Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 380

other (not just at the surface of the foam). All these factors mean that the
magnitude of force driving coalescence is different in the two types of exper-
iment and so differences in the absolute values of F c in each case might be
expected. However, what is most significant is that the overall trends in the F c
data sets are similar for both types of measurements.
In the foam experiment there is a maximum and minimum in F c at similar pH
values as in the equivalent single bubble layer experiment, except that the peak
and trough are somewhat broader. In the case of the minimum in F c (without
droplets), this is shifted to a slightly lower pH. Most significantly, in the
presence of oil droplets the foam is dramatically stabilized above the pI,
compared to without oil droplets, so that F c is practically zero, within experi-
mental error. Below pH 5.5 the SC system with oil droplets appeared to form

a mixed precipitate/gel and stable bubbles could not be formed to perform the
expansion test. The results of these experiments on model foams and individual
bubbles, as a function of pH and protein type, are in broad agreement with work
elsewhere on the whipping of emulsions, in terms of both whippability and foam
stability. 18 The results also suggest that the exact structure of the foam (bubble
sizes and packing) is not particularly important in determining its stability, but
rather the solution conditions and the mechanical properties of the interfacial
film or of the intervening aqueous phase surrounding the bubbles.
Finally, since the effect of viscosification of the aqueous phase has been
shown to affect bubble stability, some foam stability tests were also performed
with sugar syrup present in the aqueous phase. A different syrup was used in
this case, the Cerestar corn syrup, which gave an aqueous phase viscosity of 30
5 Pa s. Figure 7 compares the foam stability results from Figure 6 (in the
absence of syrup) with those obtained with syrup. Clearly the presence of the
syrup alone, in the absence of oil droplets, gives a considerable increase in

E

1.0

0.5

0.0

5 6 7

pH

Figure 7 Fraction of coalescence (F c ) with C b ¼ 1 wt% sodium caseinate as a function of
pH in foam stability experiments in the systems: sodium caseinate ( ' ); sodium
caseinate + oil droplets ( f ¼ 0.25%) ( & ); sodium caseinate + corn syrup
( E ); and sodium caseinate + corn syrup + oil droplets ( f ¼ 0.25%) ( B ).
Corn syrup viscosity ¼ 30 Pa s