2009_Book_FoodChemistry

8.15 Surface-Active Agents 457
Each emulsifier can disperse a limited amount
of an inner phase, i. e. it has a fixed capacity.
When the limit is reached, further addition of
outer phase breaks down the emulsion. The capacity
and other related parameters differ among
emulsifiers and can be measured accurately under
standardized conditions.
8.15.2 Emulsifier Action
8.15.2.1 Structure and Activity
Emulsions are made and stabilized with the aid of
a suitable tenside, usually called an emulsifier. Its
activity is based on its molecular structure. There
is a lipophilic or hydrophobic part with good solubility
in a nonaqueous phase, such as an oil or fat,
and a polar or hydrophilic part, soluble in water.
The hydrophobic part of the molecule is generally
a long-chain alkyl residue, while the hydrophilic
part consists of a dissociable group or of a number
of hydroxyl or polyglycolether groups.
In an immiscible system such as oil/water, the
emulsifier is located on the interface, where it decreases
interfacial tension. Thus, even in a very
low concentration, it facilitates a fine distribution
of one phase within the other. The emulsifier also
prevents droplets, once formed, from aggregating
and coalescing, i. e. merging into a single, large
drop (Fig. 8.13).
Ionic tensides stabilize o/w emulsions in the
following way (Fig. 8.14a): at the interface, their
alkyl residues are solubilized in oil droplets,
while the charged end groups project into the
aqueous phase. The involvement of counter
ions forms an electrostatic double layer, which
prevents oil droplet aggregation.
Nonionic, neutral tensides are oriented on the oil
droplet’s surface with the polar end of the tenside
projecting into the aqueous phase. The coalescence
of the droplets of an o/w emulsion is
prevented by an anchored “hydrate shell” built
around the polar groups.
The coalescence of water droplets in a w/o
emulsion first requires that water molecules
break through the double-layered hydrophobic
region of emulsifier molecules (Fig. 8.14 b). This
escape is only possible when sufficient energy is
supplied to rupture the emulsifier’s hydrophobic
interaction.
The stability of an emulsion is increased when
additives are added which curtail droplet mobility.
This is the basis of the stabilization effect of
hydrocolloids (cf. 4.4.3) on o/w emulsions since
they increase the viscosity of the outer, aqueous
phase.
A rise in temperature negatively affects emulsion
stability, and can be applied whenever an emulsion
has to be destroyed. Elevated temperatures
are used along with shaking, agitation or pressure
Fig. 8.13. Changes in an emulsion. 1 The droplets are
dispersed in a continuous phase. 2 The droplets form
aggregates. An increase in particle diameter results in
acceleration of their flotation or sedimentation. 3 Coalescence:
the aggregated droplets merge into larger
droplets. Finally, two continuous phases are formed; the
emulsion is destroyed
Fig. 8.14. Stabilization of an emulsion. a Activity of
an ionic emulsifier in an o/w emulsion. b Activity of
a nonpolar emulsifier in w/o emulsion. ◦ Polar groups,
∼ apolar tails of the emulsifier