Chemistry Reference
In-Depth Information
This chapter will provide a description of the structure of emulsions
stabilised by proteins, the oral processes involved during their consump-
tion (and consumption of emulsions in general) as well as how these
affect their sensory attributes. Finally, the latest developments in instru-
mental methods used to relate emulsion properties, as exhibited during
oral processes, and their sensory perception will be considered.
9.2
PROCESSING AND STABILITY OF EMULSIONS
As emulsions are thermodynamically unstable systems, a lot of effort
has been devoted towards obtaining an understanding of the mechanisms
and kinetics of instabilities, properties, as well as processing routes.
Although these phenomena have been mainly studied in the context
of processing and storage, they would be directly applicable to the
phenomena occurring in the mouth.
9.2.1
Instabilities in emulsions
In order to understand the function of proteins, in terms of the stability
they impart to oil-in-water emulsions, it is useful to briefly discuss the
main instabilities usually associated with these systems.
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Coalescence
Coalescence is the irreversible joining of two or more emulsion droplets
to form a single larger droplet. There are two mechanistic steps that
should take place for coalescence to occur: film thinning and film rup-
ture. Film thinning is a result of surface distortion between two (or more)
approaching droplets, which leads to the formation of thin liquid lamel-
lae between their surfaces. The rate and extent of film thinning depend
on the hydrodynamics of film flow and on the colloidal forces acting
across the film. Emulsion droplets will not coalesce if the thickness of
the formed lamella remains above a certain critical value. In the case
where the thickness of the lamella is reduced below this critical value,
random surface fluctuations can result in rupture of the interfacial layer
(film rupture), leading to coalescence. The kinetics of rupture depends
on the extent of these fluctuations and on the viscoelastic properties of
the adsorbed protein layer.
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Flocculation
Flocculation is the aggregation of two (or more) emulsion droplets with-
out the occurrence of any interfacial rupture events. Flocculation can
