Self-Assembly of Starch Spherulites as Induced by Inclusion Complexation with Small Ligands - Food Colloids: Self-Assembly and Material Science - page 118

crystalline material presents B-type crystallinity, while the material shifts to an
A-type crystalline packing in the later stages of the process. Both A- and B-type
starch crystals are based on double-helical starch assemblies, with the difference
being that the A-type crystal has a denser packing and only a few water
molecules in the monoclinic cell. 6 The process of split crystallization yields
highly crystalline material with spherical morphology which is resistant to-
wards enzymatic degradation in the small intestine. This type of 'resistant
starch' has found application as functional fibre in food because of promising
physiological effects in humans. 7
An alternative mechanism by which spherulitic crystallization can be initi-
ated is the formation of helical amylose inclusion complexes in the presence of
suitable ligands that induce the helix formation of amylose. Among the first
reports on starch spherulites were the investigations of Bear 8 and Schoch 9
published in 1942. It was observed that addition of butanol to starch disper-
sions leads to a selective precipitation of amylose in the form of six-segmented
spherulites with a diameter of 15-50 mm. The spherulites were found to be
birefringent under polarized light and showed an interference pattern similar to
the well-known Maltese cross of native starches. In contrast to the native starch
granules, the de novo formed spherulites are composed of amylose only, this
starch fraction being particularly prone to crystallization. The partly crystalline
amylose structures formed by amylose inclusion complexation are also known
as 'reformed amylose particles' and the process has been termed 'high-temper-
ature retrogradation'. 10 In cereal starches, endogenous lipids may promote the
formation of spherulites. 11,12 Besides linear alcohols and monoacyl lipids, other
ligands suitable for inducing the formation of starch spherulites are terpenes
and lactones. The present contribution reports on the spherulitic crystallization
of potato starch as induced by inclusion complexation with different lactones.
The latter molecules are aroma active and are relevant as naturally occurring
flavouring agents in foods.
8.2 Materials and Methods
Potato starch was obtained from Blattmann (Wa¨ denswil, Switzerland). The
flavour compounds, g-heptalactone, g-nonalactone, g-decalactone, g-dodecalac-
tone, d-decalactone and d-dodecalactone, all of purum quality, were supplied by
Fluka (Buchs, Switzerland), as was the a-amylase, isolated from pig pancreas
with an activity of 20 U mg 1 .
Starch dispersions (2 wt%) were prepared by heating native starch suspen-
sions in cans at 1211C for 30 min or in beaker at 951C for 30 min. In the latter
case, the remnants of swollen starch granules were dispersed by treating the
dispersion for 1 min with a benchtop homogenizer (Polytron, Kinematica,
Littau, Switzerland). The lactones were added to the starch dispersions at room
temperature at concentrations between 10 and 50 mmol mol 1 of anhydroglu-
cose (2% starch R 122.6 mmol mol 1 anhydroglucose) so that the extent of
amylose complexation was rather low. The complexation index of amylose in