Interactions during the Acidification of Native and Heated Milks Studied by Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 265

17.5 Discussion
These two sets of contrasting results raise questions about the interpretation of
DWS in terms of defining gelation. Clearly, the 'gelation' that is seen in
unheated milk using all of the criteria of DWS agrees with the rheology, by
any of the common definitions of a gel. The definition of a gel is difficult to
make in practice, because although we know that a gel is a space-filling
network, it is not clear what criteria can be used to establish its existence. In
terms of rheological measurements, the gel point has been identified in different
ways by different authors. The point at which G 0 and G 00 cross (where tan d ¼ 1)
has been used, 25 or where G 0 attains a value of 1 Pa, 26 or where the approx-
imately linear plot of G 0 versus pH extrapolates back to the baseline. 27
In unheated milk, the variation of the defined parameters from DWS can be
used as indicators to define the gel point. Gelation can be imagined to occur in
the region where abrupt changes are seen in the MSD slope and rapid increase
in apparent radius (or more properly, the average relaxation time t ). In
contrast, the behaviour of the heated milk suggests that the mobility of the
particles is severely restricted well in advance of the changes in the elastic and
loss moduli in rheology. So for this suspension of particles there can be no
unique definition of the gel point.
If the behaviour of 1/l*, in the region where the apparent radius begins to
increase, is dominated by the effects of S(q), then we may take the changes in
this parameter (or lack of them) to indicate that the relative positions of the
casein micelles do not change very much in heated milk, whereas they change
considerably in unheated milk, perhaps as a result of the establishment of an
aggregation equilibrium. This may be taken as an indication that the casein
micelles in heated milk cannot (or do not) approach one another as closely as
they do in unheated milk. This in turn argues that, in heated milk, in the pH
region around 5.6, we are not 'seeing' direct interactions between the casein
micelles, but rather the formation of a weak network that restricts the motion
of the particles. It should be remembered that the concentration of WP in the
milk is much less than is necessary to produce a self-supporting gel. The
diffusion of the casein micelles is not only slowed (as it would be if there were
simple aggregation reactions of freely diffusing particles), but it is also
restricted, which is typical of the formation of a network. We may then ask
of what species is this network composed: it is clear that the only candidates can
be the WP- k -cas complexes. The casein micelles are not the primary structure
forming units at this stage because the lack of change in 1/l* suggests that they
do not approach closely. It is arguable that if the WP- k -cas complexes form
links between the casein micelles, by forming chains between the large particles,
the diffusion of the micelles could be considerably affected without there being
a strong enough gel network formed to be detectable by rheology. It should be
remembered that the WP- k -cas complexes are small and of a low volume
fraction relative to the casein micelles, so that their contribution to the overall
light scattering is small; that is, we are not directly seeing the behaviour of the