Fundamentals of Water Activity
"It is now generally accepted that aw is more closely related to
the physical, chemical, and biological properties of foods and
other natural products than is total moisture content. Specific
changes in color, aroma, flavor, texture, stability, and
acceptability of raw and processed food products have been
associated with relatively narrow aw ranges."
-Rockland LB & Nishi SK, Food Tech 34:42-59 (1980)
Water is recognized as being very important, if not critical, to the stability of most products. Controlling the water within a product, by some method of drying or by chemically/structurally binding (salting or sugaring) has long been used by man for preservation. This not only controls microbial spoilage, but also chemical and physical stability.
Water Content Alone is Not a Reliable Predictor
Traditionally, discussions about water in products or ingredients focus on moisture or water content, which
is a quantitative or volumetric analysis that determines the total amount of water present. Water content
of a product is a familiar concept to most people. One measures the water content by loss on drying,
infrared, NMR or Karl Fisher titration. Moisture content determination is essential in meeting product
nutritional labeling regulations, specifying recipes and monitoring processes. However, water content alone
is not a reliable predictor of microbial responses and chemical reactions in materials.
Chemically Bound Water is Unavailable to Microbes
The limitations of water content measurement as an indicator of safety and quality are attributed to
differences in the intensity which water associates with other components in the product. The water content
of a safe product varies from product to product and from formulation to formulation. One safe, stable product
might contain 15% water while another containing just 8% water is susceptible to microbial growth. Although
the wetter product contains proportionally more water, its water is chemically bound by other components,
making it unavailable to microbes. Using only water content values, it's impossible to know how "available"
the water in the product is to support microbial growth or influence product quality.
Water Activity is Most Relevant for Quality and Safety Issues
Another more important type of water analysis is water activity (aw). Water activity describes the
energy status or escaping tendency of the water in a sample. It indicates how tightly water is "bound,"
structurally or chemically, in products. Both the water content and the water activity of a sample must be
specified to fully describe its water status. However, water activity is the property most relevant for quality
and safety issues. Water activity is closely related to the partial specific Gibbs free energy of the system.
Thus, water activity is a thermodynamic concept and has requirements for measurements. These requirements are
that the system be in equilibrium, the temperature defined, and a standard state specified. Pure water is taken
as the reference or standard state from which the energy status of water in food systems is measured. The Gibbs
free energy of free water is zero; thus, the water activity is 1.0.
Water Activity is a Ratio of Vapor Pressures
Water activity is the ratio of the vapor pressure of water in a material (p) to the vapor pressure of pure water
(po) at the same temperature. Relative humidity of air is the ratio of the vapor pressure of air to
its saturation vapor pressure. When vapor and temperature equilibrium are obtained, the water activity of the
sample is equal to the relative humidity of air surrounding the sample in a sealed measurement chamber.
Multiplication of water activity by 100 gives the equilibrium relative humidity (ERH) in percent.
aw = p/po = ERH (%) / 100
As described by the above equation, water activity is a ratio of vapor pressures and thus has no units. It ranges from 0.0aw (bone dry) to 1.0aw (pure water).
"Bound" Water is Not Totally Immobilized
Water activity is sometimes described in terms of the amounts of "bound" and "free" water in a product. Although
these terms are easier to conceptualize, they fail to define all aspects of the concept of water activity.
"Free" water is not subjected to any force that reduces its energy; therefore, all water in food is "bound"
water. The issue is not whether water is "bound," but how tightly it is "bound". Water activity is a measure of
how tightly water is "bound" and related to the work required to remove water from the system. Water that is
"bound" should not be thought of as totally immobilized. Microbial and chemical processes are related to this
"bound" energy status in a fundamental way. Because water is present in varying energy states, analytical
methods that attempt to measure total moisture in samples don't always agree or relate to safety and quality.
Water activity tells the real story.
"Bound" or "Free" Water are Not Very Useful Descriptions
There are several factors (osmotic, matrix, and capillary) that control water activity in a system. It is a
combination of these factors in a product that reduces the energy of the water and thus reduces the vapor
pressure above the sample as compared to pure water. Due to varying degrees of osmotic and matrix interactions,
water activity describes the continuum of energy states of the water in a system rather than a static
"boundness". "Bound" or "free" are not a very useful description since it is an attempt to classify a continuum
in terms of discrete states.