Food Lipids: Chemistry, Nutrition, and Biotechnology

2. Cloud Point
When a surfactant solution is heated above a certain temperature, known as the cloud
point, it becomes turbid. As the temperature is raised, the hydrophilic head groups
become increasingly dehydrated, which causes the emulsifier molecules to aggregate.
These aggregates are large enough to scatter light, and so the solution appears turbid.
At temperatures above the cloud point, the aggregates grow so large that they sediment
under the influence of gravity and form a separate phase. The cloud point
increases as the hydrophobicity of a surfactant molecule increases; that is, the length
of its hydrocarbon tail increases or the size of its hydrophilic head group decreases
[13,14].
3. Solubilization
Nonpolar molecules, which are normally insoluble or only sparingly soluble in water,
can be solubilized in an aqueous surfactant solution by incorporation into micelles
or other types of association colloid [9]. The resulting system is thermodynamically
stable; however, equilibrium may take an appreciable time to achieve because of the
activation energy associated with transferring a nonpolar molecule from a bulk phase
to a micelle. Micelles containing solubilized materials are referred to as swollen
micelles or microemulsions, whereas the material solubilized within the micelle is
referred to as the solubilizate. The ability of micellar solutions to solubilize nonpolar
molecules has a number of potentially important applications in the food industry,
including selective extraction of nonpolar molecules from oils, controlled ingredient
release, incorporation of nonpolar substances into aqueous solutions, transport of
nonpolar molecules across aqueous membranes, and modification of chemical reactions
[9]. Three important factors determine the functional properties of swollen
micellar solutions: the location of the solubilizate within the micelles, the maximum
amount of material that can be solubilized per unit mass of surfactant, and the rate
at which solubilization proceeds [9].
4. Surface Activity and Droplet Stabilization
Lipid-based emulsifiers are used widely in the food industry to enhance the formation
and stability of food emulsions. To do this they must adsorb to the surface of emulsion
droplets during homogenization and form a protective membrane that prevents
the droplets from aggregating with each other [1]. Emulsifier molecules adsorb to
oil–water interfaces because they can adopt an orientation in which the hydrophilic
part of the molecule is located in the water, while the hydrophobic part is located
in the oil. This minimizes the unfavorable free energy associated with the contact of
hydrophilic and hydrophobic regions, and therefore reduces the interfacial tension.
This reduction in interfacial tension is important because it facilitates the further
disruption of emulsion droplets; that is, less energy is needed to break up a droplet
when the interfacial tension is lowered.
Once adsorbed to the surface of a droplet, the emulsifier must provide a repulsive
force that is strong enough to prevent the droplet from aggregating with its
neighbors. Ionic surfactants provide stability by causing all the emulsion droplets to
have the same electric charge, hence to repel each other electrostatically. Nonionic
surfactants provide stability by generating a number of short-range repulsive forces
(e.g., steric overlap, hydration, thermal fluctuation interactions) that prevent the drop-
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