Emulsions, Nanoemulsions and Solid Lipid Nanoparticles as Delivery Systems in Foods - Food and Industrial Bioproducts and Bioprocessing - page 173

6.5.2 Structure of crystalline fat droplets
The type of TAG polymorph is determined by many factors and affects the crystalline
structure within a droplet. Emulsions prepared from pure TAG usually also nucleate into the
α

β
polymorph on storage, or reheating (Awad et al ., 2008 ; Bunjes and Unruh, 2007 ; Higami
et al ., 2003 ; Weiss et al ., 2008). Bunjes and co-workers (2003) reported an unusual
polymorphic form in rapidly cooled SLN corresponding to

-polymorph upon cooling, followed by a solid-solid transformation into a β ' or

α -polymorph without any

apparent long range order (i.e., no SAXS pattern). Moreover, the -crystals are aligned with

the surface (Figure 6.4) in a similar manner seen in X-ray microbeam measurements on
larger droplets (Shinohara et al ., 2008), suggesting a potential role of the surface emulsifier
in directing crystallization.
In general, the

α

form for SLN, firstly because BLI can be more
readily incorporated in the less-dense crystals and, secondly, because the formation of the

α form is preferred over β

β
form is associated with a change in droplet shape and subsequent gelation of the suspension
(see below). The rate of polymorphic transformation in fine particles is:

(1) greater for fine droplets and becomes especially significant at the nano-scale (Awad
et al ., 2008 ; Bunjes and Koch, 2005 ; Bunjes et al ., 2003 , 2007 ; Helgason et al ., 2008 ;
Higami et al ., 2003 ; Illing et al ., 2004 ; Unruh et al ., 2002 ; Westesen and Siekmann,
1997 ). The increased transformation tendency in SLN can be attributed to “the

relaxation of lattice strain developing during the transition is facilitated in smaller
crystalline domains due to the high surface-to-volume ratio, as suggested for bulk
material by Dafler ( 1977 )” (Bunjes and Westesen, 2001 ).
(2) greater for shorter chain TAGs than longer ones as the molecular mobility of small
molecules is greater (Bunjes et al ., 1996 ).
(3) lower at lower temperatures. Helgason and co-workers (2008) showed that the
polymorphic transformation rates of tripalmitin nanoparticles (r ~150 nm) increased by
increasing the temperature from 1 to 10 °C. Similarly, Awad and co-workers (2008)
showed that faster cooling rates retard the polymorphic transition.
(4) dependent on the surfactant selected (Awad and Sato, 2002; Bunjes et al ., 2002 , 2003 ).
Although the mechanism of surfactant action is not understood thoroughly, Bunjes and
co-workers (2002, 2003) suggested that, since the selection of hydrophilic surfactants
can significantly alter the transformation kinetics, the polymorphic transition starts at
the surface rather than the lipid core. This observation is reasonable assuming that TAG
molecules in the surface region interacting with surfactant molecules have higher free
energy and mobility. The capacity of different surfactants to stabilize the

α to β

form varies
considerably. For example, Bunjes and Koch (2005) showed that saturated long chain
phospholipids decreased the rate of polymorphic transitions compared to unsaturated
soy bean phospholipids. Awad and Sato (2002) saw similar results in larger droplets
with other hydrophobic emulsifiers. Bunjes and co-workers (2003) found that combining
bile salts with saturated long chain phospholipids stabilized the

α

form during
recrystallization. One of the most striking examples published to date is a nine month
shelf life in refrigerated saturated TAG nanoparticles stabilized solely with polyvinyl
alcohol (Rosenblatt and Bunjes, 2009 ).

α

In very fine droplets, the recrystallization event from less stable to more stable
polymorphic form is often associated with a change in droplet shape from spherical to plate-