Chemistry Reference
In-Depth Information
pressure, termed thermal or pressure denaturation) during emulsification
(e.g. homogenisation) (Galazka
et al
., 1996; Kim
et al
., 2002; Rampon
et al
., 2003). Galazka
et al
. (1996) investigated the effect of high pressure
on the emulsifying behaviour of β-lactoglobulin and reported that oil-
in-water emulsions stabilised by protein that was pressure-treated were
less stable than those stabilised by the native protein. More interestingly,
high-pressure treatment of emulsions prepared with the native protein
only slightly affected their stability, which suggests that the structure of
the partially unfolded β-lactoglobulin (adsorbed at the oil-water inter-
face) was more or less unaffected by the induced pressure.
In addition, emulsions stabilised by globular proteins are particularly
sensitive to thermal treatments. This is because, when the temperature
of the system is increased above a certain critical value, these pro-
teins tend to unfold and expose reactive groups originally located in the
interior of their native molecular arrangement. These reactive groups
increase the likelihood of (attractive) interactions between proteins ad-
sorbed on the same or different droplets, thus giving rise to emulsion
instabilities, such as droplet flocculation and coalescence. Emulsion sta-
bility is further compromised when salt is also present in the system,
since this results in screening of the electrostatic repulsion between the
protein-stabilised droplets. Kim
et al
. (2002) have shown that when β-
lactoglobulin-stabilised emulsions are heated above
70
◦
C (above the
denaturation temperature of the native protein) in the presence of salt
(NaCl), protein unfolding becomes much more extensive (than in the ab-
sence of salt), leading to a higher degree of droplet flocculation. A very
interesting finding from the same study was that droplet flocculation
can be significantly reduced when β-lactoglobulin-stabilised emulsions
are heated in the absence of salt, and then salt is added later, after the
system is brought down to room temperature (Kim
et al
., 2002).
Emulsion properties such as interfacial tension, interfacial concentra-
tion, and interfacial rheology are all directly affected by the denaturation
of the adsorbed protein molecules. The molecular characteristics of the
non-adsorbed proteins are also known to alter bulk physicochemical
properties of oil-in-water emulsions. The fact that the structure of ad-
sorbed and/or non-adsorbed proteins may be modified/altered during
emulsion preparation can lead to very different surface and emulsifica-
tion properties, which would, without a doubt, have important implica-
tions on the stability of oil-in-water emulsions.
∼
9.2.3
Protein-stabilised oil-in-water emulsions - Effect
of aqueous phase composition
In practice, protein functionality may have to be exhibited in oil-in-water
emulsion-based products that have a wide range of different pH values
