Water Activity and Plasticization - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 7

The GAB model has been shown to fit experimental sorption data for almost all
food materials and cover the whole a w range. 27 The model is applicable to predict
water sorption for most foods and it can also be used to calculate the GAB monolayer
value. However, the BET and GAB monolayer values are not equal and neither of
these values can be considered as a real “stability” water content. Both the BET and
GAB monolayer values are dependent on temperature and they do not reflect sta-
bility-related changes in the physical state of foods. 6,32 An alternative approach is to
determine critical , stability controlling water content and water activity , based on
the determination of the steady state water content and a w that depress the glass
transition temperature to ambient or storage temperature. 5,32

T EMPERATURE D EPENDENCEOF W ATER A CTIVITYAND S ORPTION

The temperature dependence of the vapor pressure of water may be assumed to
follow the Clausius-Clapeyron relationship. Therefore, it can be shown that the
temperature dependence of a w also follows the Clausius-Clapeyron relationship 29
according to Eq. (1.6) where the a w is a w1 and a w2 at temperatures T 1 and T 2 ,
respectively, Q s is the heat of sorption, and R is the gas constant (8.14 J/g mol).

ln a
a

Q
RT T

11

w 2

=−

S

− (1.6)

w 1 1 2

The temperature dependence of a w suggests that if the storage temperature of a
food at a constant water content, e.g., in a sealed package, increases, the a w of the
food also increases. If the a w of the food is kept constant, an increase in temperature
results in a decrease in water content. The temperature dependence of water sorption
is described in Figure 1.3 . The heat of sorption increases with decreasing water
content as more energy is needed to remove water molecules associated with the
food solids.

PHYSICAL STATE AND WATER PLASTICIZATION

S TATE T RANSITIONS

The key factors controlling quality changes and stability in food processing and
storage are temperature, time, and water content. High-moisture foods contain excess
water that provides an excellent media for diffusion and reactions. Thus, a w cannot
be used to control stability and the shelf life is determined mainly by pH, storage
temperature, and protective packaging.
Food materials at low water contents and in the freeze-concentrated frozen state
form metastable amorphous matrices. Although the stability is determined by tem-
perature and water content, it is often greatly related to the physical state of the
amorphous matrix of food solids and plasticizing water. 6 The stability in the glassy
state is based on restricted rotational mobility of molecules, while changes may
occur in the supercooled liquid state above the glass transition temperature where