Chemistry of Cereal Grains - Sourdough Biotechnology - page 14

chains, which are interconnected via a-(1,6)-glycosidic linkages, also called branch
points [ 13 ] . The a-(1,4)-linked chains have variable length of 6 to more than 100
glucose units depending on the molecular site at which they are located. The
unbranched A- or outer chains can be distinguished from the branched B- or inner
chains, which can be subdivided into B1-, B2-, B3-, and B4-chains [ 14 ] . The molecules
are “terminated” by a single C-chain containing the reducing glucose residue [ 15 ] .
Amylopectin has a tree-like structure, in which clusters of chains occur at regular
intervals along the axis of the molecule [ 16 ]. Short A- and B1-chains of 12-15 glu-
cose residues form the clusters which have double-helical structures. The longer,
less abundant B2-, B3-, and B4-chains interconnect 2, 3 or 4 clusters, respectively.
B2-chains contain approximately 35-40, B3-chains 70-80, and B4-chains up to
more than 100 glucose residues [ 12, 17 ] .

2.2.1.2 Starch Granules

In the endosperm starch is present as intracellular granules of different sizes and
shapes, depending on the cereal species. In contrast to most plant starches, wheat,
rye, and barley starches usually have two granule populations differing in size.
Small spherical B-granules with an average size of 5 mm can be distinguished
from large ellipsoid A-granules with mean diameters around 20 m m [ 18 ] . In the
polarization microscope native starch granules are birefringent indicating that
ordered, partially crystalline structures are present in the granule. The degree of
crystallinity ranges from 20 to 40% [ 19 ] and is primarily caused by the structural
features of amylopectin. It is thought that the macromolecules are oriented per-
pendicularly to the granule surface [ 12, 16 ] with the nonreducing ends of the
molecules pointing to the surface.
A model of starch granule organization from the microscopic to the nanoscopic
level has been suggested [ 12 ]. At the microscopic level alternating concentric
“growth rings” with periodicities of several hundreds of nanometers can be observed.
They reflect alternating semicrystalline and amorphous shells [ 12 ] . The latter are
less dense, enriched in amylose, and contain noncrystalline amylopectin. They fur-
ther consist of alternating amorphous and crystalline lamellae of about 9-10 nm [ 20 ] .
Crystalline regions contain amylopectin double helices of A- and B1-chains ori-
ented in parallel fashion and possibly 18 nm-wide, left-handed superhelices formed
from double helices. Amorphous regions represent the amylopectin branching sites,
which may also contain a few amylose molecules. The lamellae are organized into
larger spherical blocklets, which vary periodically in diameter between 20 and
500 nm [ 21 ]. The amylopectin double helices may be packed into different crystal
types. The very densely packed A-type is found in most cereal starches, while the
more hydrated tube-like B-type is found in some tuber starches, high amylose cereal
starches, and retrograded starch [ 12, 19 ]. Mixtures of A- and B-types are designated
C-type.