18
THEORY
3. THEORY
Water profoundly influences product attributes such as quality and safety. To completely
understand water relations in a product requires an understanding of the amount of water
(moisture content) that can be held at a given energy state (water activity). Moisture sorption
isotherms describe the relationship between water activity and moisture content at a
constant temperature. The nature of this relationship depends on the interaction between
water and other ingredients.
The amount of water vapor that a product has depends on the chemical composition,
physical-chemical state, and physical structure of the product. Consequently, the isotherm
shape is unique to each product type due to differences in capillary, surface, and colligative
effects (Figure 19). Products that lie in the low water activity portion of the isotherm are
often referred to as dry, those in the range of 0.60 a
w
to 0.90 a
w
are intermediate moisture
products, and those having water activities higher than 0.90 are high water activity products.
Figure 19 Moisture adsorption isotherms
For ease of interpretation, isotherms are often identified by Brunauer classifications
(Brunauer, 1945). Most food and pharmaceutical products fall under type I, II, or III. Type I
isotherms are typical of very hygroscopic materials. Type II (sigmoidal) isotherms are typical
for intermediate moisture products. Type III (J-shaped) isotherms are typical for crystalline
and coated materials. These general classifications proved useful when isotherms on every
product was not feasible due to time and labor constraints. However, with automation and
improved speed, isotherms can easily be conducted on any product and the uniqueness of
each isotherm often proves more valuable than placing them in a common classification.
Constructing an isotherm consists of collecting water activity and moisture content data
at various points along the water activity range. The range of water activities you use often
depends on the situation, but normally run from 0.10 a
w
up to 0.90 a
w
. You can run the
range of a
w
by controlling water activity levels using saturated salt slurries or mechanical
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