23
AQUALAB VSA
Figure 22 Spray-dried milk moisture sorption isotherms
3.5 WORKING ISOTHERMS
Full boundary isotherms give information about the physical characteristics of a product,
show hysteresis, and are important in establishing critical control points, but in many cases
a working isotherm proves very useful. A working isotherm shows how a product adsorbs and
desorbs water from its current or typical condition. To create a working isotherm, process the
product as usual, and then create a scanning curve by wetting one sample from that point
and drying a different sample from that same point. Figure 23 shows a working isotherm
(blue diamond) superimposed over a full isotherm (red square) for wood pulp. There is a
transition from the native starting point (0.60 a
w
) on the working isotherm in both adsorption
and desorption until the curves meet the bounding adsorption and desorption curves of the
full isotherm at which point the working isotherm follows the full isotherm.
The scanning curve the product initially follows depends on whether the product was
previously wetted or dried to its current state. If you wet a product to a certain water activity
and then dry it back down, there is an initial transition period as the product moves from the
adsorption curve to the desorption curve. The same is true for a product that you previously
dried and then wetted up. There is an initial transition period as the product moves from
the desorption curve to the adsorption curve. You can observe this transition region at any
point on the isotherm if you change the direction of the sorption and the product exhibits
hysteresis. (Figure 23).
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