Food Chemistry

8.15 Surface-Active Agents 457

Each emulsifier can disperse a limited amount
of an inner phase, i. e. it has a fixedcapacity.
When the limit is reached, further addition of
outer phase breaks down the emulsion. The ca-
pacity and other related parameters differ among
emulsifiers and can be measured accurately un-
der 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 solu-
bility 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 num-
ber of hydroxyl or polyglycolether groups.
In an immiscible system such as oil/water, the
emulsifier is located on the interface, where it de-
creases 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,

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 Co-
alescence: the aggregated droplets merge into larger
droplets. Finally, two continuous phases are formed; the
emulsion is destroyed

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 ten-

side projecting into the aqueous phase. The co-

alescence 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 mobil-

ity. 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 emul-

sion has to be destroyed. Elevated temperatures

are used along with shaking, agitation or pressure

Fig. 8.14.Stabilization of an emulsion.aActivity of

an ionic emulsifier in an o/w emulsion.bActivity of

a nonpolar emulsifier in w/o emulsion.◦Polar groups,

∼apolar tails of the emulsifier