Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 6

Desorption
Adsorption

WATER ACTIVITY

FIGURE 1.2 A schematic representation of sorption isotherms typical of food materials.
A hysteresis is often obtained between the adsorption and desorption isotherm.

determination of sufficient experimental data and fitting sorption models to the data.
A number of empirical and theoretical sorption models are available. 26,27 Some of
the models have proved to be useful in predicting water sorption by food materials,
particularly the Guggenheim-Anderson-DeBoer (GAB) model. 27
The well-known Brunauer-Emmett-Teller (BET) sorption model by Brunauer
et al. 28 has been applied to obtain the BET monolayer water content of foods. 29 The
BET monolayer value expresses the amount of water that theoretically may form a
layer of water molecules with the thickness of one molecule on the adsorbing surface.
The BET model is given by Eq. (1.3), where m is water content (g/100 g of solids),
m m is the monolayer value, and K is a constant.

m
m

a

=

w

(1.3)

( ) [ ]

( )

1 − a 1 +− K 1 a

m w w

The BET model can also be written into the linearized form, as suggested by
Eq. (1.4).

a
ma mK

1 K

mK a

− 1

w

= + (1.4)

( )

w

1 −

w m m

The applicability of the BET model is limited because it has proved to fit water
sorption data only over the narrow a w range from 0.1 to 0.5. 29 However, the BET
monolayer value ( Figure 1.1 ) has been suggested to be an optimal water content for
stability of low-moisture foods 4,30 and correlate with an optimum a w allowing the
longest shelf life. 25
The GAB adsorption model was introduced by van den Berg. 27,31 The GAB
model given by Eq. (1.5) is derived from the BET model, but it has an additional
parameter, C.

m
m

KCa ′

=

w

(1.5)

( ) [ ]

( )

1 − Ca 1 + K ′ − 1 Ca

m w w