40
SYSTEM
2. Compliance with government or internal regulations for specic products.
Though the water activity of most products varies by less than ±0.02 per degree Celsius,
some regulations require measurement at a specic temperature. The most common
specication is 25 °C, though 20 °C is sometimes indicated.
3. Minimization of extreme ambient temperature uctuations.
If the environmental and AQUALAB temperatures uctuate by as much as ±5 °C daily,
water activity readings may vary by ±0.01 a
w
. Temperature control eliminates variations
caused by changes in ambient conditions.
The AQUALAB models have thermoelectric components installed to allow the instrument
to maintain a set chamber temperature. Customers can set the temperature using the
Configuration tab of any of the AQUALAB models.
3.4.2.2. 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
semipermeable membrane, water tends to move from the pure water side through the
membrane to the side with the added solute. If sufficient pressure is applied to the
solute-water mixture to just stop the flow, this pressure is a measure of the osmotic
potential of the solution. Addition of 1 mol of an ideal solute to 1 kg of water produces an
osmotic pressure of 22.4 atm. This lowers the water activity of the solution from 1.0 to
0.98 a
w
. For a given amount of solute, 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 semipermeable
membranes, the osmotic effect on the free energy of the water is important for
determining microbial water relations and therefore microbe activity.
3.4.2.3. MATRIX 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 significant dilution of water.
The reduction in energy is the result of direct physical binding of water to the cellulose
or protein matrix by hydrogen bonding and van der Waals forces. At higher water activity
levels, capillary forces and surface tension can also play a role.
3.4.3 CHILLED-MIRROR DEW POINT LIMITATIONS
The limitation of the AQUALAB 4TE is its ability to accurately measure samples with high
concentrations (typically >1%) of certain volatile compounds such as ethanol or propylene
glycol, which can condense on the surface of the chilled mirror. The extent of the effect
To Next Page
Loading...
