Physical Chemistry of Foods

upon close packing. As soon as polymer molecules take part in such
crystallites, the remainder of each molecule loses much of its freedom to
reorientate, the more so if more of the material is crystalline. As becomes
clear from the figure, any further crystallization would require considerable
reorientation, also of the chain sections present in crystallites. This means
that crystallization virtually stops at a certain fraction crystalline, not
because thermodynamic equilibrium has been reached, but because crystal-
lization has become infinitely slow due to these geometric constraints. The
crystallites are not true three-dimensional crystals: there are no crystal faces
in directions roughly perpendicular to the polymer chains. Also in other
directions, the crystallites mostly have no sharp boundaries. This
imperfectness implies, for instance, that there is no sharp melting
temperature, but a melting range of at least several K (cf. Figure 6.25,
further on).
The similarity between crystallization in starch granules and in
synthetic polymers is, however, limited. The latter only show crystallite
formation if they are linear polymers, whereas in starch it concerns the
highly branched amylopectin. The crystalline structure in starch is formed
during starch synthesis and is largely irreversible. If the structure is
disrupted by melting (see Section 6.6.2), it does not reform on cooling.
The structure is intricate. Crystallites occur in two types of chain
packing, designated A (in most cereals) and B (in potato starch). Both
contain water of crystallization, 10 and 20%by mass, respectively. The
‘‘clusters’’ depicted in Figure 6.21 are predominantly crystalline. However,
the molecular chains are not straight (this is geometrically impossible: see
Figure 6.18) but form double helices, about 1 nm in diameter. Figure 6.21
may suggest that crystalline layers (thickness a) alternate with other,
amorphous layers, but the structure is more complicated. The crystalline
material is in highly curved strips that form large helices: see Figure 6.23. In
potato starch such a helix has an outer diameter of 18 nm and an inner
diameter of 8 nm; the strips are thus 5 nm wide and have a thickness
a&4 nm. The pitch of the helices, which equalsb, is about 10 nm. In the
cavity of each helix and in the space between the strips, amylose and
noncrystalline amylopectin is found; presumably, the amylose is predomi-
nantly in the cavity.
Native starch granules are thus very stiff or rigid particles. They have
crystalline regions, which will contain about half of the water present, and
the remainder of the starch and water forms a glass. The glassy state is
discussed in Section 16.1.
It may finally be mentioned that starch shows considerable variation
in properties, among granules of one source and among plant species and
cultivars. This concerns average granule size and size distribution; granule