METER AQUALAB PAWKIT, Руководство пользователя

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14
SYSTEM
2. Compliance with government or internal regulations for specific products.
Though the water activity of most products varies by less than ±0.02 per degree Celsius,
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 PAWKIT temperatures fluctuate 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 4TE and TDL models have thermoelectric components installed to allow the
instrument to maintain a set chamber temperature.
The water activity of the water in a system is influenced by factors that affect the binding
of water. They include osmotic, matrix, and pressure effects. Typically water activity is
measured at atmospheric pressure, so only the osmotic and matrix effects are important.
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 water 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.
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 Waal forces. At higher water activity levels, capillary forces and surface
tension can also play a role.