Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
aqueous medium, to reach a low energy state. The two chains in the double helix
can be arranged in either parallel or antiparallel orientation. 2,3,29 This recrystallization
or aging process is known as retrogradation, which is responsible for bread staling
and syneresis of gravy. Double helices of amylose chains are packed in a hexagonal
unit that consists of six double helices and an empty channel at the center of the
packing unit and has ca. 36 water molecules in each unit. This type of unit packing
displays a B-type x-ray pattern. 2,3,30,31 Another type polymorph, the A-type x-ray
pattern, is not found in retrograded native amylose.
In the presence of many chemicals that consist of a hydrophobic moiety, such
as alcohols and fatty acids (known as complexing agents), amylose molecules can
form helical complexes with these chemicals by having the chemicals located inside
of the hydrophobic cavity of the helix as inclusions. 32 Depending on the size of the
cross-section of the complexing agent, amylose can form single helices of different
sizes to accommodate the diameters of the complexing agents. It has been reported
that sizes of amylose single helices of six, seven, and eight glucose units per turn
are found when complexed with n-butyl alcohol (linear molecule), isobutyl alcohol
(branched molecule), and α-naphthol (bulky molecule), respectively, by using electron
microscopy and x-ray diffraction 33 and by enzymatic method. 34 X-ray diffraction
of amylose single-helical complexes gives V-type patterns. 30
7.2.2 AMYLOPECTIN
Amylopectin is a highly branched molecule, consisting of α 1-4 linked D-glucopyranose
chains that are connected by α 1-6 branch linkages. Amylopectin has a very
high molecular weight, varying between hundreds of thousands to tens of millions
anhydroglucose units (Table 7.1). 35 Figure 7.1 shows a plot of the relationship
between molecular weight and gyration radius of the amylopectin molecule. These
analytical results show that molecular weights of waxy starch amylopectins are
higher than those of normal starch counterparts.
Branch chains of amylopectin molecules can be categorized to the A chains that
are chains whose reducing ends attach to other B or C chains but do not carry any
other chains; the B chains have reducing ends attached to other B or C chains and
also carry other A or B chains; the C chain is the only chain of the molecule carrying
a reducing end. The A chains are generally short and extend within one cluster. The
B chains have different chain lengths, B 1 chains have lengths that extend within one
cluster, B 2 chains extend through two clusters, B 3 chains through three clusters, and
so on. 36 The bimodal branch-chain length distribution of amylopectin differs from
the single modal distribution of glycogen 37 and phytoglycogen. 38 Branch chain
lengths of amylopectin vary with botanical sources of the starch, which control the
polymorphic forms of crystalline structure, 39 gelatinization, pasting, and retrogradation
properties of the starch. 40
Branch chains of amylopectin are arranged in clusters. 1 After native granular
waxy maize starch was hydrolyzed by acid at room temperature, Yamaguchi et al., 41
using transmission electron microscopy, observed worm-like cluster structures that
resisted acid hydrolysis. The worm-like cluster is attributed to the crystalline structure
of outer chains of amylopectin molecules. The amorphous structure of the waxy