Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 381

stability of the foam, which is maximized at pH 5.9, both with and without

droplets. Interestingly, there is the suggestion that the systems are slightly less
stable in the presence of oil droplets + syrup, although this difference lies just
within the combined experimental scatter in the F c data. An increased viscosity
of the aqueous phase slows down thin film drainage between bubbles, and this
is known to enhance the lifetime of quiescent foams. It is not clear, however,
how the increased viscosity necessarily operates to improve stability when
interfaces are expanded in the manner of these experiments. Certainly, the
viscosity is not high enough to slow down the expansion rate: the bubbles are
observed to grow in size in response to the pressure change at the same rate as
without syrup.

E

25.4 Conclusions
We have developed two pieces of apparatus for studying the stability to
coalescence of bulk foams and individual bubbles when they are subjected to
rates and extents of pressure drop typical of many processing conditions
involving foamed food products. For the cases considered here, these test
methods appear to give complementary results, and they are also highly
discriminating in terms of distinguishing the ability of different proteins to
stabilize foams under such conditions. The proteins appear unable to stabilize
foams completely at neutral pH. For b-L and sodium caseinate, the foam
stability is considerably increased as the pH is reduced to the isoelectric point,
but below this pH the bubbles in the sodium caseinate systems become highly
unstable due to precipitation of the protein. A considerable further increase in
stability can be obtained by including a low volume fraction of oil droplets
stabilized by the same proteins. This is attributed to the formation of an even
more effective cross-linked network of droplets and/or protein + droplets at
the air-water interface as the pH is lowered towards the pI.

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