Water Activity (aw) Measurement – Some Theory Basics

Measuring water activity (aw) or equilibrium relative humidity (ERH) is a crucial parameter in the quality control of moisture-sensitive products and materials. Water activity is defined as the free or chemically unbound water present in food, pharmaceuticals, cosmetics, and other products.

Let's start with some basic theory:

What is water?

Water (H2O) is a clear liquid found everywhere on the planet. There is a water cycle: evaporation of water from streams, lakes, rivers, and oceans, then condensation of water vapor in the atmosphere, followed by precipitation in the form of rain falling back to earth.

Evaporation occurs depending on the saturation pressure of water. The vapor pressure of water, or saturated vapor pressure, indicates the amount of water vapor that can exist as a gas mixed with air and depends heavily on temperature:

Saturated vapor pressure at different temperatures

Relative humidity is the amount of water vapor pressure present in the ambient atmosphere relative to the saturated vapor pressure. Condensation occurs when the relative humidity is above 100%. Water vapor condenses in the air and water droplets appear.

Water is the main component of most living organisms. From a biological point of view, water has many distinct properties that are essential for the proliferation of life. It fulfills this role by allowing organic compounds to react in a way that enables their replication. All known forms of life depend on water.

Which products contain water?

There are two main categories:

Hygroscopic: substances that absorb water from their environment

• • Salts
• • Plant fibers (wood, paper, agricultural products, etc.) )
• • Meat products

• • Gases
• • Sand, powdered glass
• • Powdered metals

What is water activity (aw)?

Water activity is often described as a measure of “free” or “chemically unbound” water. These terms may be easy to understand, but they do not cover all aspects of water activity.

The correct definition is: "The water activity (aw) of a food is the ratio of the vapor pressure of the food itself, when in perfect equilibrium with the surrounding air, to the vapor pressure of distilled water under identical conditions. A water activity of 0.80 aw means that the vapor pressure is equal to 80% of that of pure water. Water activity increases with temperature. The moisture state of a product can be measured as equilibrium relative humidity (ERH) expressed as a percentage or as water activity expressed as a decimal. (Source: U.S. Food and Drug Administration)

The formula for aw is: aw = p / ps

Where p is the water vapor pressure above the surface of the product and ps is the water vapor pressure above a surface of pure water. Both are measured at the temperature of the product and, as a ratio, the value is always between 0 and 1. Both parameters in the aw formula depend on temperature and, therefore, aw also depends on temperature. This is because temperature changes how water interacts with the product being tested.