Water Activity Th Eory; Moisture Content; Water Activity - Decagon Devices AquaLab 4TE Operator's Manual

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3. Water Activity Th eory

3. Water Activity Th eory
Water is a major component of foods, pharmaceuticals, and cosmet-
ics. Water infl uences the texture, appearance, taste and spoilage of these
products. Th ere are two basic types of water analysis: moisture content
and water activity.

Moisture Content

Th 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. Th e limitations of moisture content measurement are attributed
to diff 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 aff ect the relationships,
water activity is the best single measure of how water aff ects these
processes.
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