amylose occurs in trace amounts, i.e. from ca. 0% to ca. 4%.
Under natural conditions, starch chains appear as left-handed
spirals (helices) with six glucose residues per turn. The
primary hydroxyl groups of glycoside residues are directed
outwards the helix, whereas the secondary ones and hydrogen
bonds are pointed inwards. The outer side of the helix
is hydrophilic and its inner tunnel – hydrophobic in character.
Owing to this, different water-insoluble substances,
including lipid compounds or iodine, may penetrate into the
starch helix, especially into amylose. The amylose-iodine
complex is navy blue.
The amylose chains and arranged in parallel amylopectin
branches (with a polymerisation degree of 10–30)
form double helices with neighbouring chains which are
strengthened with hydrogen bonds. Six neighbouring left-
-handed double helices make up crystalline forms, of ca. 10
nm in length [Eliasson & Gudmundsson, 1996]. At the sites
where bunches (clusters) of double helices with an appropriate
(over 10 glucoside residues) length appear, crystalline
regions of starch are formed. In the areas where
branches of amylopectin chains, its short chains or single
helices of free amylose or amylose bound with lipid substances
occur, either semi-crystalline or amorphous layers
are likely to form. The crystalline layers formed from
repeating subsequent amylopectin clusters together with
amorphous substance present between crystals and crystalline
layers form spherical structures (“blocklets”),
20–500 nm in size. The hard (crystalline) layers are composed
of large “blocklets” (50–500 nm), whereas the soft
(semi-crystalline) layers – of smaller ones (20–50 nm).
Regions are arranged concentrically, the “hard” ones alternately
with the “soft” ones [Kossman & Lloyd, 2000]. Starch
crystallinity, positively correlated with the content of amylopectins
and the number of double helices, ranges from
15% to 45% depending on plant species [Eliasson &
Gudmundsson, 1996].