Chemistry Reference
In-Depth Information
electrostatic interactions. Above a certain concentration of biopoly-
mer, droplet aggregation via a bridging flocculation mechanism can be
induced (Dickinson, 2003). Since the driving force is electrostatic in
origin, emulsion stability can be affected by the pH and ionic strength
conditions in the system. Bridging flocculation is most pronounced un-
der conditions of partial protein coverage of the emulsion droplets.
9.2.4
Effect of processing
Under quiescent conditions, protein-stabilised emulsions are highly
stable due to the protein interfacial film formed around the emulsion
droplets (van Aken
et al
., 2003). Nevertheless, there are a number of sit-
uations where emulsion instabilities (usually droplet coalescence) may
be promoted due to the application of mechanical stresses, e.g. during
homogenisation (Mohan and Narsimhan, 1997).
During processing, the collision frequency between droplets will in-
crease, which subsequently may also result in, for example, an increase
in the probability and frequency of coalescence events in the system.
If the protein concentration in the system is not sufficient enough to
provide full coverage of all the 'naked' oil-water interfaces created dur-
ing processing, then coalescence is more likely to occur due to the
protein 'gaps' on the surface of the droplets. Even in the case when
the protein concentration is high enough to provide full coverage of
the oil-water interfaces being created during processing, these condi-
tions of high stress can still cause emulsion instability. This is because,
during processing, oil-water interfaces can be subjected to sufficiently
large stresses that can result in 'stretching' of the protein interfacial
films. Stretching of these films may result in the creation of protein-rich
and protein-depleted interfacial regions at the surface of the emulsion
droplets in the system (Windhab
et al.,
2005). If the protein-depleted
interfacial regions of two (or more) droplets come into contact (or even
into close proximity), then coalescence can take place. This type of
coalescence event is more pronounced where protein adsorption is a
relatively slow process compared to the duration of the applied stress,
as unadsorbed protein molecules from the bulk do not have sufficient
time to adsorb at the droplet surfaces and cover the protein-depleted
gaps (Tornberg and Hermansson, 1977).
9.3
ORAL PROCESSES
Interest in the processes experienced by emulsions has been recently
extended to include those occurring in the mouth during their consump-
tion. Although the conventional processes used during the formulation
