Food Emulsifiers and Stabilizers - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 208

TABLE 7.2
Comparison of the Magnitudes of Various Quantities in Emulsions and Foams

Property Value in Emulsions Value in Foams

Particle diameter 2 × 10 -7 to 10 -5 m 10 -4 to 3 × 10 -3 m

Particle volume fraction 0.01 to 0.8 0.5 to 0.97

Density difference 10 to 100 kg m -3 10 3 kg m -3

Compressibility of dispersed phase 5 × 10 -10 N -1 m 2 10 -5 N -1 m 2

Interfacial tension 10 -3 to 10 -2 N m -1 0.03 to 0.05 N m -1

Laplace pressure e.g., 10 4 N m -2 e.g., 10 2 N m -2

Solubility of dispersed phase in continuous phase 0(O/W), 0.15 vol% (W/O) 1.1 vol%

EMULSIONS

Immiscible phases can be redispersed by mixing or homogenizing, and one phase
(oil or water) can be ruptured into droplets and dispersed into the other. With time,
the system, at equilibrium, will eventually separate into two- or three-component
phases, but for a restricted period of time the dispersed droplets will remain sus-
pended. Such thermodynamically unstable systems are referred to as emulsions. The
amphiphilic nature and concentration of the systems, along with the nature and
concentration of the oil, will determine the kinetic stability of the emulsion. The
differences between emulsions and foams are mostly due to the “easiness with which
the bubbles are deformed” in comparison to the relative rigidity of the liquid droplets,
and the differences in the surfactant organization and immobilization at the interface.
Some of the typical characteristics of foams and emulsions are summarized in
Table 7.2 . An excellent overview of emulsion and foam stability was discussed by
Walstra. 17,18

E MULSION F ORMATION

Emulsification is a set of kinetic processes that takes place during the mechanical
rupture of the film in-between the two immiscible liquid layers. The emulsifier plays
a significant role in these consequential kinetic steps by lowering interfacial tension
between the two phases and hence facilitates emulsion formation. The main role of
the emulsifier, however, is related to its ability to adsorb on the ruptured droplets
and to prevent their fast reflocculation or coalescence during the emulsification
process thereafter. The emulsifier controls the interfacial viscosity between droplets,
helps to dissipate the energy, adsorbs on the surface of the newly formed droplets,
and alters its surface charge or nature.

E MULSION S TABILITY

The stability of emulsions has been the subject of scientific debate for many
years. 19-22 Three main mechanisms of stabilization have been described involving
electrostatic, steric, and mechanical stabilization of particles.