Water activity is a measure of water in a material that is available to react with or attach itself to another material. This unattached water is referred to as “Free" water, while the unavailable water is called “Bound”.
Water Content is not Water activity but is the total amount of water in a product - both Bound Water and Free Water.
Check out this article, It deals with the differences between Water Activity and Water Content.
The typical system uses a sealed temperature controlled chamber. A sample is placed in the chamber and sealed. The free water is allowed to escape into the air in the chamber. it remains there until all the free water has left the sample. (A condition of equilibrium)
At equilibrium the relative humidity of the air in the chamber is measured. The relation of this reading to pure water is the water activity measurement expressed as the term aw. The range of water activity is from zero (no free water) to 1.0 (pure water)
Water activity (aw) is a critical factor in determining quality and safety of foods. It affects shelf life, safety, texture, flavor and smell. For example, bacteria does not grow at water activities below aw 0.9 and most molds cease to grow at water activities below aw 0.80.
By measuring water activity it is possible to predict which microorganisms will be possible sources of spoilage. In addition to influencing microbial spoilage, water activity can affect the activity of enzymes and vitamins in foods. It also has implication on color, taste and aroma.
There are three basic water activity measurement systems. These are Resistive Electrolytic Hygrometers (REH), Capacitance Hygrometers and Dew Point Hygrometers (sometimes called chilled mirror).
The equipment we offer at CSC Scientific uses the Resistive Electrolytic technique
Some Typical Food Water Activity Levels (Aw)
Fresh meat 0.98. Salami 0.83
Cheese 0.97 Dried fruit 0.88 Preserves 0.88 Honey 0.75 Pasta 0.50
Minimum Aw Values for Growth of Microorganism Groups in Foods
Most spoilage bacteria 0.90. Halophilic bacteria 0.75. Most spoilage yeasts 0.88 Xerophilic moulds 0.61. Most spoilage moulds 0.80 O. Osmophilic yeasts 0.61