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AQUALAB TDL/TDL 2
For example, a candy may have a soft chocolate center and a hard outer coating. The water
activity reading for the center and the outer coating are different, so one would need to
evaluate which part of the sample needed to be measured before crushing it. When the
candy is crushed, the water activity represents the average water activity of the entire
sample; leaving the candy whole gives a reading for the coating, which may act as a barrier to
the center.
To reduce test time when testing whole samples, see
for Low-Emitting mode.
2.2.1.2 LOW WATER-EMITTING SAMPLES
Some extremely dry, dehydrated, highly viscous, water-in-oil emulsions (e.g., butter), high fat,
or glassy compositions may require multiple readings because of their slow water-emitting
properties.The slow emission of water can be interpreted to mean that the head space is in
equilibrium with the sample and cause the instrument to exit the reading prematurely.
The most effective way to test these types of samples is to run them in the AQUALAB TDL
using Continuous or Custom mode and wait for the water activity readings to stabilize.
For faster reading, it is important to have the relative humidity of the chamber at or below
the water activity of these types of samples. This causes the sample to release water to the
vapor phase and equilibrate with the chamber. If the relative humidity of the headspace is
greater than the water activity of the sample, the sample will require a long period of time to
reach equilibrium.
To reduce test time when testing low water-emitting samples, see
to set
Low-Emitting mode.
2.2.2 SAMPLE TEMPERATURE
The ambient temperature of the AQUALAB TDL should be between 4 and 50 °C. The AQUALAB
TDL has temperature control capabilities that enable it to read samples at temperatures
different from ambient temperature but no higher than 50 °C and no lower than 15 °C.
Samples that are more than ±4 °C than the instrument chamber temperature need to
equilibrate to the instrument temperature before taking a reading, ideally within 1 or 2 °C of
the chamber temperature. Rapid changes in temperature over short periods of time cause
the water activity readings to rise or fall until the temperature stabilizes. Proceed with
normal measurements when the temperature stabilizes within an optimal 1 or 2 °C of the
chamber temperature.
High water activity samples that are warmer than the chamber temperature can cause
condensation inside the measuring chamber, which adversely affects subsequent readings.
A warning message appears if the sample temperature is more than 4 °C above chamber
temperature (
