8 SAMPLE PREPARATION AquaLab
This increases the surface area of the sample, thus decreasing read-
ing times. However, keep in mind that modifying some samples may
alter their water activity readings.
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 will represent the average water
activity of the entire sample; whereas leaving the candy whole will
give a reading for the coating, which may act as a barrier to the
center.
8.3 Slow Water-Emitting Samples
Some extremely dry, dehydrated, highly viscous water-in-oil (but-
ter), high fat, or glassy compositions may require multiple tests due
to their slow water-emitting properties. This is because the slow
emission of water decreases the change in water activity sufficiently
that the instrument determines the test to be complete, even though
changes in water activity are still occurring. The most effective way
to test these types of samples is to run them in the AquaLab using
the continuous or custom mode and wait for the water activity read-
ings to stabilize.
For faster reading, it is important to have the water activity of the
chamber at or below the water activity of these type of samples.
This causes the sample to release water to the vapor phase and equi-
librate with the chamber. If the water activity of the head-space is
greater than this type of sample, a long period of time will be re-
quired to reach equilibrium and the water activity of the sample may
be affected.
8.4 Volatile Samples
AquaLab will give accurate readings on most samples. However, sam-
ples with certain volatiles in high enough concentrations may give in-
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