METER AQUALAB PAWKIT, Руководство пользователя

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12
SYSTEM
However, moisture content alone is not a reliable indicator for predicting microbial responses
and chemical reactions in materials. The limitations of moisture content measurement are
attributed to differences in the intensity that water associates with other components.
3.3.2 WATER ACTIVITY
Water activity (
a
w
) is a measurement of the energy status of the water in a system. The
value indicates how tightly water is bound, structurally or chemically, within a substance.
The lower a sample’s water activity, the more tightly bound that water is within the sample.
The concept of water activity is of particular importance in determining product quality
and safety. It predicts safety and stability with respect to microbial growth, chemical and
biochemical reaction rates, and physical properties.
Water activity is a measure of the energy status of the water in a system and is a far better
indicator of product stability than moisture content.
Figure 12 shows how the relative
activity of microorganisms, lipids, and enzymes relate to water activity. While other factors,
such as nutrient availability and temperature, can affect the relationships, water activity is
the best single measure of how water affects these processes.
Ionic
Lipid oxidation
Br
owning reactions
Moisture sorption isother
m
Enzyme activit
y
Mold gr
owth
Yeas
t
Ba
ct
er
ia
Covalent
Solute &
Capillary
Water Activity
Water Activity—Stability Diagram
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.1
Moisture Content
Relative Reaction Rate
Figure 12 Water activity diagram adapted from Labuza (1976)
Researchers measure the water activity of a system by equilibrating the liquid phase
water in the sample with the vapor phase water in the headspace and measuring the
relative humidity of the headspace. In the PAWKIT, a sample in a sample cup is placed on
a level surface and the PAWKIT is seated on top of the cup, creating a seal. This chamber
contains an infrared thermometer and a capacitive humidity sensor. Changes in the
electrical capacitance of the polymide layer of the sensor occur as the relative humidity
of the chamber changes. The PAWKIT computes the relative humidity of the headspace