Interactions during the Acidification of Native and Heated Milks Studied by Diffusing Wave Spectroscopy - Food Colloids: Self-Assembly and Material Science - page 264

soluble WP- k -cas particles in the milk. The overall effect of this is to increase
the scattering power of the milk (because new particles are formed), and this
leads to an increase in the value of 1/l*. After the GDL is added, we see a
decrease in the particle radius with pH that is exactly the same as that seen in
unheated milk. This appears to suggest that, although the casein micelles in
heated milk have lost quantities of their k -cas, there is still enough left to form a
hairy layer that collapses as the milk is acidified. Similarly, the value of 1/l*
increases slightly during this period, as was seen for unheated casein micelles.
These changes exactly parallel those seen in unheated milk.
At pH

5.9, the apparent radius of the particles begins to increase, as does
the value of 1/l*. However, compared to the behaviour in unheated milk, the
increase in 1/l* is very small. From the plateau value at pH

E

6.0 to the value at

E

pH 5.5, where rapid increase in the apparent radius starts in the milk, the

increase in 1/l*is

E

0.1 mm 1 compared with 1.0 mm 1 in unheated milk
between the plateau and the start of the rapid change in radius. In the heated
milk, the value of 1/l* is at a plateau at pH

B

5.5, although there is an
indication of an increase for pH o 5.4. Unfortunately, because of experimental
noise, reliable results are difficult to obtain beyond this point. Over this pH
range, the apparent radius of the particles increases to about double the
original value. This is essentially similar to the behaviour of unheated milk,
apart from the much smaller change in 1/l* and the higher pH at which the
changes occur. At pH

E

5.6 the MSD slope begins to decrease as the particles
are inhibited from diffusing freely, and the very rapid increase in particle size
begins. It should be noted, however, that, in contrast to the apparently very
rapid transition between slow and rapid aggregation that is seen in unheated
milk, there is a somewhat longer transition stage in the heated milk. These
phenomena are taking place at pH values around 0.5 units higher than those in
unheated milk.
A remarkable difference between the unheated and heated milks is that, for
the latter systems, the changes in the rheological parameters G 0 and G 00 do not
occur at the same pH as the changes in the DWS parameters. The rheological
gel point is found to be at pH

E

5.3, as has been shown by other studies. 13,23
The G 0 measured for heated milk is much higher than for unheated milk, and
the final gel obtained is about 20 times more elastic than for unheated milk; the
data shown in Figure 2 refer to the earliest stages of the gelation only. Whereas
in unheated milk the G 0 value becomes higher than G 00 , and increases in parallel
with the apparent particle radius and the change in MSD slope, in heated milk
the light scattering changes are well established before there are detectable
changes in the rheological parameters. We have consistently observed in DWS
experiments a major increase in the apparent particle radius and changes in the
MSD slope at higher pH values than the point where gelation is observed in the
rheometer. 24 This seems to present us with the paradox that, at higher pH
values, there is extensive aggregation of the particles in the system, and
restricted diffusion, even though there is no evidence of gel formation registered
by the rheometer.

E