What is Water Activity?

What It Is - And What It Isn't

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.

How is Water Activity Measured?

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 to 1.0 aw.

Why is Water Activity Important?  

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.

Measurement Systems

There are three basic water activity measurement systems. These are Resistive Electrolytic Hygrometers (REH), Capacitance Hygrometers and Dew Point Hygrometers (sometimes called chilled mirror).  

An Easy to Use, Portable Water Activity Test System

The Schaller Humimeter Water activity Measuring system is based on a propriety relativity humidity sensor. This sensor is placed on the top of a sample vessel, after loading a sample, and creates an airtight seal.  When the space above the sample reaches equilibrium the aw reading is displayed on the meter. The system comes with optional Kits.

Scigiene AW3 Kit  A Open

The Schaller Humimeter Water Activity Meter

Water actoivity metter Schaller Load the Sample,  Closed Sample Jar, Wait for stabilization, then read the Water Activity (AW)

See The Optional Kit details.

Water Activity is used to Check Safety of Foods

                  Some Typical Food Water Activity Levels (Aw)traffic_light_green_go_400_clr_3989


                            F
resh meat 0.98.                                         Salami 0.83 
                           
Cheese 0.97                                                Dried fruit  0.88                                                                                  Preserves 0.88                 Honey 0.75         Pasta 0.50                                               

Here are some of the water activity levels that start to become dangerous.

                Minimum Aw Values for Growth of  Microorganism Groups in Food

 

                                  Most spoilage bacteria 0.90.                              Halophilic bacteria 0.75.                                                                                  Most spoilage yeasts 0.88                                  Xerophilic moulds 0.61.                                                                                    Most spoilage moulds 0.80                              Osmophilic yeasts 0.61

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