Enhancement of Stability of Bubbles to Disproportionation Using Hydrophilic Silica Particles Mixed with Surfactants or Proteins - Food Colloids: Self-Assembly and Material Science - page 367

100

50

0

05 0 100

t / min

Figure 12 Comparison of bubble radius R as a function of time t for bubbles stabilized by
different silica particles + lecithin or b -LG at pH ¼ 7: J , 1.5 wt% colloidal
silica + 1.3 10 4 mol dm 3 lecithin; E , 1 wt% fumed silica + 0.05 wt% b -
LG. The solid line indicates the expected behaviour of 0.05 wt% b -LG on its
own 13

24.3.6 Colloidal Silica Particles + Lecithin
Experiments were conducted with 5.5-nm colloidal silica particles + lecithin.
At the same lecithin concentrations as used in the fumed silica + lecithin
experiments, there were problems in generating enough bubbles for study, i.e.,
the foamability of systems was poor. Figure 12 illustrates the typical radius
evolution of the few bubbles that could be formed. The bubbles were consid-
erably more stable than the same-size bubbles stabilized by pure b-LG, but they
were only slightly more stable than bubbles stabilized by particles + b-LG.
Overall the performance of lecithin with the colloidal silica seemed to be
slightly worse than with the fumed silica. As discussed above, the reason for
this may be poorer adsorption efficiency of the lecithin. Since the specific
surface area of the colloidal silica is considerably greater than that of the fumed
silica, it may be that the required level of surface hydrophobicity of the particles
was not reached.

24.4 Conclusions
It has been shown that hydrophilic inorganic particles, of a size compatible with
ingestion, can stabilize air bubbles very effectively against disproportionation,
provided their surfaces are physically modified by adsorption of surfactants
with positively charged head groups. In general, the smaller (colloidal) silica is
more effective, probably because it remains better dispersed. Above a minimum
particle concentration, DDAB is very effective, provided the surfactant con-
centration is not too high or too low. It is necessary to have a low surface
coverage of the silica, in the region of 3-11% coverage, and hence only partial
particle hydrophobicity. Analogous experiments with lecithin showed that this
zwitterionic surfactant was not as effective as DDAB in modifying the surface
properties of the silica to enhance bubble stability. Foamability was particu-
larly inefficient with lecithin + silica particles. This is most probably due to the
poorer adsorption efficiency of lecithin on silica under the neutral solution
conditions considered here.