Food Emulsifiers and Stabilizers - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 211

the oil and a hydrophilic surface toward the water, and it is quite obvious that this
phase possesses ideal interfacial properties needed to reduce the surface energy of
oil-water interfaces.
The rheological properties of monolayers of binary surfactant systems have also
been related to emulsion stability and to the structural properties of the lamellar liq-
uid-crystalline phases formed by the surfactants in water. It was also suggested that the
emulsifier molecules adsorbed at an oil-water interface will adopt the same hydrocarbon
chain structure as they have in the bimolecular lipid layer of lamellar mesophases.

Particle Stabilization — Mechanical Stabilization

Stabilization of foams and emulsions by particulate material was originally described
by Pickering. 28 The mechanism involves particles adsorbing at the interface, which
in turn requires an appropriate balance of interfacial free energies such that particles
are wetted preferentially by the continuous phase. The contact angle between the
three interfaces defines the ability of the particle to stabilize or destabilize the colloid.
The angle in Figure 7.2 should lie between 0 and 90 o . Pragmatically, the most

effective stabilization occurs when

θ

is in the range of 60 to 70°. If the angle is too
close to 90°, physical pertubations at the interface can lead to destabilization.
A classical food product, in which the air phase is stabilized by particles, is
whipped cream. Fat droplets adhere to air bubbles during the shipping process,
forming a protective layer and preventing bubbles from coalescing. Stability of
whipped cream is then described in terms of the interfacial energies between air,
fat, and aqueous phases. Increasing the oil-water or air-water interfacial tension
increases adhesion of droplets to air bubbles, and so enhances stability. This can be
achieved in practice through the use of oil- or water-soluble emulsifiers. Mayonnaise
is another example of particle stabilization, although here the size of the adsorbing
particle is much smaller: egg-yolk particles (ca. 30 nm diameter) are considered the

θ

(a) (b)

Water

Water

γ pw

Particle

θ

Oil

θ

γ ow

Oil

γ po

(c)

Oil

Water

θ

FIGURE 7.2 Illustration of emulsion stabilization by solid particles. (A) Definition of
contact angle θ for a single spherical particle at the oil-water interface. In terms of g po , γ pw
at the particle-oil, particle-water, and oil-water interfaces, respectively. (B) Oil-in-water;
droplet stabilized by particles preferentially wetted by water (θ < 90°). (C) Water-in-oil droplet
stabilized by particles preferentially wetted by oil (θ > 90°).