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water activity studies where temperature control is critical. There are many shelf-life,
packaging, and isotherm studies in which temperature control would be very beneficial.
2. Compliance with government or internal regulations for specific products.
Though the water activity of most products varies by less than ±0.02 per 1 °C, some
regulations require measurement at a specific temperature. The most common
specification is 25 °C, though 20 °C is sometimes indicated.
3. Minimization of extreme ambient temperature fluctuations.
If the environmental and AQUALAB TDL temperatures fluctuate by as much as ±5 °C
daily, water activity readings vary by ±0.01
a
w
. Temperature control eliminates variations
due to changes in ambient conditions.
The AQUALAB TDL has thermoelectric components installed to allow the instrument to
maintain a set chamber temperature. The temperature of the instrument can be changed in
the Configuration tab.
3.3.1.2 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.
The 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.
3.3.1.3 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
