Agriculture Reference
In-Depth Information
with elevated levels of amylose (Neuffer et al.,
1997). The
ae
locus encodes a starch branching
enzyme (SBEII). When this gene is nonfunc-
tional, not only is starch amylose elevated, but the
branching pattern of the remaining amylopectin
is altered (Neuffer et al., 1997). Yamamori et al.
(2000) were the fi rst to report high-amylose
mutants in wheat. Wheat grain lacking starch
granule protein-1 (SGP-1) is reported to have an
apparent amylose content of approximately 35%
elevated above the typical 25% amylose content
of wild-type wheat (Yamamori 2005). Li et al.
(1999) reported SGP-1 to be a soluble starch syn-
thase homologous to SSIIa of maize, which plays
a role in synthesis of maize amylopectin (Zhang
et al., 2004).
An alternate approach was used to develop
wheat with an amylose content of approximately
70%. Regina et al. (2006) used RNA interference
to down-regulate wheat starch-branching enzymes
SBEIIa and SBEIIb. Suppression of only SBEIIb
did not affect amylose content, but simultaneous
suppression of both forms resulted in starch with
70% amylose.
et al., 2006). Protein content of waxy fl our can
vary from 9.4% to 14.2% (Sayaslan et al., 2006).
Sayaslan et al. (2006) reported that waxy wheat
fl our had about 3%-7% less total starch than
normal or wild-type wheat fl our, though in some
cases waxy fl our might be equivalent to normal
wheat fl our (Guan et al., 2007b).
Dry-milling of various waxy hard, soft, and
spring wheats gave 3%-20% lower fl our yields
compared with the wild-type wheat (Yasui et al.,
1999; Graybosch et al., 2003; Kim et al., 2003;
Chibbar and Chakraborty 2005; Sayaslan et al.,
2006). In some cases lower test weight accounted
for lower fl our yield; in others, the low fl our yield
could not be fully explained. Low fl our yield does
not seem to be related to gluten strength but has
been attributed to elevated lipid and β-glucan
content in waxy wheat (Yasui et al., 1999). These
factors could cause poor bolting of fl our (Neel and
Hoseney 1984) or fl aking of middling fractions
during grinding on smooth rolls, especially for
soft waxy wheat. Waxy wheat also produces
higher levels of damaged starch during dry-
milling (Bettge et al., 2000; Kim et al., 2003).
Graybosch et al. (2003) suggested that waxy wheat
endosperm might have higher crystallinity due to
lack of amylose, and that could affect dry-milling
of waxy wheat. The same fl our yield can be
obtained from waxy hard wheat as from normal
hard wheat by roller-milling on a Buhler experi-
mental mill, but the feed rate of waxy wheat
kernels must be reduced or the waxy fl our would
clog the system during dry-milling (S. Garimella,
pers. comm.). To realize the full potential of waxy
wheat, fl our yield needs to improve. Further
research is needed to investigate the cause of
reduced fl our yield and how to improve dry-
milling of waxy wheat.
The wet-milling process can produce two valu-
able products from waxy wheat fl our: vital wheat
gluten and waxy wheat starch. However, waxy
hard wheat fl our is not always suited for the wet-
milling process, which fractionates the fl our into
vital wheat gluten, prime starch, tailings (B-grade
starch), and water-soluble components. From
1999 to 2002, researchers at Kansas State Univer-
sity examined the wet-milling of 15 waxy wheat
lines (Sayaslan 2002; Sayaslan et al., 2006). Wet-
Sweet wheat
Nakamura et al. (2006) combined the waxy mutant
(lacking functional GBSS) with the
sgp-1
mutant
(lacking soluble starch synthase SSIIa) to form a
double-mutant type. Surprisingly, grains of this
line had elevated levels of maltose and sucrose but
also displayed shrunken kernels. Kernels were
reported to have a sweet taste, similar to that of
sweet corn. Flour from this line also had elevated
sugar content, and incorporating it into baked
goods resulted in sweeter-tasting products.
Altered starch characteristics
Unique waxy wheat fl our properties
Waxy wheat starch consists mainly of amylopec-
tin (a branched glucose polymer). In addition,
waxy wheat kernels contain approximately 20%
more lipids, 35% more arabinoxylan (pentosan),
and 30% more β-glucan in their endosperm than
wild-type wheat (Yasui et al., 1999; Sayaslan
