A water molecule is two hydrogen atoms bound to one oxygen.

Water Activity

What is Water Activity?

Water activity (αw) is a measurement that denotes the microbiological activity of a food, as well as implies the storage stability of food items.

For example, a dry good such as a cracker or pretzel would have a low (αw) value and exhibit high storage stability, whereas a cake with a higher (αw) would exhibit a short shelf life.

The water activity (αw) of a food is the ratio between the vapor pressure of the food itself, when in a completely undisturbed balance with the surrounding air media, and the vapor pressure of distilled water under identical conditions.1 A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. It is relevant to food quality and safety issues.1


Preservation of food with low moisture has been used to protect food quality and safety for thousands of years. In recent decades, water activity has become the traditional approach used universally to try to answer the question, “how dry is dry enough?”

How does it work?

The αw of a food describes the degree to which water is “bound” in the food, its availability to participate in chemical/biochemical reactions and its availability to facilitate growth of microorganisms. Low water activity can help inhibit microorganism growth. The bacteria cell can only transfer nutrients in and waste materials out through the cell wall. The materials must be in soluble form to permeate the cell wall. Therefore certain “available” water is required for microorganism’s growth.


There is no device that can directly measure the water activity. Rather, αw is measured with an indirect method. It is measured by equilibrating the liquid phase water in a sample with the vapor phase water in the headspace of a closed chamber and measuring the relative humidity of the headspace. Methods for water activity determination are detailed in the Official Methods of Analysis of AOAC International.


Water activity (αw) is the most commonly used criterion for safety and quality.2 Figure 1 shows stability in terms of microbial growth limits and rates of degradative reactions as a function of water activity.2

A diagram of water activity vs. food stability.

Figure 1: Diagram of water activity vs. food stability.2

By measuring and controlling the water activity, it is possible to:2

  • Predict which microorganisms will be potential sources of spoilage and infection
  • Maintain the chemical stability of products
  • Minimize non-enzymatic browning reactions and spontaneous autocatalytic lipid oxidation reactions
  • Prolong the activity of enzymes and vitamins
  • Optimize the physical properties of products such as moisture migration, texture and shelf life.

Several points need to be paid attention to. It may be combined with other preservative factors, such as temperature, pH, redox potential, etc., to establish conditions that inhibit microorganisms.3

The αw can be manipulated in foods by a number of means, including addition of solutes such as salt or sugar, physical removal of water through drying or baking, or binding of water to various macromolecular components in the food.3

Limitations and new approach

Water activity measurement needs a thermodynamic equilibrium system. However, the food system is in kinetically metastable, dynamically constrained glassy states.4 The glassy state is a transition between two different equilibrium states.

Solutes can vary in their ability to inhibit microorganisms at the same αw value. For example, the lower αw limit for the growth of Clostridium botulinum type A has been found to be 0.94 with NaCl as the solute versus 0.92 with glycerol as the solute.3

Although measurement has limitations, it is still a good measurement for food safety and quality protection.

FDA Regulation

FDA defines low-acid and acidified canned foods with pH and αw and gives the guidance for commercial processors of acidified and low-acid canned foods.5  FDA also regulates that food are not considered as Potentially Hazardous Food (PHF) when their αw is 0.85 or lower.6


  1. “Water Activity (aw) in Foods.” U.S. Food and Drug Administration, 27 Jan. 2015. Accessed 02 Feb. 2017.
  2. “Fundamentals of Water Activity.” Pullman, Washington: Decagon Device, 2006. Accessed 02 Feb. 2017.
  3. “Evaluation and Definition of Potentially Hazardous Foods – Chapter 3. Factors That Influence Microbial Growth.” U.S. Food and Drug Administration, 19 Mar. 2015. Accessed 03 Feb. 2017.
  4. Slade, Louise, Harry Levine, and David S. Reid. ” Beyond Water Activity: Recent Advances Based on an Alternative Approach to the Assessment of Food Quality and Safety.” Critical Reviews in Food Science and Nutrition 30.2-3 (1991): 115-360.
  5. “Guidance for Commercial Processors of Acidified & Low-Acid Canned Foods.” U.S. Food and Drug Administration, 08 Nov. 2016. Accessed 02 Feb. 2017.
  6. “Evaluation and Definition of Potentially Hazardous Foods – Chapter 2. Current and Proposed Definitions of “Potentially Hazardous Foods”.” U.S. Food and Drug Administration, 26 Nov. 2014. Accessed 03 Feb. 2017.