Biomedical Engineering Reference
In-Depth Information
FIGURE 4.1
Representative moisture sorption isotherms for spray dried eggs. Adapted
from Iglesias and Chirife.
20
of the water affinity of that food.
19
In terms of packaging considerations, the water
activity of a food product, at various moisture contents and temperatures, will
determine whether this product will gain or lose moisture when exposed to an
atmosphere of a given relative humidity. Water activity can be related to moisture
content of a food through moisture sorption isotherms. As shown in
Figure 4.1
,
the
moisture sorption isotherm is a plot of the moisture content of a food in equilibrium
with different water activities, and describes the temperature dependent water adsorp-
tion or desorption characteristics of that product. Moisture adsorbed, for example,
by an inadequately protected dry food material (low a
w
) exposed to a more humid
environment, may initiate deteriorative reactions that lead to a reduced shelf life.
The permeability of packaging materials to moisture is, therefore, a critical factor
in controlling changes in the moisture content and a
w
of packaged foods and, hence,
their shelf life.
The water vapor transmission rate (WVTR) is the commonly used term to
describe the permeability of polymer materials to water:
WVTR
=
QL
/
At
where
Q
is the amount of moisture passing through the barrier,
L
is the thickness,
A
is the surface area, and
t
is time. Previously compiled WVTR data
14
indicate that
typical food packaging polymers exhibit a wide range of water vapor transmission
sides of the film must be stated as both affect permeability. Since WVTR values are
usually specified for temperature and RH conditions, which apply to standardized
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