Self-Assembly of Starch Spherulites as Induced by Inclusion Complexation with Small Ligands - Food Colloids: Self-Assembly and Material Science - page 119

the range 5-30% was measured by iodine titration. 13,14 Selected samples were
complexed at higher lactone concentration to saturate the amylose-binding
sites. 13 The samples were mixed for 20 s and kept at 251C for 24 h before
analysis. In one case a sample was annealed by heating the suspension of starch
spherulites to 551C for 60 min before further analysis by differential scanning
calorimetry (DSC).
Spherulites were characterized by light microscopy by placing on a slide a
drop of the starch dispersion, preferably from the precipitate. The sample was
observed with an Axioskop microscope (Zeiss, Germany) equipped with a
Hamamatsu 58103 CCD camera under polarized light, in phase contrast, or in
the differential interference contrast (DIC) mode.
The dynamics of spherulitic starch crystallization was also observed in situ
by light microscopy. For this purpose a potato starch dispersion was mixed with
d-dodecalactone in a syringe and injected onto the sample holder of a hot stage at
351C (Linkam LTS 350 hot stage controlled by the LinkSys32 software, Linkam,
Surrey, UK). The same system was also used to follow the structural changes of
spherulites upon heating from 30 to 1151C with a heating rate of 51Cmin 1 .
DSC and wide-angle X-ray diffraction measurements were performed on
samples of extensively complexed starch at high lactone concentration. The
samples were freeze-dried and measured as described elsewhere. 13 For DSC
measurements the rehydrated samples (30% dry matter) were weighed ( 40 mg)

into pressure pans (Perkin Elmer, Norwalk, CT, USA) and measured in a DSC
instrument (TA Instruments 2920, USA) at 101Cmin 1 from 4 to 1601C. After
cooling the sample to 41C with an average cooling rate of

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201Cmin 1 ,a
second run was performed. For the wide-angle X-ray diffraction measurements,
the moisture content of freeze-dried samples was adjusted to 10-15%. The
samples were compressed into tablets and measured in the transmission mode on
a powder diffractometer (Siemens Kristalloflex D500, Karlsruhe, Germany)
using CuKa radiation (1.54 A ˚ ) with 35 mA and 40 kV. A divergence slit of
2 mm and a receiving slit of 11 were chosen.
The susceptibility of spherulites towards a-amylase was assessed by treating
a suspension of spherulites with a-amylase (activity 100 U g 1 dry starch) for
24 h at 251C. The changes at the molecular level were followed by measuring
the iodine-binding capacity by amperometric iodine titration. 14 The thermal
stability and the microstructure of the spherulites were followed by DSC and
light microscopy, respectively.

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8.3 Results and Discussion
These experiments were carried out with potato starch because it is a lipid-free
starch, which therefore does not contain pre-existing complexes. Upon addition
of lactones at concentrations leading to a partial complexation of starch
dispersions, 13,14 the spherulites could be observed by light microscopy after
24 h as shown in Figure 1. Spherocrystalline structures with diameters ranging
from 10 to 90 mm were found. Three different morphologies are present: small