Emulsions, Nanoemulsions and Solid Lipid Nanoparticles as Delivery Systems in Foods - Food and Industrial Bioproducts and Bioprocessing - page 170

have greater oxidative stability than those prepared with whey protein. This is despite the fact
that whey protein-stabilized emulsions droplets are more positively charged (+55.9 mV) than
their casein counterparts (+29.9 mV) at the low pH studied (Chaiyasit et al ., 2007 ).
Smaller droplets (larger interfacial area) increase the exposure of the encapsulated BLI
to reactive molecules, so would be expected to reduce their chemical stability. Despite
this, comparisons of BLI degradation rates at different particle sizes are relatively few and
the postulated relationship is not always seen. For example, Kiokias and co-workers
(2007) did not report any effects of droplet size (d 32 = 0.5-2 μm) on the rate of lipid
oxidation in model food emulsions as a function of temperature, while Let and co-workers
(2007), and Nakazawa and co-workers (2008) showed fine

3 fatty acid-rich oil and
methyl linolenate emulsions oxidized more slowly than coarse ones (d 32 = 0.5-1.5 μm and
median = 0.02-1 μm, respectively). Droplet size distribution affects not only the chemical
stability of BLI and the physical stability of the EBDS but also the bioavailability of BLI
within the EBDS.
Bioavailability, the amount of bioactive ingredient metabolized at the appropriate part of
the living body, is believed to increase with decreasing particle size in the EBDS. Acosta
(2009) reviewed various bioavailability studies and showed that, in general, there was an
increased bioavailability in emulsions with droplets smaller than 500 nm; this was even
greater for droplets less than 100 nm (Acosta, 2009). Various mechanisms have been
suggested to explain this effect and perhaps the most universal is the increased solubility of
BLI in aqueous phase around the very fine particles (i.e., Laplace pressure effects). However,
it has also been shown that residence time of the carrier particles in the intestine increases
with decreasing particle size, which in turn increases the amount absorbed (Kreuter, 1991).
The shorter diffusion paths in fine particles have been suggested as an alternative reason for
increased bioavailability (Acosta, 2009; McClements et al ., 2007 ). However, while the
diffusion paths in the lipid phase are certainly reduced, it is unclear if they are rate limiting
in the overall process of absorption. Finally, Jani and co-workers (1990) proposed that very
fine particles (d < 100 nm) could be adsorbed whole by living cells.

ω

6.5 CRYSTALLIZATION IN EMULSIONS

The equilibrium solid fat content (SFC) of a lipid is determined by the temperature and the
type of molecules present (e.g., longer saturated or trans-unsaturated fats have higher
melting points). The vegetable oils most commonly used in conventional food emulsion
preparation are liquid at both storage and use temperatures. Other food lipids, notably milk
fat, partially hydrogenated vegetable oil and some tropical fats are semi-crystalline at room
temperature and below. Finally, hard fats (e.g., stearin fractions from tropical oils and animal
fats, fully hydrogenated vegetable oils, certain waxes) are (almost) completely crystalline at
room temperatures. The physical properties of lipids can be manipulated by changing the
TAG composition by either chemical (e.g., hydrogenation or inter-esterification) or physical
means (e.g., fractionation or blending) (Haumann, 1994). In all cases, commercial food
lipids are a complex mixture of different TAG molecules.
TAG molecules can crystallize typically as one of three polymorphic crystal structures,

α , each with different subcell structures (i.e., hexagonal, orthorhombic-

perpendicular, and triclinic-parallel, respectively) and with increasing melting points and
enthalpies of fusion (Sato, 1999). If a liquid oil is cooled quickly, it will tend to nucleate

, β ' and β