Pawkit
Theory
44
cohesive forces that hold water in pores and capil-
laries, and to particle surfaces. If cellulose or protein
were added to water, the energy status of the water
would be reduced. Work would need to be done to
extract the water from this matrix. This reduction in
energy status of the water is not osmotic, because
the cellulose or protein concentrations are far too
low to produce any significant dilution of water.
The reduction in energy is the result of direct physi-
cal binding of water to the cellulose or protein
matrix by hydrogen bonding and van der Waal
forces. At higher water activity levels, capillary
forces and surface tension can also play a role.
Sorption Isotherms—relating
a
w
to
water content
Changes in water content affect both the osmotic
and matric binding of water in a product. Thus a
relationship exists between the water activity and
water content of a product. This relationship is
called the sorption isotherm, and is unique for each
product.
Figure 1
shows a typical isotherm. Besides
being unique to each product, the isotherm changes
depending on whether it was obtained by drying or
wetting the sample. These factors need to be kept in
mind if one tries to use water content to infer the
stability or safety of a product. Typically, large
