Answer
It is readily calculated that the specific surface area was 0.33 m^2 per mL of oil, and
the amount of protein available thus was 3 mg per m^2 of O–W interface. A glance at
Figure 11.15b suggests that this would not have been enough to provide the drops
with a plateau value surface load. Hence some coalescence may indeed have
occurred. This would also lead to a decrease in interfacial area, hence to an increase
in surface load and thereby, maybe, to an increase in stability. Stability may also
increase owing to slow cross-linking of the adsorbed protein molecules, and it has
been shown thatb-lactoglobulin, the major component of whey protein, is prone to
the formation of intermolecular 22 S 22 S 22 bridges in an adsorbed layer.
13.4.3 Foams
The lifetime of a foam is typically some orders of magnitude smaller than
that of an emulsion: say, an hour versus several months. This will primarily
be due to thenumber of film rupturesneeded to break a foam being smaller
by a factor of, say, 10^5 than in an emulsion. Moreover, thefilmsbetween
bubblesare quite large(We 4 1) andnontransient, greatly promoting film
rupture. On the other hand, theinterfacial tension A–W is largerthan the
O–W tension by a factor of about 5. Another important aspect is that the
various instabilities in a foam may reinforce each other. For instance,
Ostwald ripening (Section 13.6) gives larger bubbles and hence larger films
and faster drainage, hence a decreased film stability.
It may even be questioned whether there is a close correlation at all
between film stability and the lifetime of a foam. Much of the research on
film rupture concerns quite large films, several mm in radius, whereas those
in food systems are rarely above 50mm. Most of the films studied were
stabilized by small-molecule surfactants that are never or rarely used in
foods. In the author’s opinion these studies, however interesting they may be
in general, are hardly relevant for foods. Moreover, Ostwald ripening tends
to be the dominant instability in most food foams.
When considering foam film stability, three rather different cases
should be distinguished. Since film thickness is an essential parameter, it is
useful to consult Section 11.2.2 on drainage.
- Thick films. These are often young films. They tend to be quite
stable according to the Vrij theory, unless the surfactant concentration is
very small. Consequently, an option to stabilize a foam against coalescence
is to retard film thinning, i.e., drainage. Drainage is counteracted by the
development of a surface tension gradient on the film surfaces, and the
gradient can be greater for smaller films and for surfactants of high surface