Water Activity (aw) Measurement – Some Theory Basics

The measurement of 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 non-chemically bound water in foods, pharmaceuticals, cosmetics and other products.

Let's start with some basic theories:

What is water?

Water (H2O) is a transparent liquid found throughout the planet. Awater cycle exists: evaporation of water from streams, lakes, rivers and oceans, then condensation of the water vapor in the atmosphere followed by precipitation with rain falling back down to earth.

Evaporation occurs based upon the saturation pressure of water. The vapor pressure of water, or saturation vapor pressure, indicates the amount of water vapor that can exist as gas mixed in the air and is very temperature dependent:

Temperature (°C)	Water Vapor Pressure (kPa)
0	0.61
10	1.23
20	2.34
30	4.24
40	7.37
50	12.33
60	19.92
70	31.18
80	47.34
90	70.11
100	101.33 = 1 atm

Saturation vapor pressure at different temperatures

Relative humidity is the amount of water vapor pressure present in the ambient atmosphere compared to the saturation vapor pressure.

Condensation occurs when the relative humidity is above 100%. Water vapor will condense from the air and water droplets will appear.

Water is the major component of most living organisms. From a biological standpoint, water has many distinct properties that are critical for the proliferation of life. It carries out this role by allowing organic compounds to react in ways that ultimately allow replication. All known forms of life depend on water.

What products contain water?

Two broad categories exist:

Hygroscopic: substances that absorb water from their surroundings

• •Salts
• •Vegetal fibres (wood, paper, produce…)
• •Meat products

Hydrophobic: substances that don't absorb water from their surroundings

• •Gases
• •Sand, glass in powder form
• •Metals in powder form

What is water activity (aw)

Water activity is often described as a measure of the “free” or “non-chemically bound” water. These terms might be easy to understand, but do not cover all aspects of water activity.

The correct definition is: “The water activity (aw) 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. A water activity of 0.80 aw means the vapor pressure is 80 percent of that of pure water. The water activity increases with temperature. The moisture condition of a product can be measured as the equilibrium relative humidity (ERH) expressed in percentage or as the 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 product surface and ps the water vapor pressure above a surface of pure water. Both are measured at the product temperature and as a ratio the value is always between 0…1. Both parameters in the aw formula are temperature dependent and as such aw is temperature dependent. This is because temperature changes how water interacts with the product being tested.

Learn more about water activity on this video.

Watch out for more information about “Why Measure Water Activity” soon on this blog.