Slide 1

Jangam, Mujumdar - Basic Concepts and Definition

Figure 1.3. Deterioration rates as a function of water activity for food systems

Water activity is temperature dependent. Temperature changes water activity due
to changes in water binding, dissociation of water, solubility of solutes in water, or the
state of the matrix. Although solubility of solutes can be a controlling factor, control is
usually from the state of the matrix. Since, the state of the matrix (glassy vs. rubbery
state) is dependent on temperature, one should not be surprised that temperature af-
fects the water activity of the food. The effect of temperature on the water activity of a
food is product specific. Some products shows an increase in water activity with increas-
ing temperature, others decrease aw with increasing temperature, while most high mois-
ture foods have negligible change with temperature. One can therefore not predict even
the direction of the change of water activity with temperature, since it depends on how
temperature affects the factors that control water activity in the food.

1.3.1.3. Equilibrium Moisture Content and Sorption Isotherms

The moisture content of a wet solid in equilibrium with air of given humidity and
temperature is termed the equilibrium moisture content (EMC). A plot of EMC at a given
temperature versus the relative humidity is termed sorption isotherm. An isotherm ob-
tained by exposing the solid to air of increasing humidity gives the adsorption isotherm.
That obtained by exposing the solid to air of decreasing humidity is known as the de-
sorption isotherm. Clearly, the latter is of interest in drying as the moisture content of
the solids progressively decreases. Most drying materials display “hysteresis” in that the
two isotherms are not identical.

Figure 1.4 shows the general shape of the typical sorption isotherms. They are cha-
racterized by three distinct zones, A, B and C, which are indicative of different water
binding mechanisms at individual sites on the solid matrix. In region A, water is tightly
bound to the sites and is unavailable for reaction. In this region, there is essentially mo-
nolayer adsorption of water vapor and no distinction exists between the adsorption and
desorption isotherms. In region B, the water is more loosely bound. The vapor pressure

Lipid oxidation

Browning reaction

Moisture isotherm

Enzymes Molds Yeast Bacteria

Relative

activity

Water activity, aw

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