Chemistry Reference
In-Depth Information
Chapter 8
Self-Assembly of Starch Spherulites as
Induced by Inclusion Complexation with
Small Ligands
Be´ atrice Conde-Petit,
1
Stephan Handschin,
1
Cornelia
Heinemann
2
and Felix Escher
1
1
INSTITUTE OF FOOD SCIENCE AND NUTRITION, ETH ZURICH,
CH-8092 ZURICH, SWITZERLAND
2
NESTLE PTC KONOLFINGEN, NESTLESTRASSE 3, CH-3510
KONOLFINGEN, SWITZERLAND
8.1 Introduction
Starch is synthesized in higher plants in the form of partly crystalline granules
with different morphologies and sizes between 1 and 100 mm. The linear
amylose and the branched amylopectin are the starch polymers, and most
common starches are composed of 25% amylose and 75% amylopectin. The
formation of starch granules in the plant is a process governed by genetics
1
and
thermodynamics.
2
The process yields starch assemblies with a more or less
spherical morphology and with alternating shells of amorphous and crystalline
material.
To a certain extent native starch granules can be considered as natural
spherulites. Starch granule assembly is thought to start from the hilum. This
'nucleus' of the granule and its surrounding core contain less well-organized
polymers and sometimes an amylose-rich core. Spherulitic starch crystallization
has been proposed as a model for starch granule initiation in vivo.
3,4
Ziegler
et al.
4
have demonstrated the spontaneous formation of spherulites in aqueous
mung bean starch dispersions at concentrations between 10% and 20%. This
type of spherulitic assembly presents a B-type crystallinity, which corresponds
to a double-helical arrangement of starch in the crystals. The spherulites are
mainly composed of amylose or slightly branched material, and their crystal-
lization is preceded by a phase-separation process.
Aqueous solutions of maltodextrins obtained by enzymatic degradation and
debranching may also present spherulitic crystallization on holding at moder-
ately elevated temperature (521C).
5
In the early stages of the reaction the
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