Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 374

25.3 Results and Discussion

25.3.1 Effect of Protein Type
Figure 3 shows the results of single bubble layer experiments with solutions of
b-L, WPI, SC and OA over a range of bulk protein concentration C b . In the
absence of expansion under the conditions described, there was no significant
coalescence over the experimental time scale. As an indicator of stability, the
final number of fraction of bubbles coalescing due to the expansion was
calculated. This parameter (F c ) does not take any account of differences in
stability due to bubble size or to the position of bubbles in the interface relative
to the expanding walls, but it turns out in practice (see below) that this is not
particularly important in interpreting the results presented here. The rate of
expansion was kept constant at (dlnA/dt) ¼ 0.07 s 1 , where A is the surface
area of the bubbles (or the planar interface), and the extent of expansion was set
at A/A 0 ¼ 3, where A 0 is the initial area. This corresponds to reducing the
pressure in the system from 1 to 0.2 atm in 12 s. The systems contained invert
syrup diluted to 40% total solids to give a viscosity of 7.8 10 3 Pa s
(i.e., approximately eight times higher than without sugar). For every set of
conditions, the experiment was repeated at least four times, in each case with
typically 50-100 bubbles present before the expansion. The mean values of F c
are plotted, with the error bars representing the standard deviations about the
means.
The first key feature to note in Figure 3 is the significant variation in the
ability of different proteins to stabilize the bubbles under these conditions, even
at quite high values of C b . Slightly different bubble-size distributions were
produced with the different proteins, and detailed analysis has shown 1 that the

0.6

0.4

0.2

0.0

0 2 4 6

C b / wt%

Figure 3 Fraction of coalescence (F c ) in single bubble layer experiments at different bulk
protein concentrations (C b )atpH ¼ 7, for rate of expansion (d ln A/dt) ¼ 0.07
s 1 and extent of expansion (A/A 0 ) ¼ 3: OA ( n ); sodium caseinate ( J ); WPI
( m ) and b -L ( ' ). All systems contained 40% invert syrup with a viscosity of
7.8 10 3 Pa s