Chemistry Reference
In-Depth Information
differentiate between the critical water content of isolated lactose and skim
milk solids of 6.8 and 7.6 g/100 g solids, respectively.
The effect of non-lactose solids on the T
g
of powder may be predicted
using the extended Couchman-Karasz equation, i.e., more than three
constituents (water, amorphous lactose, glucose and galactose, casein
and whey proteins) are considered (Schuck, 2005). The Couchman-Karasz
equation for tertiary mixtures (casein, carbohydrates and water) takes
the form
T
g
¼
W
1
Cp
1
T
g1
þ
W
2
Cp
2
T
g2
þ
W
3
Cp
3
T
g3
W
1
Cp
1
þ
W
2
Cp
2
þ
W
3
Cp
3
;
where T
gi
is the glass transition temperature (K) of component i, Cp
i
is the
change in heat capacity (J/kg/8C) of this component at T
gi
and W
i
is its weight
fraction.
IV.4.2.
Caking
Caking is a problem that arises when a low-moisture, free-flowing
powder first becomes lumpy, then agglomerates into a solid and finally
transforms into a sticky mass. These physical and microstructural changes
take place during caking of skim milk powder while equilibrated at 43-94%
RH (Aguilera et al., 1995). Initial changes are evident when powder reaches
>
54% RH while at
74% RH, lactose crystallization as well as bridging
between particles occurs. The caking of milk powder is preceded by viscous
flow on the surface of particles as amorphous lactose becomes sticky on
exposure to humid air. Viscous flow was measured as an increase in density
of lactose plugs within a cylindrical compaction apparatus after incubation
under defined temperature/time conditions (Lloyd et al., 1996). The onset
temperature of viscous flow decreased with increasing a
w
and corresponded
to the onset temperature of T
g
(Lloyd et al., 1996). Viscous flow at T
>
T
g
increases the potential for caking to occur. It is speculated that other forms
of lactose (
-lactose monohydrate, anhydrous
-lactose and the compound
crystals of
/
-lactose) in addition to electrostatic forces between particles,
the presence of mineral salts and proteins, surface wetting followed by water
equilibration or cooling and pressure also play significant roles. In the case
of fat-containing powders, fat-induced caking becomes a problem when
total or surface fat exceeds 41 and 12.6%, respectively (Foster et al., 2005).
The caking in this instance is attributed to fat crystallization in the liquid
bridges between particles, crystallization being triggered by cooling of the
powder.
