22
THEORY
3. Structural changes within a solid matrix occur when the material swells and polar
sites once obscured are now exposed to bind with water. For example, hydrated protein
contains many sites for water “binding” before desorption while dehydrated protein has
limited polar sites for water binding prior to adsorption.
4. Some solutes may supersaturate below their crystallization water activity
(nonequilibrium condition) and thus, hold more water as a
w
declines. Foods with high
sugar content frequently exhibit this phenomenon.
3.3 NONEQUILIBRIUM
If diffusion of water into (adsorption) or out of (desorption) a material is slow and you do
not allow sufficient time for complete diffusion, then there is probably a large amount of
apparent hysteresis that you can reduce by allowing sample equilibration.
3.4 MATRIX CHANGES
Figure 22 shows three different isotherm curves for spray-dried milk powder, each with
unique maximum water activities and different sorption histories. The moisture sorption
isotherms for spray-dried milk powder show a change in hysteresis due to a phase change
at 0.43 a
w
. When the isotherm ends prior to the phase change at 0.43 a
w
(red square), there is
little hysteresis. When the isotherm ends above the phase change (blue diamond), there is
apparent, but not actual hysteresis due to the phase change. Subsequent isotherms run on
the sample after experiencing a phase transition (green triangle) do not show an inflection
point, are repeatable, and exhibit only small levels of hysteresis. The boundary isotherm with
a 0.80 a
w
maximum experienced a phase change at 0.43 a
w
, indicated by a sharp inflection
point in the curve. The desorption curve for this isotherm appears to show hysteresis,
especially below 0.60 a
w
. However, an isotherm, run on a sample wetted to a maximum water
activity below the phase change, exhibits very little hysteresis. The lack of hysteresis in this
isotherm indicates that the matrix changes that occur at 0.43 a
w
are completely responsible
for the apparent hysteresis.
Various bonding mechanisms bind water in a sample to particle surfaces. When the
configuration of particle surfaces changes due to a phase change, binding sites change
and the amount of water which can be bound at a given energy of water also changes. An
isotherm curve of the phase changed sample does not show further phase transitions since
simply drying the sample does not return it to an amorphous state.
These matrix changes represent a true physical change in the material. They are not
reversible by drying, no matter how many drying cycles occur. Differences between the
adsorption and desorption curves in the initial isotherm is not true hysteresis since the
sample matrix has experienced a physical change. Differences between sorption curves in
the subsequent isotherms represent true hysteresis.
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