Emulsions, Nanoemulsions and Solid Lipid Nanoparticles as Delivery Systems in Foods - Food and Industrial Bioproducts and Bioprocessing - page 171

into a less stable polymorphic form and then possibly transform into a more stable
polymorphic form with time, particularly if stored at higher temperatures (Himawan et al .,
2006). In some cases, different yet physically similar TAG molecules can co-crystallize to
form a compound crystal (Sato, 2001). If a lipid is processed into colloidal size droplets,
the crystallization process is considerably different, affecting both the kinetics of
crystallization, crystalline microstructure, and the structure of the fat crystal matrix
(Coupland, 2002 ).
6.5.1 Kinetics of crystallization in fine droplets
The process of crystallization proceeds via two distinct processes: crystal nucleation and
growth (Garside, 1985). The nucleation kinetics in fine droplets is often different from
nucleation in the same liquid in bulk. In a fine emulsion, the number of droplets exceeds the
number of potential nucleation catalysts (impurities) present in the liquid oil. Thus, a
proportion of the lipid is effectively catalyst free and must nucleate by other mechanisms.
This may either be completely spontaneous homogeneous nucleation or, more probably,
some catalytic effect of the droplet surface (Coupland, 2002). In either case, the crystallization
temperature is greatly reduced below the melting point and depends both on particle size
and on the nature of the emulsifier selected. For example, Higami and co-workers (2003)
showed that the crystallization temperature of trilaurin molecules decreased from 18.9 °C,
the crystallization temperature of the bulk lipid, to -9.5 °C when emulsified into droplets
smaller than 100 nm.
In a fine droplet, droplets crystallization is most often governed by surface effects, i.e.,
surface heterogeneous nucleation. The most generally accepted model for surface
heterogeneous nucleation is based on an ordering of lipid molecules by the hydrophobic
portion of the emulsifier at the interface. If the hydrophobic tail of the surfactant molecule
has a similar molecular structure to the lipid, it may catalyze the surface nucleation event
(Coupland, 2002). A good example of surfactant-directed interfacial heterogeneous
nucleation is provided by Awad and Sato (2002) and Kaneko and co-workers (1999), who
showed that hydrophobic sucrose oligoesters of stearic acid and palmitic acid accelerated
the crystallization rate in emulsified palm kernel oil and hexadecane, respectively. They
argue the hydrophobic surfactant absorbs to the interior surface of the droplet, where it
provides a template for crystal formation.
The ordering effect preceding interfacial heterogeneous nucleation is believed to take
place via the formation of rotator phase; a transient structure corresponding to TAG
molecules orientated along their long axis but retaining some rotational mobility (Sirota,
1998). This phase is believed to act as an interfacial frozen monolayer, which in turn acts
as a template for crystallization of the remainder of the lipid droplet (Wu et al ., 1993 ;
Sirota, 1998). However, Gulseren and Coupland (2008) argue that the region of lipid
affected by the surfactant is much greater than a monolayer. Some recent and elegant
support for this mechanism is provided by Shinohara and co-workers (2008), who used a
focused X-ray beam to study hexadecane emulsions stabilized with different polysorbate
surfactants. They showed that when the size and shape of the hydrophobic tail of the
surfactant is similar to hexadecane, then the rotator of the lipid was detectable before
crystallization and produces a radial distribution of crystals after crystallization is complete
(Figure 6.3 ).