25
AQUALAB VSA
Where m is the moisture in g/100 solids or g/g solids, k
b
is a constant in the range of
0.70to1 and c is a constant in the range of 1 to 2,000. In addition, m
o
is the monolayer
moisture content in the same units as m and a
w
is the water activity at moisture m.
Equation 8
Where m is the moisture in g/100 solids or g/g solids, χ = In[−In(a
w
)] and b
0
− b
3
are empirical
constants.
3.7 USES FOR MOISTURE SORPTION ISOTHERMS
Moisture sorption isotherms provide valuable information. For anyone who dries or wets
their product, the sorption isotherm serves as a drying and wetting curve that provides
information about the moisture content of a product when dried or wetted to a specific water
activity. Customers can use the sorption isotherm to assist in process control by determining
drying rates and optimal endpoints.
If a product exhibits hysteresis, the isotherm shows what impact the hysteresis has on the
moisture content after drying to a given water activity.
An additional function of the isotherm is moisture content prediction. Although water
activity is a much better predictor of safety and quality than moisture content, there are
times when it is necessary to know both water activity and moisture content as well as the
relationship between the two parameters for a given product. Water content measurements
can be inaccurate, time-consuming and require a precision balance. As an alternative to
moisture content measurement methods, you can use the sorption isotherm to determine
moisture content based on water activity, usually with better precision than actually running
a moisture content analysis and in much less time.
Customers can utilize isotherms to determine the effect of temperature on water activity and
moisture content readings. Isotherms conducted at several different temperatures show
the temperature at which a product, in a sealed package (at constant moisture content), is at
unstable water activity levels.
Isotherms can be very valuable for formulation and product development. By comparing
the isotherms of different formulations, it is possible to determine if a product can be
adjusted to allow higher moisture content at a given water activity or a lower water activity
at given moisture content. The result can be a moister product that is still shelf stable. For
those producing multi-component products, it is possible using the isotherms of the two
components to determine the final water activity of the mixture without actually making the
product. For dried products, the isotherm predicts the moisture content of the product when
it is dried to a shelf stable water activity level.
Finally, sorption isotherms are valuable for shelf life prediction. You can use the product
isotherm to determine package requirements depending on the products sensitivity to
moisture or to determine the monolayer moisture content, which represents the most stable
state of your product. The shape of the isotherm can provide information about the level of
amorphous to crystalline material in a product.
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