Biomedical Engineering Reference
In-Depth Information
or ouzo). Various technologies are used for particle size reduction in emulsions; the most
common are high-pressure valve homogenizers, although other devices, including high
intensity ultrasound, membrane homogenizers, and microfluidic devices, are also capable of
generating very fine particles with uniform droplet size distributions (Solans et al ., 2005 ).
Although the efficiency of droplet formation (i.e., size and uniformity of the droplets)
depends on many factors, in general there is an approximately linear inverse relationship
between the homogenization pressure and particle size (McClements, 2005).
Emulsions are thermodynamically unstable structures given a degree of kinetic stability
by an adsorbed interfacial layer of amphiphilic emulsifiers. The emulsifiers serve to lower
the interfacial tension and provide some inter-droplet repulsive forces to stabilize the
dispersions (e.g., steric and electrostatic). The interfacial layer is typically between about
1 and 10 nm thick for food grade emulsifiers, such as surfactants, phospholipids, proteins, or
polysaccharides, and the interfacial concentration is in the order of a few mg per square
meter of surface (McClements, 2005; McClements and Decker, 2000).
The various mechanisms for emulsion destabilization include flocculation, coalescence
(in fluid particles), partial coalescence (in semi-crystalline particles), Ostwald ripening and
gravitational separation (creaming or sedimentation) (Claesson et al ., 2004 ; Dickinson,
1992 ; McClements, 2005 ; Vanapalli and Coupland, 2004 ). In general, the instability
processes are slower for fine particles (e.g., the rate of creaming by Stokes Law is inversely
proportional to the square of particle diameter). The major exception is destabilization via
Ostwald ripening. Ostwald ripening is the growth of large particles at the expense of fine
particles due to diffusion of molecules from one droplet to another. Ostwald ripening is
driven by differences in surface curvature and is, therefore, faster in very fine particles.
Ripening processes are particularly important when molecules in the lipid phase have some
solubility in the aqueous phase, so while triacylglycerols (TAGs) typically ripen slowly, the
transport of more polar emulsified BLI may be faster. Aqueous surfactant micelles can
increase the solubility of lipids in the aqueous phase and thus increase the rate of ripening
(Weiss et al ., 1996 ).
An emulsion can be envisaged as consisting of at least three distinct microenvironments for
added BLI molecules (Figure 6.1): the continuous aqueous phase, the lipid core of the
droplets, and the surface of the droplets as the amphiphilic region. The relative volume of
the aqueous and lipid phases can be altered by changing the formulation of the emulsion,
while the surface to lipid volume ratios can be altered by changing the particle size
(Figure 6.2). BLI localized in the lipid core or aqueous phases will be predominantly
surrounded by TAG and water molecules, respectively, while BLI molecules in the interfacial
region will interact with both to some extent as well as with the high concentration of
emulsifiers on the surface.
BLI molecules will partition between the different microenvironments depending on
their chemical structure and intermolecular interactions. As they are lipophilic molecules,
the aqueous concentration will be very low. Molecular lipophilicity is most commonly
expressed in terms of the log P value, where P is the ratio of equilibrium concentrations in
octanol and water. Large log P values suggest the molecule will exist predominantly inside
the emulsion droplets (Table 6.1). One weakness of the log P approach is that it neglects the
Search WWH ::

Custom Search