Food Emulsifiers and Stabilizers - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 209

Electrostatic Stabilization-DLVO Theory
Electrostatic stabilization is based on the DLVO theory, 23,24 which describes the
electrostatic repulsion forces arising from the diffused, electric double-layer present
around the surface of charged particles. Opposing the repulsive forces are the Van
der Waals interactions between particles, arising from the main types of attraction
forces: dipole-dipole, dipole-induced dipole, and induced dipole-induced dipole.
The attraction energy is proportional to the particle size, its charge, and the medium
(Hamaker constant A) and is inversely proportional to the distance between two
approaching particles.
The repulsive term is derived from the local accumulation of counter ions at a
charged surface; the concentration of these ions being dependent on the ionic strength
of the medium. The presence of a layer of counter ions around a particle is associated
with an electrostatic potential which results in two like particles experiencing a
repulsive force when they approach one another. Only a few food systems rely solely
on electrostatic repulsion forces for their stability.
Adsorbed proteins undoubtedly carry charges, but they are also macromolecules
that have the capacity for enthalpy and entropy stabilizing effects which are probably
more important. Simple ionic surfactants, which are both functional and permissible
in foods, are rare. One such emulsifier which makes stable oil-in-water emulsions
is sodium steroyl lactylate (SSL).
Electrostatic repulsive forces must undoubtedly be involved in food colloid
stability, particularly when proteins are present. In practice, however, other forces
appear to predominate. Thus, dairy-cream emulsion droplets, which are stabilized
by milk proteins, do not flocculate at the isoelectric point (pH 4.6) provided the
temperature is kept below 10°C. This observation is exploited in the manufacture
of cultured creams and related dairy products.
Steric Stabilization
When two colloidal particles approach closely, the adsorbed surfactant layers inter-
act. With adsorbed macromolecules, the interaction can involve a reduction in con-
figurational entropy as molecular chains overlap. Additionally, hydration of adsorbed
hydrophilic components can lead to an enthalpy repulsion term when two particles
are in close proximity, leading to a local osmotic gradient which tends to force the
particles apart. With small-molecule emulsifiers, such as monoglycerides and other
esters, the entropy term is insignificant and enthalpy forces are often too weak to
provide adequate stability. Consequently, few food colloids are stabilized by small
molecule non-ionic emulsifiers. Notable exceptions to this, however, are the poly-
oxyethylene derivatives of sorbitan fatty acid esters (Tweens), which are capable of
providing stable oil-in-water emulsions at only mono-layer coverage. Use of this
fact is made when stabilizing flavored oils for fruit juices and other drinks are present,
where Tweens are the class of emulsifiers capable of imparting the necessary long-
term emulsion stability.
For food emulsions, it is not small-molecule surfactants, but macromolecules
(usually proteins) that are the universal stabilizers of oil-in-water emulsions and