Decagon Devices pawkit Operator's Manual page 15

Pawkit User Manual
3. Water Activity Theory
Osmotic Effects
Osmotic effects are well known from biology and physical
chemistry. Water is diluted when a solute is added. If this
diluted water is separated from pure water by a semi-perme-
able membrane, water tends to move from the pure water
side through the membrane to the side with the added sol-
ute. If sufficient pressure is applied to the solute-water mix-
ture to just stop the flow, 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. This lowers the water activity of the
a
solution from 1.0 to 0.98 . For a given amount of solute,
w
increasing the moisture content of the systems dilutes the
solute, decreasing the osmotic pressure, and increasing the
water activity. Since microbial cells are high concentrations
of solute surrounded by semi-permeable membranes, the
osmotic effect on the free energy of the water is important
for determining microbial water relations and therefore
their activity.
Matric Effects
The sample matrix affects 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. This reduction in
energy status of the water is not osmotic, because the cellulose
or protein concentrations are far too low to produce any sig-
nificant dilution of water. The reduction in energy is the result
of direct physical binding of water to the cellulose or protein
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