Agriculture Reference
In-Depth Information
Video-recorded images revealed that waxy wheat
starch granules swelled greatly at 60-70 ºC and
disintegrated into many small fragments at 70-
80 ºC. No granular residues remained at 90 ºC. In
contrast, normal wheat starch granules started
increasing in size at 57 ºC, continued to swell
above that temperature, and retained a rounded
shape even at 90 ºC. The easy fragmentation of
waxy wheat granules explains the large break-
down in viscosity, greater susceptibility to α-
amylase degradation, and low falling numbers of
waxy wheat fl our.
Limited work has been done on chemical mod-
ifi cations of waxy wheat starch (Reddy and Seib
1999; Bertolini et al., 2003; Hansen et al., 2007).
Cross-linked waxy wheat starch showed higher
thickening power compared with cross-linked
waxy maize starch (Reddy and Seib 1999). Fur-
thermore, the same modifi cation produced better
freeze-thaw stability in waxy wheat starch than in
waxy maize starch. Bertolini et al. (2003) reported
that waxy wheat starch generally exhibited higher
reactivity with a cross-linking reagent (phospho-
rus oxychloride) than normal or partial waxy
wheat starch, but with the substitution reagent
propylene oxide, no differences in reactivity were
observed among wheat starch types differing in
amylose content. Higher levels of phosphoryl
chloride were required to effect changes in pasting
behavior of waxy maize starch compared with
waxy wheat starch (Reddy and Seib 2000). Waxy
wheat starch cross-linked at a low level gave a
higher pasting consistency compared with cross-
linked waxy maize starch. Cross-linked and
hydroxypropylated or acetylated waxy wheat
starch had similar consistency compared with
similarly modifi ed waxy maize starch, but the
modifi ed waxy wheat starch had lower gelatiniza-
tion temperature and better freeze-thaw stability
(Reddy and Seib 2000).
Partial waxy wheat has been shown to provide
superior quality in certain Asian (wet) noodle
applications (Epstein et al., 2002). Blends of waxy
and wild-type wheat can give superior quality to
Asian noodles and fresh fl our tortillas (Guo et al.,
2003a,b) and can be used to retard staling of baked
products (Bhattacharya et al., 2002). Partial waxy
wheat starch also may provide a better substrate
than wild-type wheat in the production of modi-
fi ed food starch (Reddy and Seib 2000).
High-amylose wheat
High-amylose wheat and its potential applications
are discussed in two reviews by Hung et al. (2006)
and Regina et al. (2007). The fi rst high-amylose
wheat mutants had an apparent amylose content
of 30.8 to 37.4% (proportion of total starch),
as determined by colorimetric measurement,
amperometric titration, and concanavalin A
methods (Yamamori et al., 2000). Amylopectin
structure in the amylose mutants was altered.
Levels of chains with DP 6-10 increased, while
chains with DP 11-25 decreased.
Starch granules from high-amylose wheat
mutants have a number of unique features: (i)
A-type starch granules are deformed, (ii) the x-
ray diffraction pattern does not show any major
peaks, and (iii) the DSC thermogram is broad
and does not have a peak due to the melting of
the amylose-lipid complex (Yamamori et al.,
2000). Starch granule morphology was also
altered in high-amylose wheat endosperm (>70%
amylose) produced by RNA interference (Regina
et al., 2006). Under polarized light more than
90% of the starch granules were not birefringent.
The majority of high-amylose potato starch gran-
ules also show less birefringence under polarized
light, and many have irregular surfaces with deep
fi ssures in the center of the granules (Schwall
et al., 2000). In contrast, high-amylose maize
starch granules (>90%) typically show birefrin-
gence in the form of a typical Maltese cross under
polarized light (Shi and Jeffcoat 2001). There are
some elongated, tubular, or rodlike starch gran-
ules in high-amylose maize starch, and those
granules do not give a Maltese cross under polar-
ized light.
Dough properties and baking qualities of the
high-amylose
sgp-1
wheat mutant were compared
to that of both waxy and wild types (Morita et al.,
2002; Hung et al., 2005). Bread baked from high-
amylose fl our had signifi cantly lower loaf volume
than bread from wild-type or waxy fl our, and
