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SYSTEM
reduced. Work would need to be done to extract the water from this matrix. This reduction in
energy status of the water is not osmotic, because the cellulose or protein concentrations
are far too low to produce any significant dilution of water. The reduction in energy is the
result of direct physical binding of water to the cellulose or protein matrix by hydrogen
bonding and van der Waals forces. At higher water activity levels, capillary forces and surface
tension can also play a role.
CHILLEDMIRROR DEW POINT LIMITATIONS
The limitation of the AQUALAB3 is its ability to accurately measure samples with high
concentrations (typically >1%) of certain volatile compounds such as ethanol or propylene
glycol, which can condense on the surface of the chilled mirror. The extent of the effect
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 AQUALAB3 provides a solution for products containing volatile materials. The
AQUALAB3 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.
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 AQUALAB3 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
isotherm and infer the moisture content using the measured water activity of samples and
that isotherm.
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