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AQUALAB
is determined by how readily the material volatilizes, which is both concentration and
matrix dependent. Therefore, even if a sample contains materials that could volatilize, it
may still be possible to make accurate readings using the chilled-mirror dew point sensor.
The AQUALAB TDL provides a solution for products containing volatile materials. The TDL
uses a tunable diode laser which shines light of a controlled wavelength, specific to water
vapor, at a detector that receives light from the laser. The vapor pressure of the headspace
of the instrument in equilibrium is determined by the loss of signal strength from the laser
caused by the presence of water vapor in the headspace. For more information, contact
Customer Support.
3.4.4 WATER POTENTIAL
Water activity is closely related to a thermodynamic property called the water potential,
or chemical potential ( µ) of water, which is the change in Gibbs free energy (∆G
) when
water concentration changes. Equilibrium occurs in a system when µ is the same
everywhere in the system. Equilibrium between the liquid and the vapor phases implies
that µ is the same in both phases. This means the measured water potential of the vapor
phase can be used to determine the water potential of the liquid phase. Gradients in µ
are driving forces for moisture movement. In an isothermal system, water tends to move
from regions of high water potential (high a
w
) to regions of low water potential (low a
w
).
Moisture content is not a driving force for water movement and, therefore, cannot be
used to predict the direction of water movement, except in homogeneous materials.
3.4.5 SORPTION ISOTHERMS
Changes in moisture content affect both the osmotic and matric binding of water in a
product. Thus, a relationship exists between the water activity and moisture content.
This relationship is called the sorption isotherm and is unique for each product. Besides
being unique to each product, the isotherm changes depending on whether it was
obtained by drying or wetting the sample. Typically, large safety margins are built into
moisture content specifications to allow for these uncertainties.
While the sorption isotherm is often used to infer water activity from moisture content,
one could easily go the other direction and infer moisture content from water activity.
This is particularly attractive because water activity can be measured much more quickly
than moisture content. This method gives particularly good precision in the center of the
isotherm. In order to infer moisture content from water activity, an isotherm for the particular
product is needed. METER sells an isotherm generator called the AQUALAB Vapor Sorption
Analyzer (VSA) or METER can run the isotherm for a fee. Visit metergroup.com/food/services/
water-activity-and-isotherm-testing for more information.
For example, if using the AQUALAB to monitor the moisture content of dried potato flakes,
measure the water activity and moisture content of potato flakes dried to varying degrees
using the standard drying process for those flakes. That data is used to construct an
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