Sources for starch
Sources for starch
Starch is the major carbohydrate reserve in plant tubers and seed
endosperm where it is found as granules [330, 1758] each typically containing several million amylopectin molecules
accompanied by a much larger number of smaller amylose molecules.
By far the largest source of starch is corn (maize) with other commonly
used sources being wheat, potato, tapioca and rice. Amylopectin
(without amylose) can be isolated from 'waxy' maize starch whereas
amylose (without amylopectin) is best isolated after specifically
hydrolyzing the amylopectin with pullulanase .
Genetic modification of starch crops has recently led to the development
of starches with improved and targeted functionality .
Starch consists of two types of molecules, amylose
(normally 20-30%) and amylopectin (normally 70-80%). Both consist
of polymers of α-D-glucose units in the 4C1 conformation. In amylose these are linked -(14)-,
with the ring oxygen atoms all on the same side, whereas in amylopectin
about one residue in every twenty or so is also linked -(16)-
forming branch-points. The relative proportions of amylose to amylopectin
and -(16)- branch-points both depend
on the source of the starch, for example, amylomaizes contain over
50% amylose whereas 'waxy' maize has almost none (~3%) .
structure of amylose
structure of amylopectin
[Back to Top ]
Amylose and amylopectin are inherently incompatible
molecules; amylose having lower molecular weight with a relatively
extended shape whereas amylopectin has huge but compact molecules.
Determination of the molecular weight distribution of starch molecules presents several problems . The presence of amylose tends to reduce the crystallinity of the amylopectin and influence the ease of water penetration into the granules. Most of their structure consists of α-(14)-D-glucose
units. Although the α-(14)
links are capable of relatively free rotation around the (φ) phi and (ψ) psi torsions, hydrogen bonding between
the O3' and O2 oxygen atoms of sequential residues tends to encourage
a helical conformation. These helical structures are relatively
stiff and may present contiguous hydrophobic surfaces.
Amylose molecules consist of single mostly-unbranched chains
with 500-20,000 α-(14)-D-glucose
units dependent on source (a very few α-16
branches and linked phosphate groups may be found ,
but these have little influence on the molecule's behavior ).
Amylose can form an extended shape (hydrodynamic radius 7-22
nm ) but generally
tends to wind up into a rather stiff left-handed single helix
or form even stiffer parallel left-handed double helical junction
zones (Jmol, 39 KB, ).
Single helical amylose has hydrogen-bonding O2 and O6 atoms
on outside surface of the helix with only the ring oxygen pointing
inwards. Hydrogen bonding between aligned chains causes retrogradation
and releases some of the bound water (syneresis). The aligned
chains may then form double stranded crystallites that are resistant
to amylases. These possess extensive inter- and intra-strand
hydrogen bonding, resulting in a fairly hydrophobic structure
of low solubility. The amylose content of starches is thus the
major cause of resistant starch formation (RS3, see below).
Single helix amylose behaves similarly to the cyclodextrins by possessing a relatively hydrophobic inner surface that holds
a spiral of water molecules, which are relatively easily lost
to be replaced by hydrophobic lipid or aroma molecules. It is
also responsible for the characteristic binding of amylose to
chains of charged iodine molecules (for example, the polyiodides;
chains of I3- and I5- forming structures such as I93- and I153-; note that neutral I2 molecules may give polyiodides in aqueous solution and there
is no interaction with I2 molecules except under
strictly anhydrous conditions) where each turn of the helix
holds about two iodine atoms and a blue color is produced due
to donor-acceptor interaction between water and the electron
deficient polyiodides. [Back to Top ]
Amylopectin is formed by non-random α-16
branching of the amylose-type α-(14)-D-glucose
structure. This branching is determined by branching enzymes
that leave each chain with up to 30 glucose residues. Each
amylopectin molecule contains a million or so residues, about
5% of which form the branch points. There are usually slightly
more 'outer' unbranched chains (called A-chains) than 'inner'
branched chains (called B-chains). There is only one chain
(called the C-chain) containing the single reducing group.
A-chains generally consist of between 13-23 residues . There are two main fractions of long and short internal B-chains with the longer chains (greater than about 23-35 residues) connecting between clusters and the shorter chains similar in length to the terminal A-chains .
Each amylopectin molecule contains up to two million glucose residues
in a compact structure with hydrodynamic radius 21-75 nm  (waxy maize amylopectin >300 nm ).
The molecules are oriented radially in the starch granule and as
the radius increases so does the number of branches required to
fill up the space, with the consequent formation of concentric regions
of alternating amorphous and crystalline structure. In the diagram
below: A - shows the essential features of amylopectin. B - shows
the organization of the amorphous and crystalline regions (or domains)
of the structure generating the concentric layers that contribute
to the “growth rings“ that are visible by light microscopy.
C - shows the orientation of the amylopectin molecules in a cross
section of an idealized entire granule. D - shows the likely double
helix structure taken up by neighboring chains and giving rise to
the extensive degree of crystallinity in granule. There is some
debate over the form of the crystalline structure but it appears
most likely that it consists of parallel left-handed helices with
six residues per turn. An alternative arrangement of interconnecting clusters has been described for some amylopectins .
Some amylopectin (for example, from potato) has phosphate groups attached to some hydroxyl
groups, which increase its hydrophilicity and swelling
double-helical chains can either form the more open hydrated
Type B hexagonal crystallites or the
denser Type A crystallites, with staggered
monoclinic packing, dependent on the plant source of the
granules . Type A,
with unbroken chain lengths of about 23-29 glucose units
is found in most cereals.
, with slightly longer unbroken chain lengths
of about 30-44 glucose units is found in banana, some tubers
such as potato and high amylose cereal starches. There is also
a type C
structure, which is a combination
of types A and B and found in peas and beans. Starch granule architecture has been recently described [1008
]. [Back to Top
Starch is a versatile and cheap, and has many uses as thickener,
water binder, emulsion stabilizer and gelling agent. Its form and functionality have recently been reviewed . Starch is often
used as an inherent natural ingredient but it is also added for
its functionality. It is naturally found tightly and radially packed
into dehydrated granules (about one water per glucose) with origin-specific
shape and size (maize, 2-30 μm; wheat,
1-45 µm; potato, 5-100 μm
). The size distribution determines
its swelling functionality with granules being generally either
larger and lenticular (lens-like, A-starch) or
smaller and spherical (B-starch)  with less swelling
powera. Granules contain 'blocklets'
of amylopectin containing both crystalline (~30%) and amorphous
areas. As they absorb water, they swell, lose crystallinity and
leach amylose. The higher the amylose content, the lower is the
swelling power and the smaller is the gel strength for the same
starch concentration. To a certain extent, however, a smaller swelling
power due to high amylose content can be counteracted by a larger
granule size . Although the properties
of starch are naturally inconsistent, being dependent on the vagaries
of agriculture, there are several suppliers of consistently uniform
starches as functional
Of the two components of starch, amylose has the most
useful functions as a hydrocolloid. Its extended conformation causes
the high viscosity of water-soluble starch and varies relatively
little with temperature. The extended loosely helical chains possess
a relatively hydrophobic inner surface that is not able to hold
water well and more hydrophobic molecules such as lipids and aroma
compounds can easily replace this. Amylose forms useful gels and
films. Its association and crystallization (retrogradation) on cooling
and storage decreases storage stability causing shrinkage and the
release of water (syneresis). Increasing amylose concentration decreases
gel stickiness but increases gel firmness. Retrogradation is affected by lipid content, amylose/amylopectin ratio, chain length of amylose and amylopectin, and solid concentration . Amylopectin interferes
with the interaction between amylose chains (and retrogradation)
and its solution can lead to an initial loss in viscosity and followed
by a more slimy consistency. Mixing with κ-carrageenan, alginate, xanthan
gum and low molecular weight sugars can also reduce retrogradation.
At high concentrations, starch gels are both pseudoplastic and thixotropic with greater storage
stability. Their water binding ability (high but relatively weak)
can provide body and texture to foodstuffs and is encouraging its
use as a fat replacement.
A significant proportion of starch in the normal
diet escapes degradation in the stomach and small intestine
and is labeled 'resistant starch' (for recent reviews see ),
but this portion is difficult to measure (see  for methods) and depends on a number
of factors including the form of starch and the method of cooking
prior to consumption. Nevertheless resistant starch serves as
a primary source of substrate for colonic microflora, and may
have several important physiological roles (see hydrocolloids
and health). Resistant starch has been categorized as physically
inaccessible (RS1), (raw) ungelatinized starch (for example, in banana; RS2 b ), thermally stable retrograded starch
(for example, as found in bread, especially stale bread, mainly
amylose; RS3) and chemically modified starch (RS4).
Resistant starch should be considered a dietary fiber. Although
not exactly quantifiable due to its heterogeneous nature, some
is determined by the official Association of Official Agricultural
Chemists (AOAC) method. Starch with structure intermediate between the more crystalline resistant starch (for example, RS3 in staled bread) and more amorphous rapidly digestible starch (for example, in boiled potato) is slowly digestible starch  (for example, in boiled millet). Slowly digestible starch gives reduced postprandial blood glucose peaks and is therefore useful in the diabetic diet.
Many functional derivatives of starch are marketed including cross-linked,
oxidized, acetylated, hydroxypropylated and partially hydrolyzed
material. For example, partially hydrolyzed (that is, about
two bonds hydrolyzed out of eleven) starch (dextrin ) is used in
sauces to control viscosity.
Interactive structures are available (Jmol). [Back to Top ]
a Swelling power is determined
after heating the starch in excess water and is the ratio of the
wet weight of the (sedimented) gel formed to its dry weight. It
will depend on the processing conditions (temperature, time, stirring,
centrifugation) and may be thought of as its water binding capacity.
b The amount of resistant starch is highest in unripe green bananas (~15%) and drops during ripening to much lower values as the starch is converted to glucose.