Agriculture Reference
In-Depth Information
has been recognized (Rahman et al., 2000). The
main chemical and physical starch characteristics
of direct importance to quality are the amylose
content, the granule-size distribution, the
temperature of gelatinization, the viscosity of
starch gels, and swelling power. Some of these
properties are linked to others; the two basic
properties are amylose content and granule size.
protein may be bound within the granules
(granule-bound proteins) or on the surface. The
granule-bound proteins are enzymes that have
been involved in starch synthesis, and have
become trapped in the matrix while taking part in
forming the granule. The surface-bound proteins
often include artifacts, captured there during the
isolation of starch from fl our or wheat. One of the
surface proteins has been linked with grain
softness (Greenwell and Schofi eld 1986), although
this “grain softness protein” or “friabilin” has
since been shown to be a mixture of several closely
related proteins known as puroindolines (Morris
et al., 1994).
One of the granule-bound proteins, granule-
bound starch synthase (GBSS), is associated with
amylose synthesis in wheat. The enzyme is
encoded by loci on chromosomes 7A, 7D, and 4A
(Nakamura et al., 1995). The locus on chromosome
4A, known as
Wx-B1
, was translocated naturally
from chromosome 7B during the evolution of
primitive wheats (Table 21.2). The presence of
the null allele at this locus has been linked to high
noodle quality, especially for Japanese udon
noodles (Zhao et al., 1998). The presence of all
three GBSS enzymes encoded at their respective
loci introduces a degree of redundancy in the
synthesis of amylose, which usually comprises
about 25%-30% of the starch. Removal of one of
the enzymes encoded by any of the loci (to give
what is known as a single null) results in starch
containing less amylose, usually from 18% to
25%. Removal of a second enzyme reduces the
amylose content further to about 12%-18%.
Wheat with this combination is known as a
double-null genotype, and both single- and
double-null genotypes are also referred to as
partial waxy. When all three forms of the enzyme
are absent, the amylose content is reduced to
almost zero in what is known as waxy wheat
(Nakamura et al., 1995; Kiribuchi-Otobe et al.,
1997). Other granule-bound proteins have been
associated with branching and debranching of
glucose chains during the biosynthesis of
amylopectin (Rahman et al., 2000).
The size distribution of wheat starch granules
is also important for processing quality. The
distribution is bimodal, with a minimum in the
Amylose content
The two chemical components of starch are
amylose and amylopectin. Amylose is an essentially
linear chain of glucose units linked by α-1,4-
bonds between the reducing group of carbon-1
and the hydroxyl group of C-4 of the next glucose
molecule. This reducing group in glucose itself
normally exists in a cyclic structure with a hemi-
acetal formed between the hydroxyl of C-5 and
C-1. At the reducing end of the chain, C-1 is not
bound to other glucose units. There are thousands
of glucose units in an amylose chain, giving it a
molecular weight of several hundred thousand
Daltons. Amylopectin also consists of glucose
units joined together in the same way as amylose.
However, there are also occasional side branches
produced when a bond is formed between the
hydroxyl on C-6 and C-1 of another glucose
molecule or chain. On these chains, additional
branches may form, giving a highly branched
structure with a molecular weight over one million
Daltons, although most of the individual side
chains contain less than thirty glucose units. Like
amylose, amylopectin contains only a single
reducing group in each molecule. The chain that
contains the reducing group is known as the C-
chain. Bound to it are B-chains and A-chains. A-
chains have no further branches, while B-chains
have one or more branches along their length.
Starch granules
In its native form, starch is deposited in granular
form, ranging in size from <1 μm to about 40 μm
(Briarty et al., 1979). These granules contain
not only amylose and amylopectin, but also
small amounts of protein, lipid, and nonstarch
polysaccharides (Rahman et al., 2000). The
