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1.5. STARCHES
1.5.1. General
Starches are produced by all plants, and the starch-
like glycogens are produced in animals (liver). In both
cases they function mainly as energy reserves.
Starches from tubers (e.g. potatoes) and seeds (e.g.
corn, rice, wheat, rye..) are main agricultural products,
and therefore main energy sources for humans in all
parts of the world. Modern civilization totally depends
on starches.
In addition to the importance as food and food additives, starches and starch
derivatives are basis for industrial applications in paper treatment (‘sizing’),
glues, or as fabric stiffeners. Novel applications include low-calorie dietary
fibers, biodegradable packaging materials, thin films, and thermoplastic
materials.
Native starches are found in
granules (figure: wheat starch
granules). Different plants have
granules of different, but
characteristic sizes and shapes
(Rice: ca 2
µ
m, potato: up to 100
µ
m). They contain almost
exclusively starch, in a dry and
compact – and semi-crystalline -
form. Upon heating (cooking), the
granules absorb water. The
process is called gelatinization,
and involves swelling
subsequently followed by bursting. By prolonged heating, the starch
macromolecules dissolve in water. For scientific purposes, starches can also
be dissolved in DMSO (dimethyl sulfoxide), or in alkali. The latter is a general
procedure, which can be employed for many non-water soluble
polysaccharides as long as they do not contain alkali labile linkages.
Cooling of swollen (or even dissolved) starch leads to thickening, as observed
by making a sauce. The macromolecules re-associate and expel water. This
is called retrogradation.
Dissolved starches can be fractionated into two classes of biopolymers,
namely
amylose
and
amylopectin
. Despite they both consist only one sugar -
α
-D-glucose – they have very different macromolecular architectures and
hence, very different properties.
The biosynthesis of amylose and amylopectin is a highly complex, but well
coordinated, process. It involves many genes and many enzymes, both by