Chemistry Reference
In-Depth Information
particle surface, increasing the contact area (Rennie et al., 1999; Ozkan et
al., 2002; Fitzpatrick et al., 2004, 2007).
11.5.5.2.
Crystallization
Crystallization is a most important process regarding various aspects of
the quality of dairy food powders. At an advanced stage of caking, crystal-
lization of lactose can reinforce the already formed liquid bridges and then
consolidate the caking (Chen, 2007). Controlled crystallization of lactose can
be used to reduce the hygroscopicity and caking tendency of dairy powders.
Crystallization of lactose impairs the solubility of milk powder and acceler-
ates damaging chemical changes (see Buera et al., 2005, for a review). Crystal-
lization of lactose is responsible for the undesirable sandy texture in ice cream
and condensed milk, while it is considered desirable in milk chocolate, where
the free fat content is proportional to the fraction of crystallized lactose in the
concentrate (Vuattaz, 1999). The size of ice crystals in ice cream is also a
major factor in consumer acceptability.
Crystallization comprises two steps: nucleation and growth. According
to classical crystallization theories, both processes are controlled by thermo-
dynamic effects (formation of nuclei of the critical size, incorporation of
molecules into the crystal lattice) and kinetic ones (transport of molecules
from the bulk solution to the nucleus or to the growing crystal interface).
Both types of effect have opposite temperature dependence, resulting in nuclea-
tion and growth rates with bell-shaped curves in a temperature range between
T
g
and T
m
. The overall crystallization rate is therefore expected to show a
similar behaviour. The (time-dependent) crystallization temperature (T
cr
)in
amorphous sugars determined by DSC was reported in a number of studies to
be approximately halfway between their respective T
g
and T
m
values (Roos and
Karel, 1991b,c, 1992; Saleki-Gerhardt and Zografi, 1994; Gabarra and Hartel,
1998). The effect of water was found to be about the same on T
cr
and T
g
as
indicated by a fairly constant value of (T
cr
-T
g
) (Roos and Karel, 1991b,c). The
time to complete crystallization (t
cr
) in samples of lactose with a water content
between about 1 and 8% could be fitted to a WLF-type equation: This is the
behaviour to be anticipated for experimental temperature ranges near T
g
,
where the crystallization rate is controlled by kinetic effects. For broader
temperature ranges, extending to smaller undercooling (T close to T
m
), bell-
shaped curves were reported for sucrose, lactose (Kedward et al., 1998, 2000)
and starch (Marsh and Blanshard, 1988; Farhat et al., 2000) (Figure 11.18).
The shift in the maximum crystallization rate with water content could be
explained by changes in T
g
and T
m
(Kedward et al., 2000).
Lactose crystallization can be retarded, not only by maintaining low-
temperature/low-water-content conditions but also by adapting product
