Thin Liquid Films - Surface and Colloid Chemistry: Principles and Applications - page 167

uid crystalline structures). In fact, in technical formulations, SDS can be deliberately
used with some (less than 1%) C 12 H 25 OH to enhance the foaming properties.
The foam drainage, surface viscosity, and bubble size distributions have been
reported for different systems consisting of detergents and proteins. Foam drainage
was investigated by using an incident light interference microscope technique.
The foaming of a protein solution is of theoretical interest and also has wide
application in the food industry (Friberg, 2003) and to firefighting practices. Further,
in the fermentation industry where foaming is undesirable, the foam is generally
caused by proteins. Since mechanical defoaming is expensive due to the high power
required, antifoam agents are generally used. On the other hand, antifoam agents are
not desirable in some of these systems, for instance, in food products. Further, anti-
foam agents deteriorate gas dispersion due to increased coalescence of the bubbles. It
has been known for a long time that foams are stabilized by proteins, and that these
are dependent on pH and electrolytes.
High foaming capacity is explained by the stability of the gas-liquid interface
due to the denaturation of proteins, especially due to their strong adsorption at the
interface, which gives rise to stable monomolecular films at the interface. The foam
stability is caused by film cohesion and elasticity. Further, studies have shown that
the degree of foaming of bovine serum albumin (BSA) aqueous solution was inves-
tigated. The effect of electrolytes and alcohol was investigated. A good correlation
was found between the adsorption kinetics and foaming properties. The effect of
partial denaturation on the surface properties of ovalbumin and lysozyme has been
reported. Most protein molecules exhibit increased hydrophobicity at the interface as
denaturation proceeds, due to the exposure to the outer surface of the buried hydro-
phobic residues (in the native state). The hydrophobicity of proteins (as described
in Chapter 5) has been found to give fairly good correlation to emulsion stability in
food proteins. The surface tension of these proteins decreased greatly as denatur-
ation proceeded. The emulsifying and foaming properties of proteins were remark-
ably improved by heat denaturation without coagulation. The emulsifying properties
increased and were found to exhibit correlation with surface hydrophobicity. The
protein-foaming properties increased with denaturation. The foaming power and
foam stability of C 12 H 25 SO 4 Na (SDS)-ovalbumin complexes did not improve as much
as with heat-denatured protein. The surface hydrophobicity showed an increase. It is
thus safe to conclude that the heat- and detergent-denatured proteins are unfolded by
different mechanisms. These studies are in accord with the unfolding studies carried
out comparing urea or SDS unfolding by fluorescence.
8.3.2 f o a m S T r u c T u r e
The foam as TLF has a very intriguing structure. If (1) two bubbles of the same
radius come into contact with each other, this leads to (2) the formation of contact
area and subsequently to (3) formation of one large bubble.
In stage 2, the energy of the system is higher than that in stage 1, since the system
has formed a contact area (dAc). The energy difference between (2) and (1) is γ dAc.
When the final stage is reached (3), there will be a decrease in the total area by 41%