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200
150
100
50
0
0
100
200
300
t / min
Figure 4 Comparison of typical bubble radius R versus time t for 1 wt% fumed silica +
0.05 wt%
b
-LG at pH
¼
7. The solid lines indicate the expected behaviour of
0.05 wt%
b
-LG on its own, taken from modelling of previous experimental data
in the same bubble-size range
13
100
50
0
0
200
400
t
/ min
Figure 5 Comparison of bubble radius R as a function of time t stabilized by fumed silica
+ DDAB or
b
-LG at pH
¼
7:
n
, 3 wt% silica + 10
5
mol dm
3
DDAB;
E
,1
wt% silica + 0.05 wt%
b
-LG. The solid lines indicate the expected behaviour of
0.05 wt%
b
-LG
13
which means that there is not enough b-casein available to stabilize the bubbles.
While it is not clear which of these explanations is the more likely, it is
nonetheless noteworthy that there is this pronounced difference in behaviour
for two milk proteins.
24.3.3 Fumed Silica Particles+Lecithin
There are few natural cationic surfactants present in food colloids, and to our
knowledge no artificial ones are permitted as food additives. Phospholipids are
zwitterionic, carrying both a negative and a positive charge at neutral pH. The
latter should favour adsorption to a negatively charged surface such as silica.
For comparison with DDAB, experiments were therefore conducted with egg-
yolk lecithin, as an example of a natural surfactant that might aid hydrophilic
particle adsorption at the A-W interface.
Bubbles formed at the A-W interface in a mixed dispersion of silica particles
+ lecithin were not particularly stable, especially in comparison to the sil-
ica+DDAB system. Figure 6 illustrates typical results from several experi-
ments with lecithin + silica where the fraction of surviving bubbles, F,is
plotted as a function of time. Whereas with all the other systems studied here
the rate of decrease of F was significantly lower than the initial value after two
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