Chemistry Reference
In-Depth Information
Table IV.1
Calculation of Stickiness and Caking Index (SCSI)*
modified from Schuck et al (2005)
T-T
g
(8C)
Number of points
Cp (J/g/8C)
Number of points
5
<
0.1
0
0
>
5;
10
1
0.1;
<
0.2
1
>
10;
15
2
0.2;
<
0.3
2
>
15;
20
3
0.3;
<
0.4
3
>
20;
30
4
0.4;
<
0.5
4
>
30
5
0.5
5
*SCSI
¼
Sum of the number of points for [T-T
g
]
þ
number of points for [Cp]. For
example, a powder with a [T-T
g
]
¼
168C and Cp = 0.25 (J/g/8C) has a SCSI of 5.
A particular challenge occurs when considering the drying profile of a range
of particles. One of the difficulties is to be able to predict surface moisture
content and temperature, which are important variables for determining
whether the T
g
of surface lactose has been exceeded and what are the pro-
spects of the particles becoming sticky (Nijdam and Langrish, 2006).
IV.4.4.
Instantization/Agglomeration
Regular spray-dried milk powders do not disperse readily or completely
when poured onto water.
In order to improve reconstitution properties, agglomeration was devel-
oped as a process whereby larger powder particles (more likely as particle
clusters) are created, largely by the adhesion of smaller to the larger particles -
the smaller particles limit the instant dispersion of powders in water. The
resulting more open powder structure allows greater penetration of water in
the course of subsequent wetting and dispersing.
The key to agglomeration in practice is to create an environment for
adequate mixing of ''wet'', sticky partially dried powder particles with
recycled dried fines (i.e., fine powder particles collected by cyclonic separa-
tion), so that the smaller particles adhere to the larger ones and form clusters
around them. The surface properties of partially dried powder in the wet zone
of the spray drying chamber influence the nature of particle bridges that may
be formed. If the starting material is a dried powder, then partially rewetting
and heating in excess of T
g
will provide a viscous surface for adjacent particles
to adhere to and form strong bridges (Bhandari and Howes, 1999). There is a
dependence on the presence of fat and amorphous lactose to provide the
necessary stickiness to facilitate agglomeration. Nijdam and Langrish (2006)
expressed a concern that inadequate amounts of lactose may be present
because
of
competitive
displacement
by
protein.
However,
since
most
