged foods, the water-vapor permeability of the packaging material is a decisive factor in controlling changes in moisture content and thus the water activity of packaged foods.
What is the importance of water activity in food?