Structure of starch granules from different sources.pdf

3d structure of starch granules found in various plants

1. Structure of starch
granules from
different sources
Assignment on Principles
of Food Science &
Nutrition
Submitted by-
SOUMIK DAS [A-2019-51-B]
Course no. PPT-352, Faculty-
Agriculture
6
th
term, 3
rd
year, UBKV

2. Structure of starch granules from
different sources
Introduction
Starch constitutes a major energy supply for
humans worldwide and is produced as a
reserve carbohydrate in plants. The most
important sources for humans are diverse
cereals, rhizomes, roots and tubers. Storage
starch is produced in amyloplasts as discrete
granules with distinct morphology in
different plants, ranging from round, oval,
ogival or elongated to flat, lenticular or
polyhedral, and sizes from sub-microns to
more than 100 µm in diameter. In some
cereals, notably wheat and barley, there are
two major populations of granules
distinguished by their size; the diameter cut-
off being approximately at 10 µm. In barley, the average diameter of the large granules (also called A-
granules) is between 15 and 19 µm depending on the variety, and the diameter of the small (B) granules is
between 3.1–3.7 µm. The starch granules consist almost entirely of two major polysaccharides, namely
amylose and amylopectin. Both consist of chains of α-(1, 4)-linked D-glucose residues, which are
interconnected through α-(1, 6)-glucosidic linkages, thus forming branches in the polymers. Although
amylose traditionally is considered to be linear, branched amylose molecules contains few branches , but
both branched and linear amylose have long chains with several hundreds or even thousand glucosyl units
, whereas amylopectin is extensively branched and have comparatively short chains . The branches in
amylopectin constitute about 5% of the molecule, which results in a very complex molecular structure.
Short chains of amylopectin form double-helices, which crystallize and contribute to the semi-crystalline
nature of the starch granules. In most “normal” starch granules, amylopectin constitutes the major
component by weight, whereas amylose constitutes 15–30%; however, many exceptions exist. Waxy
starches, so named because of the waxy appearance of the endosperm in waxy cereals, contain no or very
little amylose. Genetically modified starches may contain 50–80% amylose. A report presented recently
an engineered barley starch reaching almost 100% apparent amylose.
Minor non-carbohydrate compounds in starch granules contribute at most only a few per cent by weight.
Proteins in the granules are mostly related to the biosynthesis of the starch. Some proteins are only
loosely associated with the granules, whereas others are firmly bound inside the granules, notably
granule-bound starch synthase I (GBSSI), which is required for amylose synthesis. Lipids are rare in
many root and tuber starches, but more abundant in cereals in the form of free fatty acids or
lysophospholipids, which make inclusion complexes with amylose. In many root and tuber starches,
especially potato, the phosphate is not associated with lipids, however, but covalently bound to the

3. amylopectin component. In contrast, cereal and grass starches have only very little covalently bound
phosphate. Nevertheless, this phosphate is important as it is involved in starch turnover in the plant. In
addition to phosphorus, trace amounts of other elements are also found, of which potassium and
magnesium are comparatively abundant
Structure of Starch Granules
In view of the great diversity in starch granule morphology, it is remarkable to find that their internal
architectural features are shared universally among the plants and regardless the plant organ (endosperm,
root, stem, etc.). When observed in cross-polarized light in an optical microscope, a “Maltese cross” is
typically seen extending the arms from the so-called hilum, which is believed to be the origin of growth
of the granule. This birefringence pattern shows that the molecules, or a large part of the molecules, are
arranged in a radial fashion and suggests a high degree of order inside the granules.
 Crystallinity
As mentioned above, starch granules are semi-
crystalline, i.e., they contain both crystalline
and amorphous parts. If starch granules are
treated in dilute hydrochloric acid, or sulphuric
acid, the amorphous parts in the granules are
removed and the crystalline parts remain. The
“Maltese cross” also remains [48], which
shows that the organized molecular segments
are confined to the crystallites. The crystallites
are formed by short, external chain segments of
amylopectin with a degree of polymerization
(DP) approximately 10–20 glucosyl units. As
two chains unite into a double-helix with 6
glucose residues per turn of each strand and a
pitch of 2.1 nm , the length of these double-
helices is about 4~6 nm. Wide-angle X-ray scattering (WAXS, also known as X-ray diffraction, XRD)
has shown that the double-helices crystallizes into either of two polymorphs, the so-called A- or B-types.
Some plants, such as peas and many other legumes, possess granules with a mixed pattern assigned C-
type. In the A-type crystal, the double-helices are closely packed into a monoclinic unit cell (with
dimensions a = 20.83 Å, b = 11.45 Å, c = 10.58 Å, space group B2) containing 8 water molecules. In the
B-type crystal, the double-helices are packed in a hexagonal unit cell (dimensions a = b = 18.5 Å, c = 10.4
Å, space group P61) with 36 water molecules. In this crystalline lattice the water molecules fill up a
channel, which does not exist in the A-type. The relative crystallinity in starch granules greatly varies
between plant varieties in the range 17~50%, most often being higher in waxy starches compared to their
normal, amylose-containing counterparts. A starch granule is a spherocrystalline assembly of amylopectin
molecules oriented radially.
Crystallite Packing of Starches
Cairns et al., Carbohydr. Polym, 32 (1997) 275;

4. Buleon et al., Int. J. Biol. Macromol., 23 (1998)
85.
A-type starch
α (%)
Waxy maize
38-48
Normal maize
38-43
Rice
38-39
Wheat
36-39
B-type starch
Potato
25-40
High-amylose maize
25
Cassava/tapioca
24
C-type
Smooth pea (A: B = 56:44) 21
 Lamellar Structure
Small-angle X-ray scattering (SAXS) shows a 9–10 nm repeat distance in all starch granules. This was
suggested to stem from stacks of repeating crystalline and amorphous lamellae, of which the former are
represented by the double-helices. The crystalline lamellae have been isolated from acid-treated granules.
In transmission electron microscopy (TEM) images, waxy maize granules with A-type crystallinity show
regularly formed parallelepiped blocks having acute angles of 60~65◦ and with lengths and widths of 20–
40 and 15–30 nm, respectively . The dimensions suggest that a single nanocrystal contains between 150
and 300 double-helices. Amylose-containing starches give rise to nanocrystals with less perfect
symmetry, suggesting interference of amylose on the crystal structure. Nanocrystals isolated from B-type
starch granules (e.g., potato granules) possess irregular structures, probably a result of the organisation of
the double-helices in the B-crystallites. As the repeat distance is 9–10 nm and the crystalline lamellae fill
up 4–6 nm, the amorphous lamellae are 3–6 nm thick. This part consists of longer, internal chains of
amylopectin and probably amylose. Unexpectedly, the amorphous lamellae tend to be thicker in the
absence of amylose (i.e., in waxy starches) than in its presence, whereas the crystalline lamellae are
thinner. The molecular density in the crystalline lamellae is lower in potato starch (1.10 gcm−3) than in
waxy maize (1.22 gcm−3), probably due to the higher water content in the B-polymorph crystals, whereas
the molecular density of the amorphous lamellae was found to be fairly similar in potato and waxy maize
(0.52 and 0.46 gcm−3, respectively, the latter being A-crystalline). Interestingly, only minor differences
appeared to exist between waxy maize and normal maize, as well as between waxy maize and rice or
cassava starch, all being A-type crystalline starches.
 Granular Rings
The stacks of amorphous and crystalline lamellae form larger shells, or rings, being in the order of
100~400 nm thick. The rings are semi-crystalline in nature as they contain both the amorphous and the

5. crystalline lamellae [66], but they
have also been considered as
“crystalline” or “hard” shells. The
rings are embedded in an amorphous
matrix with a lower molecular density
in potato (0.49 gcm−3) than in waxy
maize (0.68 gcm−3) and other A-
crystalline starches. The matrix was
described as an “amorphous
background”, “semi-crystalline shell”,
or “soft shell”. The granular rings,
often named “growth rings”, are generally thinner at the periphery of the granules and thicker in their
interior parts. In cereals (but not in potato the semi-crystalline rings has been considered as layers formed
during photosynthesis in daylight, as old results had shown that the crystalline ring was formed only if the
plant (wheat or barley) was grown under constant light supply . Surprisingly, however, this could not be
confirmed in a recent investigation, wherein the rings remained in barley starch grown under constant
light. In fact, the relative crystallinity decreased somewhat, instead of increased, in granules from constant
light exposure. Around the granule’s hilum, the rings might be absent and replaced by a void (filled with
water, if the granule has not been dried). In some starch granules, especially in many cereals (e.g., maize,
wheat, barley and sorghum), channels penetrate the granules from the surface. Some of these channels
even connect the surface to the interior voids, if present. The channels are filled with proteins and also
phospholipids were found there. The channels are important sites of penetration of different enzymes,
such as α-amylases or glucoamylases, and diverse chemicals during modification of starch in vitro.
 BLOCKLETS
Scanning electron microscopy (SEM) and atomic force microscopy (AFM) have revealed protrusions,
named blocklets, on the surface of the granules
with sizes ranging approximately between
10~300 nm. The size appears to be partly
species specific. Potato granules have larger
blocklets (50~300 nm) [88] than wheat
granules (10~60 nm). Blocklets have also been
observed within the growth rings in the
interior parts of starch granules using diverse
microscopic techniques and appear to
transverse the entire rings. The exact nature of
the blocklets remains unclear, however.
Blocklets were clearly distinguished by AFM
in dry, acid-treated potato starch, but upon in
situ humidification blocklets in the semi-
crystalline rings collapsed and simultaneously
fused together, suggesting a flexible structure. It is known that the relative crystallinity in starch granules
as measured by WAXS increases with the water content. Moreover, the lamellar structure is also only
identified in humidified granules by SAXS. Thus, it appears that when blocklets fuse together, the

6. crystalline structure simultaneously emerges. Blocklets have been found in both semi-crystalline and
amorphous rings and Tang et al. Suggested the latter rings (or soft shells) consist of “defective”
blocklets, in contrast to “normal” blocklets in the former, hard shells. The dimensions of the blocklets
suggest that a single blocklet might represent a single, or a few, amylopectin molecules. Also amylose
might be a constituent of the blocklets and/or functioning as an interconnecting material outside the
blocklets. Park et al. Found that iodine-amylose complexes extend like “hairs” from blocklets on the
granules’ surface in potato and maize starches, and exclusively in maize also from gaps between the
blocklets. This confirmed the “hairy billiard ball” hypothesis earlier proposed by Line back. Zobel and
later Saibene et al. Concluded that amylose interacts with amylopectin in potato starch granules, but not
in maize. Possibly, this relates to how, or to which degree, amylose is involved in the blocklet structures.
Starch Granules from different sources


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Starch Granules at higher magnification on
polarized light

7. 1. Wheat:
Wheat (Triticum aestivum L.) is an important cereal crop and a staple food for humans and animals
worldwide. Starch is the major storage compound in wheat endosperm, accounting for 65–75% of the
final dry weight of a grain, and is synthesized in the amyloplast of endosperm cells since 4 days after
anthesis (DAA) and the endosperm structure no longer changes after 33 DAA. Starch is mainly composed
of linear amylose and highly branched amylopectin, which assemble to form a semicrystalline granule.
For linear amylose, the glucose units are joined through α-(1,4)-glycosidic linkages which are mainly
catalyzed and elongated by granule-bound starch synthase (GBSS). Amylopectin mainly consists of long
chains of α-(1,4)-linked D-glucopyranosyl units with occasional branching α-(1,6)-linkages that form
branched structure. The α-(1,6)-glycosidic linkages are catalyzed and elongated by starch branching
enzymes (SBE) and soluble starch synthase (SSS), respectively. Amylose content and amylopectin fine
structure greatly influence physicochemical properties that affect grain quality, flour quality, and starch
properties. Wheat starch granules are classified into three groups regarding their size; A-type , B- type
and C-type granules. The A-type granules have a lenticular shape and a diameter greater than 15 µm,
while the B-type (5-15 µm) and C-type (less than 5 µm) granules are spherical (Peng et al., 1999;Bechtel
et al., 1990) . The components that make up wheat starch granules are amylose and amylopectin, the
relative proportions of which differ between starches. Wheat starch normally contains 25-28% amylose
and 72-75% amylopectin.
2. Potato:
Potato starch granules consist primarily of two tightly packed polysaccharides, amylose and
amylopectin. Amylose, which amount for 20-30%, is the principal linear component, but a fraction is in
fact slightly branched. Amylopectin is typically the major component and is extensively branched,
containing short chains with an average length of 22-25 glucosyl residues. The branching pattern is not
well known, but branch point clustering guides chains to determine the overall starch granule architecture
and starch functionality. The clusters consist of 5-10 grouped short chains, which are interconnected by
long chains with more than 36 residues. The clusters consist of still smaller, very tightly branched
building blocks. The clusters direct the semicrystalline structures found inside the starch granules. The
crystals, which are ~5.2. nm thick, contain double helices formed from the external chains extending from
the clusters. A range of enzymes is involved in the biosynthesis of the cluster structures and linear
segments. These are required for sugar activation, chain elongation, branching, and trimming of the final
branching pattern. As an interesting feature, potato amylopectin is substituted with low amounts of
phosphate groups monoesterified to the C-3 and the C-6 carbons of the glucose units. They seem to align
well in the granular structure and have tremendous effects on starch degradation in the potato and
functionality of the refined starch. A specific dikinase catalyzes the phosphorylation of amylopectin.

3. Maize:
Normal maize starch was fractionated into two sizes:  large granules with diameters more than 5 μm and
small granules with diameters less than 5 μm. The large granules were surface gelatinized by treating
them with an aqueous LiCl solution (13 M) at 22−23 °C. Surface-gelatinized remaining granules were
obtained by mechanical blending, and gelatinized surface starch was obtained by grinding with a mortar

8. and a pestle. Starches of different granular sizes and radial locations, obtained after different degrees of
surface gelatinization, were subjected to scanning electron microscopy, iodine potentiometric titration,
gel-permeation chromatography, and amylopectin branch chain length analysis. Results showed that the
remaining granules had a rough surface with a lamella structure. Amylose was more concentrated at the
periphery than at the core of the granule. Amylopectin had longer long B-chains at the core than at the
periphery of the granule. Greater proportions of the long B-chains were present at the core than at the
periphery of the granule.

4. Barley:
Waxy barley starch granules composed of a mixture of large spherical, disc and lenticular shaped whereas
normal and waxy starches showed small irregularly shaped starch granules. A characteristic of cereal
starches, A-type crystallinity of barley starches were observed by X-ray diffractions. The relative
crystallinity (RC) of normal, high-amylose and waxy barley starches are in the range 20–36%, 37–42%
and 33–44%, respectively. The wide difference in RC among normal, waxy and high-amylose starches
can be attributed to varietal differences and/or to starch moisture content. In normal, waxy and high-
amylose maize starches, RC ranges are 17.6–28.0%, 17.2–21.8% and 25–42%, respectively. Large
granules of barley starch generally exhibit a higher RC than small granules (Figure 16.4). Irrespective of
amylose content, barley starches exhibit a V-amylose–lipid complex peak centered at
20Μ°2θ representing crystalline V-amylose–lipid complexes. However, this peak has been shown to be
absent in waxy maize starch.
5. Oats:
The starch granules in the endosperm of most grasses are compound i.e. they are composed of multiple,
separately-initiated granules that have grown together within the same plastid and partially fused. Oat
(Avena sativa) endosperm contains compound starch granules, but unusually for grasses, oat endosperm
cells also contain smaller, simple starch granules. The simple granules pack between the larger,
compound granules.

The Aveneae and the Triticeae tribes are closely related taxonomically and unlike other grasses, both have
two types of starch granule that differ in size and morphology in their endosperm cells. The small
granules in oats are simple (i.e. generated from a single initiation) and they appear from microscopy to be
quite similar in shape (roughly spherical) to the B-type starch granules seen in the endosperm of wheat
(Triticum aestivum) and other members of the Triticeae tribe. The large granules in oat endosperm,
however, are very different in morphology from the large A-type granules in Triticeae. In oats, the large
granules are ovoid in shape and compound (comprising many separately-initiated and partially-fused sub-
granules) whereas in Triticeae such as wheat, the large granules are disc-shaped and simple (comprising a
single granule). The compound granules in oats have sometimes been referred to as A-type granules and
the simple granules as B-type. However, due to the different morphologies of the large granules in wheat
and oats, here, we will refer to the two sorts of granules in Triticeae as A- and B-type but to those in oats
as ‘compound’ and ‘simple’.

9. 6. Cassava
Cassava starch granules are round with a truncated end and a well-defined hilum. The granule size is
between 5 and 35 μm. The starch has an A-type X-ray diffraction pattern, usually characteristic of cereals,
and not the B type found in other root and tuber starches. The C-type spectrum, intermediate between A
and B types, has also been reported. The nonglucosidic fraction of cassava starch is very low: the protein
and lipid content is below 0.2%. There is thus no formation of an amylose complex with lipids in native
starch. Amylose contents of 8–28% have been reported, but most values lie within the range of 16–18%.


Conclusion
Starches of diverse sources differ in their chemical composition and in structural characteristics of their
glucans and granules due to genetic, environmental and nutritional factors to which plants are exposed
during their development. In the inner part of the starch granules amylose, lipids, phosphorylated
residues, and long lateral chain amylopectin interact between them avoiding the water uptake. In contrast,

10. a high percentage of amylopectin, especially that with short lateral chains, allow hydration via hydrogen
bond more freely which produces gels susceptible to retrogradation when stored for a long time. Smaller
starch granules have a bigger superficial area that allows fast hydration; however, surface pores and
channels on the surface of starch granules enhance the water uptake. Thus, higher hydration rates increase
the swelling, viscosity, and gelatinization capacity of starch granules. However, the set of chemical
compounds found on the starch granule and their interactions are also involved in the thermal properties
of starches.



ACKNOWLEDGEMENT


I would like to convey my cordial gratitude to the teachers of our course PPT-352 for giving me such an
opportunity to do the assignment by which I am able to nourish my intellect as well as built an immense
sense on this assignment topic.


Thanking you, yours faithfully,
SD/-…………..
SOUMIK DAS