Protein-Stabilised Emulsions and Rheological Aspects of Structure and Mouthfeel - Practical Food Rheology - page 198

The rate and mechanism of adsorption also depends on the type
of protein (Beverung et al ., 1999). Flexible proteins (e.g. β-casein)
display overall more rapid adsorption dynamics than globular proteins
(e.g. ovalbumin) and also the ability to attain an apparent equilibrium
interfacial tension sooner; equilibrium interfacial conformation in oil-
in-water emulsions stabilised by globular proteins (e.g. ovalbumin) takes
place after significantly long times. Because of their rapid adsorption
at the interface, flexible proteins usually do not display a diffusional
induction period (the first regime in the protein adsorption mechanism),
unless they are present in the system at very low concentrations. The
adsorption dynamics of flexible proteins may be explained by their
disordered structure in solution, which promotes rapid adsorption and
tension equilibration in the second and third regimes. Conversely, the
slow interfacial unfolding and rearrangement of the globular proteins are
the molecular processes that dominate the adsorption kinetics beyond
the induction regime.

9 . 2 . 2 . 1 Protein displacement

Although protein adsorption at the oil-water interface can be regarded
as an irreversible process, protein desorption can take place due to
protein displacement by either other proteins or surfactants subsequently
adsorbing at the same interface. Mutual displacement of proteins by
other proteins relates to the difference in the ability of each of them
to change their conformation upon adsorption. Since flexible proteins
undergo conformational changes more easily and rapidly than globular
proteins, the former tend to displace the latter from an interface (Arai
and Norde, 1990a, 1990b).
The displacement of proteins by lmw surfactants, from both solid
and liquid surfaces, usually takes place by 'solubilisation' of the protein
or 'direct replacement' (Mackie et al. , 1999). One of the most inter-
esting areas in this field has been the interaction between proteins and
emulsifiers (including surfactants and lipids). The reason for this is that
although both proteins and emulsifiers can stabilise foams and emulsions
alone, their individual mechanisms of stabilisation are incompatible, of-
ten resulting in dramatic destabilisation when both species are present
at the interface. This process is commonly known as competitive desta-
bilisation (Wilde et al ., 2004). Water-soluble lmw surfactant can bind to
proteins and form a soluble protein-surfactant complex, which desorbs
from the interface to the bulk. This mechanism does not require the
lmw surfactant to adsorb at the interface, but it must have a strong ten-
dency to interact with the protein. Displacement by direct replacement
occurs due to the fact that the interfacial energy (interfacial tension)
for the surfactant is lower than that for the protein. Protein replacement