What is Water Activity?

Water activity (Aw) refers to the amount of free water that is available in food for microbial growth. It is a measurement of the water that is not bound to components in the food, and therefore available for microbial growth. 

Water activity is measured in values from 0.0 to 1.0. The lower the Aw value, the more "dry" a food item is considered. However, water activity should not be regarded as moisture content. The scientific definition of water activity refers to its measurement in equilibrium relative humidity. You can think of water activity as the humidity of a food.

When water activity is above .85, several pathogenic bacteria can grow and may cause food poisoning. MN's cottage foods law requires cottage foods to have a Aw value of .85 or less to prevent the growth of potentially harmful bacteria.

The water activity of a food product can be lowered in a variety of ways.

• Salt and/or sugar can be added. Both salt and sugar work by binding with free water in the food product, making that water no longer available to microorganisms for growth. Salt is more effective at binding water than sugar, but the influence of taste should be considered before choosing to alter your recipe.
• Water activity may also be decreased by removing water from the food product. This can be amount of time and allowing more water to evaporate.
• Water can also be removed post-cooking through dehydration. This method is often used to preserve fruits and meats. Dehydration works by removing free water, reducing the amount that is available for microorganisms to grow.

Excerpted from "Understanding the Water Activity of Your Food," Virginia Cooperative Extension, 2012.

Using The Elitech GSP-6 Meter to Measure Water Activity in Cottage Foods

• USER GUIDE: instructions to annually CALIBRATE and USE the GSP-6 to test cottage foods (print)

• Email info@mfma.org for your copy of the Water Activity Excel Correction Tool (ECT) for the Elitech GSP-6. 

• Cottage Foods Testing Log Keep your testing log in a place where you can find it if a food inspector ever asks to see the results of your tests.   

Order the Elitech GSP-6 Water Activity Kit

You order these kits from our MFMA Store.

Full starter kit,

$85 (includes taxes, S & H)

Partial kit (if you already have the Elitech GSP-6), $37 (includes taxes, S & H)

Calibration standards (for annual calibration of Elitech GSP-6), $24 (includes taxes, S & H)