refractive index is higher. The sign observed in a starch granule indicates
that the molecules have a roughly radial orientation. This fits with the
growth of a granule being in the radial direction, starting at the so-called
hilum, which becomes its center.
Whether a material is crystalline or not can be established by x-ray
diffraction. X-rays have a very small wavelength, of the order of 0.1 nm,
which implies that individual atoms may cause scattering. If the atoms (or
small molecules) occur at regular distances, sharp diffraction maxima occur,
and the crystal structure can be derived from the diffraction pattern. Native
starch indeed shows a distinct diffraction pattern. It can be concluded from
these and other data that 20 to 45%of the starch is in a crystalline form.
Since this concerns almost exclusively amylopectin, this material would be
crystalline for 30 to 50%. A starch granule thus hascrystalline regions, the
remainder being amorphous. The crystalline regions are small (order of
10 nm) and are called (micro)crystallites.
The starch structure is similar to that of concentrated synthetic
polymers below a given temperature. Above this temperature we have a
polymer melt, and upon cooling, microcrystallites are formed, in which
parts of the long molecules have a parallel orientation, as illustrated in
Figure 6.22. The driving force is the lowering of contact enthalpy occurring
FIGURE6.22 Highly schematic picture of microcrystalline regions in a mass of a
linear polymer at very high concentration (little or no solvent). The reader should
remember that a two-dimensional picture cannot give a fully realistic representation
of a three-dimensional structure in a case like this.