AquaLab TDL 3 WATER ACTIVITY THEORY
3 Water Activity Theory
Water is a major component of foods, pharmaceuticals, and cosmet-
ics. Water influences the texture, appearance, taste and spoilage of
these products. There are two basic types of water analysis: moisture
content and water activity.
3.1 Moisture Content
The meaning of the term moisture content is familiar to most people.
It implies a quantitative analysis to determine the total amount of
water present in a sample. There are two primary methods for deter-
mining moisture content: loss on drying and Karl Fisher titration,
but you can also use secondary methods such as infrared and NMR.
Moisture content determination is essential in meeting product nu-
tritional labeling regulations, specifying recipes and monitoring pro-
cesses. However, moisture content alone is not a reliable indicator for
predicting microbial responses and chemical reactions in materials.
The limitations of moisture content measurement are attributed to
differences in the intensity with which water associates with other
components.
3.2 Water Activity
Water activity is a measure of the energy status of the water in a
system, and thus is a far better indicator of perishability than water
content. Figure 1 shows how the relative activity of microorganisms,
lipids and enzymes relate to water activity. While other factors, such
as nutrient availability and temperature, can affect the relationships,
water activity is the best single measure of how water affects these
processes. Researchers measure the water activity of a system by
equilibrating the liquid phase water in the sample with the vapor
phase water in the headspace and measuring the relative humidity
of the head-space. First place a sample in a sample cup that seals
inside the TDL sample chamber. Inside the sample chamber is a tun-
able diode laser and an infrared thermometer. The TDL determines
the vapor pressure in the headspace and the infrared thermometer
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