Chemistry Reference
In-Depth Information
Methods for quantifying the degree of caking (flowability test, shear
and tensile strength measurement, visual observation of lumps, microscopic
and image analyses, penetration force test and crushing test) are largely
empirical, and not always reproducible. A caking index, developed by
Aguilera et al. (1995), is defined as the state of the system at any time relative
to that of the initial state. Two morphological indicators of the state of the
system are the ratio of the porosity of the instant system to that of the initial
system p
(t)
/p
o
; and the ratio of inter-particle bridge diameter to particle
diameter, D
bridge
/D
particle
. These authors proposed that bridging occurs at
the onset of caking as a result of surface deformation and sticking at contact
points between particles, without a measurable decrease in system porosity.
During this early phase, small inter-particle bridges may disintegrate under
even mild shaking. Agglomeration follows and involves an irreversible con-
solidation of bridges but the high porosity of the particulate system is main-
tained. The compaction that occurs at an advanced stage of caking is
associated with a pronounced loss of system integrity as a result of thickening
of inter-particle bridges owing to flow, reduction of inter-particle spaces and
deformation of particle clumps under pressure.
Many methods have been proposed for eliminating, minimizing or
controlling caking, e.g. (i) controlling storage conditions below a
w
0.57 if no
amorphous lactose is present, and below a
w
0.25 if it is present (Bronlund,
1997); (ii) avoiding mixing of particles with different initial humidity and
temperature (Fito et al., 1994); (iii) cooling the product immediately to an
appropriate temperature, well below T
g
before packaging (Bronlund, 1997;
Bhandari and Howes, 1999); (iv) minimizing temperature variation in
different parts of powder silos or bags during storage (Fito et al., 1994);
(v) conditioning of powders by ventilation in silos during storage (Bagster,
1970; Lenniger and Beverloo, 1975); (vi) adding a glidant or flow conditioner
to improve powder flow (Lai et al., 1986), high molecular weight carbohy-
drates (e.g., maltodextrin) in infant formula (Aguilera et al., 1995) to raise the
T
g
of the samples, and dessicant, e.g., silicon dioxide and sodium silicoalu-
minate (Lai et al., 1986) to improve dehydration characteristics and to reduce
stickiness (Chuy and Labuza, 1994).
Technological options (i)-(iv) above target the stabilization of amor-
phous lactose during storage. These are common techniques to delay caking
but they do not prevent it. The availability of accurate data on free moisture,
water of crystallization and amorphous lactose in lactose powders may assist
in selecting the suitable parameters for processing and storage conditions for
controlling caking. Method (v) is intended to eliminate any amorphous
material and to allow sufficient time for the removal of bound moisture in
bulk powder. The final attempt (vi) is desirable to reduce caking and to
improve the physical properties of milk and lactose powders.
