Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 379

It should be emphasized that in none of these experiments did we observe any
oil droplet instability. At this C b value, and at the rates of interfacial expansion
applied, the work of Hotrum et al. 11 suggests that this is indeed what would be
expected. Therefore, the droplets did not appear to coalesce, or to enter and
spread at the planar air-water interface to form lenses or larger oil droplets.
This means that droplet instability factors cannot explain the changes observed;
in fact, their occurrence would tend anyway to reduce bubble stability. Thus we
propose that the additional stabilizing effect of the added oil droplets is due to
the droplets themselves forming a flocculated network as the pH is lowered.
Whether this network forms part of the protein network already at the bubble
surface - either within it or adsorbed to it - is not yet so clear.
The enhancement in bubble stability on addition of emulsion droplets at
pH ¼ 7, where protein flocculation is not expected, suggests that the mere
presence of oil droplets at the interface may also be a factor in improving
stability. The oil droplets will naturally tend to rise up to the planar air-water
interface and so accumulate around the air bubbles located there. Confocal
images (not shown) of some aerated (whipped) systems do suggest that the
emulsion droplets can become positively associated with the bubbles surfaces
even at pH ¼ 7. Partly for this reason, it was also considered necessary to develop
a method for looking at bulk foam stability under such expansion conditions.
Figure 6 compares the results from the single bubble layer experiments for
SC (Figure 5) with the corresponding results from the foam stability test under
the same solution conditions. In the latter case the bubbles were formed at 3
atm and the pressure was dropped to 1 atm in 12 s; but this was still a similar
expansion rate to that in the single bubble layer experiments. Note also that in
the foam experiments the value of F c was calculated from the number of
bubbles visible at the wall of the cell, and from bubbles coalesced with each

1.0

0.5

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5 6 7

pH

Figure 6 Fraction of coalescence (F c ) with C b ¼ 1 wt% sodium caseinate as a function of
pH in: (a) single bubble layer experiments in the systems sodium caseinate ( m )
and sodium caseinate + oil droplets ( f ¼ 0.25%) ( n ); (b) foam stability
experiments in the systems sodium caseinate ( ' ); and sodium caseinate + oil
droplets ( f ¼ 0.25%) ( & )