Manipulation of Adsorption Behaviour at Liquid Interfaces by Changing Protein–Polysaccharide Electrostatic Interactions - Food Colloids: Self-Assembly and Material Science - page 205

the presence of LMP decreases the dilatational modulus, in the presence of HMP
the dilatational modulus is higher than that for the pure protein layer. The same
trend was observed at the air-water interface. A possible explanation is that,
since HMP has a lower charge density than LMP, and therefore the binding of
protein molecules to HMP is weaker, it is unable to prevent the protein molecules
from forming a dense protein layer at the interface. An adsorbed complex
structure with HMP, as in layer II of Figure 7, could therefore quickly transform
into a structure like layer IV, i.e., a dense protein layer, reinforced by coadsorp-
tion of HMP, and possibly even containing more b-Lg than the pure protein
layer. This hypothesis is supported by the observation that, when a protein layer
was formed at the air-water interface from a pure protein solution, and subse-
quently LMP was injected into the bulk solution, the dilatational modulus
increased to a value higher than that of the pure protein layer. The resulting
mixed b-Lg + LMP adsorbed structure could be depicted as layers IV or V in
Figure 7, depending on the protein/polysaccharide mixing ratio in the bulk
solution. Apparently, when LMP is present from the beginning of adsorption, it
can prevent the formation of a dense protein layer at the air-water interface (and
presumably also at the oil-water interface). Conversely, it can reinforce a protein
layer formed prior to LMP addition.
The effect of different mixing ratios with LMP, as well as the effect of the
sequence of adsorption, has been shown to influence the surface shear rheology
at the air-water interface even stronger than dilatational rheology. The build-up

protein
monolayer

I I AIR/OIL AIR/OIL

WATER

II II III III

single
complex
layer

IV IV V V

protein layer
with complex
layer
underneath

negatively
charged
complexes

negatively
charged
complexes

neutral
complexes

neutral
complexes

Figure 7 Schematic representation of various kinds of adsorbed layers at the air-water or
oil-water interface: I, protein monolayer; II and III, mixed layers from simul-
taneous adsorption, where layer II is from negatively charged complexes and
layer III is from neutral complexes; IV and V, complexes adsorbed at a
previously formed protein layer, where layer IV is for negatively charged
complexes and layer V is for neutral complexes