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the gel point as indicated by the crossover of G
0
and G
00
. The elastic modulus
increases also almost exactly in synchronization with the change in apparent
radius (or more correctly, the average
t
) in the system. At the beginning of this
gelation process, the value of 1/l* appears to stabilize. If we interpret the change
in l* as being defined by S(q), this must imply that, at the moment of gelation,
this function has stopped changing because the particles have reached their final
positions in the gel, and that subsequently no change in their relative positions
occurs.
Thus, for the unheated milk, we are seeing agreement between the two
experimental methods used, showing that a single network is formed. The
situation is more complex in milk that has been heated before acidification.
17.4.2 Heated Milk
Observation of the heated milk before acidification shows that the particle size
is very similar to that of unheated milk (Figure 2). This has generally been
found to be the case in measurements made with milk that has been heated at
around its natural pH.
22
However, the initial value of 1/l* is larger than it is in
unheated milk. This also has been commonly observed in the experiments that
we have described previously. Since the overall concentration of protein in the
milk does not change during heating, the increase in 1/l* must arise from
changes in the optical properties of the particles, and this may in turn result
from the binding of denatured WP to the casein micelles, and the formation of
10
4000
1.1
200
3.2
8
1.0
175
3000
2.8
6
0.9
150
2.4
2000
4
0.8
125
2.0
1000
2
0.7
100
1.6
0
0
0.6
75
6.5
6.0
5.5
5.0
pH
Figure 2 Changes in the parameters derived from DWS and rheology during acidification of
heated milk with 1.5% GDL. Various quantities are plotted against pH:
K
,
apparent radius from value of
t
calculated from correlation function, and con-
verted using the Stokes-Einstein relation (left scale);
&
,apparentradiusmeas-
ured during early part of the reaction (far right scale);
J
, slope of MSD against
time, calculated from correlation functions (middle right scale);
'
, 1/l* calcu-
lated from turbidity (right scale);
B
,G
0
from bulk rheometry (far left scale)
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