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AQUALAB 3
Another effect of temperature on water activity occurs when samples are near saturation.
A sample that is close to 1.0 a
w
and is warmer than the sensor block will cause water to
condense within the chamber. This causes errors in the measurement and in subsequent
measurements until the condensation disappears. For example, a sample at 0.75 a
w
only
needs to be 4 °C above the sample chamber temperature to cause condensation to form. The
AQUALAB3 warns the user if a sample temperature is more than 4 °C above the chamber
temperature, but for high water activity samples, the operator needs to be aware that
condensation can occur if a sample is warmer than the sample block.
There are several advantages in having a temperature-controlled water activity meter. A few
major reasons are detailed here.
1. Research purposes.
Researchers can use temperature control to study the effects of temperature on the
water activity of a sample, make a comparison of the water activity of different samples
independent of temperature, and conduct accelerated shelf-life studies or other
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 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 AQUALAB3 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.
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.
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
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