Sorption Isotherms - Decagon Devices AquaLab 4TE Operator's Manual

AquaLab
3. Water Activity Th eory
separated from pure water by a semi-permeable membrane, water
tends to move from the pure water side through the membrane to
the side with the added solute. If suffi cient pressure is applied to the
solute-water mixture to just stop the fl ow, this pressure is a measure
of the osmotic potential of the solution. Addition of one mole of an
ideal solute to a kilogram of water produces an osmotic pressure of
22.4 atm. Th is lowers the water activity of the solution from 1.0 to
0.98 a . For a given amount of solute, increasing the water content
w
of the systems dilutes the solute, decreasing the osmotic pressure,
and increasing the water activity. Since microbial cells are high con-
centrations of solute surrounded by semi-permeable membranes,
the osmotic eff ect on the free energy of the water is important for
determining microbial water relations and therefore their activity.
Matric Eff ects
Th e sample matrix aff ects water activity by physically binding water
within its structure through adhesive and cohesive forces that hold water
in pores and capillaries, 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. Th is
reduction in energy status of the water is not osmotic, because the cellu-
lose or protein concentrations are far too low to produce any signifi cant
dilution of water. Th e reduction in energy is the result of direct physical
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 Water Activity to Water Content
Changes in water content aff ect both the osmotic and matric bind-
ing of water in a product. Th us a relationship exists between the
water activity and water content of a product. Th is relationship is
13