AquaLab
3. Water Activity eory
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3. Water Activity eory
Water is a major component of foods, pharmaceuticals, and cosmet-
ics. Water in uences the texture, appearance, taste and spoilage of these
products. ere are two basic types of water analysis: moisture content
and water activity.
Moisture Content
e meaning of the term moisture content is familiar to most peo-
ple. It implies a quantitative analysis to determine the total amount
of water present in a sample. Primary methods for determining
moisture content are loss on drying and Karl Fisher titration, but
secondary methods such as infrared and NMR are also used. Mois-
ture content determination is essential in meeting product nutri-
tional 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 materi-
als. e limitations of moisture content measurement are attributed
to di erences in the intensity with which water associates with other
components.
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 a ect the relationships,
water activity is the best single measure of how water a ects these
processes.
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