Agriculture Reference
In-Depth Information
makes it clear that dough weakening is inevitable
due to the action of proteases deposited in the
developing grain by
Eurygaster
insects. As a result,
grain that is severely damaged by
Eurygaster
maura
is unsuitable for breadmaking purposes,
and is thus downgraded at harvest (Sivri et al.,
1999, 2004). If, in this case, dough quality has
been lost due to a major reduction in the molecu-
lar-weight distribution of the gluten proteins, it
should be possible to restore dough strength by
re-forming cross-links between the glutenin poly-
mers. This restoration has been demonstrated
with the use of transglutaminase as a means of
producing new cross-links between glutenin
chains (Koksel et al., 2001).
Dough strength may also be reduced by the
environmental infl uence of heat stress during
grain fi lling (Table 21.6), due to interference with
the process of disulfi de-bond formation after
polypeptide synthesis, thereby reducing the pro-
portion of large glutenin polymers (Blumenthal et
al., 1995). The mechanism of this defect is illus-
trated (Fig. 21.2) by the shift toward a lower
molecular-weight distribution in the FFF profi le
of proteins from grain samples of plants that had
been heat-stressed during grain fi lling (Daqiq
2002).
These examples illustrate the importance of
disulfi de bonds between the polymeric glutenins
(Wrigley and Bekes 1999). The process of disul-
fi de-bond formation is understood to continue
slowly after the initial synthesis of the glutenin
proteins, even continuing during grain storage if
under warm conditions (e.g., at over 30 ºC for a
few months) when fl our milled from the grain is
found to have increased dough strength (Wrigley
and Bekes 1999). Lack of awareness of this phe-
nomenon has caused diffi culties in the past for
grain handlers and millers.
LIPID COMPOSITION AND
WHEAT QUALITY
In contrast to protein and starch, the lipid frac-
tion is a minor component of the wheat grain,
constituting about 3%-4% of whole grain weight,
and even less (1%-2.5% by weight) in the endo-
sperm (straight-run milled fl our). When protein
is removed from wheat fl our, dough and bread-
making properties are completely lost. In con-
trast, dough properties and breadmaking capacity
are largely retained when the lipids are removed
from the fl our. However, fl our lipids do play an
important role in the dough-mixing and baking
processes. They interact and form complexes
with gluten protein and contribute to the
stabilization of gas-cell structure, thus having
signifi cant effects on loaf volume and on fi nal
texture.
Lipid composition and distribution
Wheat lipids are a complex mixture of compo-
nents. They are unevenly distributed throughout
the various parts of the wheat kernel. One-third
of the total lipid fraction is located in the germ,
which accounts for only about 4% of the total
grain by weight; therefore, the germ has the
highest lipid content (Fig. 21.3).
More than 20 classes of lipids exist in wheat,
each containing numbers of separate entities
(Morrison 1978; Hargin and Morrison 1980).
They can be divided into two groups, the nonpo-
lar and polar lipids. Triglycerides are the major
nonpolar lipids, representing about 50% of total
nonpolar lipids in wheat. They are deposited in
spherosomes (oil droplets) bounded by a mono-
layer membrane, and this is the form in which
plants usually store lipids. The remainder of the
Table 21.6
Loss of dough strength (as R
max
in Brabender units) for grain harvested with or without three days of heat stress
at two sites in northern NSW, Australia.
Variety
(
HMW
Alleles)
a
No Heat Stress
Heat Stressed
Change in Dough Strength Due to Heat
Sunco (
aua
)
550
205
63% loss
Sunstate (
aid
)
660
380
42% loss
a
Glu-1
alleles are indicated in order for the A, B, and D genomes.
