Chemistry Reference
In-Depth Information
9.2.2
Protein functionality at liquid interfaces
Proteins have been extensively used to assist the formation of oil-in-
water emulsions and also to stabilise their bulk microstructure. This
dual functionality of proteins in emulsions stems from their surface-
active nature, and thus their ability to adsorb at the oil-water interface,
but is also a direct consequence of their relatively 'large' molecular size,
in comparison to the smaller emulsifiers used in foods (e.g. Tweens),
which renders the interfaces they adsorb onto to be much 'thicker'.
By doing so, proteins lower the interfacial tension of the fluid inter-
faces (that they absorb onto) and thus aid the formation of oil-in-water
emulsions by facilitating droplet break-up during emulsification. The
equilibrium interfacial tension of protein-stabilised interfaces, depend-
ing on the type of the oil phase, is roughly between 10-25 mN/m
for both flexible (e.g. β-casein) and globular proteins (e.g. ovalbumin,
bovine serum albumin) (Beverung
et al.
, 1999).
In terms of their ability to lower the interfacial tension of oil-in-water
emulsions, proteins are less effective when compared to low molecular
weight (
lmw
) surfactants. Nonetheless, proteins are far more superior in
stabilising the formed emulsion droplets, with respect to droplet coal-
escence and flocculation, than
lmw
surfactants (van Aken
et al
., 2003).
Although
lmw
surfactants have higher adsorption energies per square
metre than proteins, the latter can adsorb at the interface with several seg-
ments, and changes in their conformation allow for even more segments
to adsorb. Therefore, because of their overall high energy of adsorp-
tion, proteins, forming a saturated monolayer covering the emulsion
droplets, can be considered as being irreversibly bound to the oil-water
interface.
The protein concentration required to provide a saturated monolayer
covering the emulsion droplets (surface concentration) usually depends
on the pH and ionic strength of the system; surface concentration in-
creases at pH values close to the isoelectric point of the protein (Graham
and Phillips, 1979). Surface concentration also depends on the type of
protein; saturation for flexible proteins occurs at values of 2-3 mg/m
2
,
whereas for globular proteins, it is usually in the range of 1-2 mg/m
2
(Beverung
et al.
, 1999). In both cases, the thickness of the resulting
interfacial films is between 5 and 6 nm (Graham and Phillips, 1979).
Further adsorption of protein molecules, on the initially formed pro-
tein monolayer, can take place but will only affect the thickness of the
interfacial film and not its interfacial properties (interfacial tension).
Although protein molecules are now reversibly adsorbed, with respect
to aqueous substrate exchange, the thickness of the interfacial film can
increase to more than 10 nm (Graham and Phillips, 1979). It is the
thickness of these protein-saturated interfaces that provides a sterically
