Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
The branch-chain lengths of phytoglycogen display a single-modal chain-length
distribution, which is different from the bimodal distribution of amylopectin.
7.3 ORGANIZATION OF STARCH GRANULES
Starch is synthesized in granules inside amyloplasts. Starch granules isolated from
different botanical sources and different organs display different shapes and sizes. 55
Diameters of starch granules vary from submicrons to more than 100 µm. Examples
of diameters of starch granules are potato starch 15 to 75 µm, wheat A-granules 18
to 33 µm and B-granules 2 to 5 µm, maize starch 5 to 20 µm, rice starch 3 to 8 µm,
and amaranth starch 0.5 to 2 µm. 55 Starch granules display spherical, oval, disk,
polygonal, elongated, kidney, and lobe shapes. Leaf starch has flat-shaped small
granules that are submicron in diameter. 4 Wheat, barley, rye, and triticale starches
display bimodal granule size distributions: the disk-shaped, large A-granules and
the spherical, small B-granules. 55 Rice and oats starches are known as compound
starches, which are defined as multiple granules synthesized within a single amyloplast.
55 Thus, starch granules are tightly packed together and develop into polygonal
irregular shapes.
When viewed under a polarized light-microscope, starch granules display birefringence,
known as the Maltese cross. The Maltese cross birefringence reflects the
radial arrangement of starch molecules in the starch granule. The center of the
Maltese cross, the hilum, is the organic center of the granule where biosynthesis of
the starch granule initiates. The hilum is not necessarily at the geometric center of
the granule; some can be located close to the end of the granule, reflecting the
initiating site of the starch biosynthesis.
Many starch varieties, particularly those having the A-type x-ray pattern and
short average branch-chain length (e.g., sorghum and maize), display pinholes on
the surface of starch granules. The pinholes are open ends of serpentine-like channels
penetrating into the granule 56 and are attributed to enzyme hydrolysis of starch
granules. Scanning electron micrographs of cracked corn kernels shows enzymedegraded
starch granules located around germs, indicating that enzyme hydrolysis
of starch initiates at the location surrounding the germ to generate energy for
germination. 57
Surface gelatinization of starch granules by saturated neutral salt solutions, such
as LiCl and CaCl 2, enables separation of starch molecules by layers from the
periphery of the granule. Therefore, one can investigate structures of starch molecules
at different radial locations of a granule. 58,59 The amylose content is higher at
the periphery of the granule than at the hilum. This difference in the amylose content
is consistent with a known fact that amylose content of starch increases with size
of starch granules. The same studies also reveal that amylopectin molecules at the
hilum have longer branch chains than do those at the periphery. 58,59
Structures of amylopectin molecules isolated from disk-shaped, large A-granules
and from spherical, small B-granules of barley, wheat, and other starches have been
analyzed. Large disk-shaped granules consist of amylopectin that has fewer short
branch chains than does that of small spherical granules. 57 Differences in branch
structures determine the shape of amylopectin molecules, which, in turn, governs