Self-Assembly of Starch Spherulites as Induced by Inclusion Complexation with Small Ligands - Food Colloids: Self-Assembly and Material Science - page 122

of potato starch in the presence of g-heptalactone suggests that the ligand was
not able to form stable complexes, whereas all other lactones were good
complexing partners. Depending on the rate of complexation, the formation
of amylose complexes leads to the formation of a network gel or spherulitic
crystallization accompanied by solid liquid bulk phase separation. 14 The
complexation rate is influenced by the type of ligand, the dispersion state and
concentration of the ligand, and the temperature and shearing conditions. It
can be assumed that starch spherulites are primarily composed of amylose as
suggested by blue staining with iodine (results not shown). This implies that the
formation of spherulites is the result of an amylase + amylopectin phase-
separation process. Amylose and amylopectin possess limited miscibility in
aqueous systems and the spontaneous phase-separation process is promoted by
the formation of amylose complexes. Spherulitic crystallization is favourable
if the continuity of the amylose phase falls below a critical level before extensive
amylose complexation is reached. On the other hand, if the spontaneous
amylase + amylopectin phase separation progresses faster than amylose
aggregation, the formation of a kinetically stabilized amylose network is
promoted. 14
The dynamics of spherulite formation upon complexation of potato starch
with d-dodecalactone was followed with light microscopy. Like most good
complexing partners, d-dodecalactone possesses a low solubility in water which
is revealed by the emulsion character of the starch-d-dodecalactone system. The
micrographs in Figure 3(a-d) show that the growth of spherulites starts in the
vicinity of a drop of d-dodecalactone, where a ring-like structure consisting of
small crystals or a gel phase appears after a few minutes. This ring forms a kind
of envelope around the ligand droplet and serves as nucleation point for the
formation of spherulites. Large spherulites that present birefringence are visible
after 24 h at the interface of the lactone droplet [Figure 3(e,f)]. Numerous
spherulites also develop in the bulk aqueous phase, which implies that lactone
molecules diffuse into the aqueous phase. The size and the degree of perfection
of the spherulites increase with the distance to the lactone droplets. This is in
line with the hypothesis that a low complexation rate - in this case as a
consequence of the low ligand concentration - favours self-assembly of the
complexes into well-ordered structures. 14,16
The thermal stability of amylose lactone spherulites was analysed by DSC
and hot-stage microscopy. The results are presented in Figure 4. Spherulites
formed with d-decalactone exhibited a broad multiple melting transitions with
a peak temperature around 801C and a melting range of

401C. The annealing
of spherulites at 551C for 60 min did not lead to significant changes in the
melting range, but the curve shape suggests that more thermostable crystals are
formed on annealing at the expense of less stable ones. 17 The existence of
different thermodynamic states of complexes with different thermal stabilities
has also been described for amylose monoglyceride complexes. 18 Similar to
the melting of native starch granules, the melting of amylose spherulites
promotes the disappearance of the birefringence of spherulites. At the end of
the melting process, remnants of spherulitic structures could be seen in phase

B