Coalescence of Expanding Bubbles: Effects of Protein Type and Included Oil Droplets - Food Colloids: Self-Assembly and Material Science - page 376

These combined observations lead us to speculate that the main mechanism
leading to coalescence in this situation is some sort of catastrophic fracture of
the adsorbed protein film at a certain amount of expansion, or a range of
interfacial strains. This is substantiated by other recent work. 11-14 For example,
measurements of the surface tension g of adsorbed films of b-L, WPI and SC,
when undergoing similar expansions, showed that for the globular proteins
there was a marked increase in the maximum change (increase) in g as the films
were aged. This was explained 14 as being due to increased cross-linking within
the films with time, which resulted in the films being more liable to fracture on
expansion, rather than adjusting to a homogeneous redistribution of protein
within the interface. The work of Hotrum et al. 11 clearly indicates a greater
tendency for film fracture with more rigid protein films, such as those formed
from b-L and soy glycinin, in comparison with b-casein. Bos et al. 12,13 have also
observed such fracture, and have measured yield stresses of protein films in
relation to foamability and foam stability.
It seems noteworthy that the least stable bubbles were those formed in the
solution of OA, which is well known for its tendency to form highly aggregated
films at the air-water interface. The WPI system is more stable than that
containing OA, but less stable than that containing pure b-L. Likewise, it has
been shown that WPI has a greater tendency to form more highly aggregated
interfacial films than pure b-L, which are therefore more likely to fracture on
expansion. 14
One of the clear conclusions from the above is that it seems difficult to find a
food protein that, on its own, can prevent bubble coalescence under these
conditions of expansion. The only way to reduce F c close to zero seems to be to
increase the aqueous phase bulk viscosity (viscoelasticity) to a point that it is
practically in a gelled state. This we have demonstrated 1,15 by incorporating a
range of polysaccharide thickeners. Under such circumstances, the bubbles are
effectively immobilized in a thin film of bulk elastic gel beneath the planar
interface. This is an important result technologically because such thickeners
are frequently part of the recipe of foamed products. But such bubble stability
results obtained with hydrocolloids present are probably not strictly relevant to
the behaviour of a flowable foam undergoing a pressure drop.
25.3.2 Effect of Oil Droplets
Another key ingredient of many dairy-based whipped products is the presence
of oil droplets. These are often partly crystallized, and their partial coalescence
into a network around the surface of bubbles appears to be the major factor
influencing the foam stability. Certainly, dispersions of solid particles can be
used to form bubbles that are extremely stable to disproportionation, 16,17 but
the stability of such systems to systematic pressure variation does not appear to
have been tested. As partially crystalline oil droplets are difficult to control in
terms of their size and aggregation, we have decided deliberately to test first the
effect of completely fluid emulsion droplets on coalescence stability. In this we