Water activity is simply the ratio of the water vapor pressure in any kind of food system to the water vapor pressure of pure water. aw = p product/p water
This simple mathematical equation bas a very important meaning in food technology and in many biological areas. In this article three aspects of the application of water activity in food preservation are discussed.

1. The relationship between water activity and food preservation from the point of view of microbial growth, one of the most important aspects of food preservation
2. Water activity prediction using theoretical equations.
3. Experimental determination of water activity using the NOVASINA Thermoconstanter measuring instrument

A number of food processing operations involve the transfer of water from one point to another effective dehydration, for example, is contingent on a knowledge of the water activity in food.

A further point of significance is moisture gain and loss in packaged foods. In order to predict the shelf life of foods the water activity also bas to be predicted.
Finally, food processing in the form of salting or sugaring also depends on a knowledge of water activity

"osmotic dehydration", the moisture transfer process which occurs during the salting of fish or cheese, etc.
The food with its specific water activity is surrounded by a solution with lower water activity The driving force between the internal and external water activity produces water flow which is known as osmotic dehydration. There is also a certain amount of solute diffusion into the food resulting in a reduction of water activity to a level sufficient to prevent microbial growth and hence to preserve the food.

Moisture migration in a dry food mix is also a very important problem and a good example of the application of water activity. In a dry soup containing dried chicken pieces, beef, carrots, potatoes and rice, for example, and packed in a moisture-proof package, water transfer takes place between the different ingredients. If all the ingredients have different moisture contents and levels of water activity the water transfer will continue until a state of equilibrium is reached, i.e. until all ingredients have the same water activity levels, but not necessarily the same moisture content.

A major application of water activity concerns the control of microbial growth. Other aspects such as quality or organoleptic properties are important, but safety in food is the first and most significant criterion, and this means the control of microbial growth. The lowest limit for growth in foods or any other item is around aw 0.6. In the narrow range between aw 1 and aw 0.6 a large number of micro organisms can grow which are potentially dangerous to food. As a result, the regulatory agencies in many countries are now beginning to define water activity standards for processed foods.