Agriculture Reference
In-Depth Information
milling was accomplished by the dough-washing
method. All 15 samples were mixed to a stiff
dough, but only two dough samples remained
cohesive during washing under a stream of water
to remove starch. During washing, most of the
dough balls lost their cohesiveness and became
soft and runny. The soft, runny consistency made
it impossible to remove the waxy wheat starch
from the dough mass. In a similar experiment,
fl our from six waxy hard wheats, the wild-type
HRW cultivar Karl 92, and the partial waxy HW
cultivar Trego harvested in 2006 was examined
by the dough-washing method (Guan et al.,
2007b). Among the six waxy wheat samples, four
provided starch recovery comparable to wild-type
hard wheat fl our. However, the waxy wheat fl our
dough was more sticky during the early dough-
washing stages.
Seven waxy wheat fl our samples and two wild-
type fl our samples also were processed by a fl our-
dispersion process (Sayaslan 2002; Sayaslan et al.,
2006). Except for one soft waxy fl our sample,
recovery of gluten protein (79%-87%) and their
purities (82%-88% protein) were comparable
to wild-type fl our in the dispersion process.
However, purifi cation of prime starch was some-
what more diffi cult and required more process-
ing. Even with added washing the prime waxy
wheat starch still contained 0.4%-0.5% protein
compared with 0.3% protein in normal wheat
starch.
Exact reasons for generally inferior wet-milling
properties of waxy wheat are not known. To date,
no published results report on protein structure
in waxy hard wheat versus wild-type hard wheat.
Some studies suggested elevated pentosan content
in waxy fl our might interfere with separation and
purifi cation of waxy wheat starch (Sayaslan 2002;
Sayaslan et al., 2006). Others (Bettge et al., 2000)
reported that waxy wheat starch granules are
easily damaged by physical forces, which also
might explain some of the diffi culty in purifying
waxy wheat starch granules. Guan et al. (2007b)
reported that when a small amount of hemicellu-
lase was added, waxy hard wheat fl our dough
became stronger, and during dough washing, the
dough became less sticky. More research is needed
to determine the factors that give good versus
poor fractionation and to develop improved wet-
milling processes to produce starch and vital
wheat gluten from hard waxy wheat.
Waxy wheat starch structure and properties
Waxy wheat starch granules give an A-type x-ray
diffraction pattern as do normal wheat starch
granules (Hayakawa et al., 1997; Fujita et al.,
1998). Waxy wheat starch has a higher degree of
crystallinity (37%-44%) than normal wheat
starch (29%-36%), presumably due to the higher
percentage of amylopectin in waxy wheat starch
(Fujita et al., 1998). The amylopectin fraction is
responsible for the crystalline phase in wild-type
and waxy starch (French 1984).
Thermal properties of waxy wheat starch,
including gelatinization and retrogradation, have
been determined by differential scanning calo-
rimetry (DSC) and compared with those of
normal wheat starch (Yasui et al., 1996;
Hayakawa et al., 1997; Fujita et al., 1998; Sasaki
et al., 2000; Yoo and Jane 2002). Gelatinization
peak temperatures are 2-5 ºC higher in waxy
wheat starch (Yasui et al., 1996; Fujita et al.,
1998; Hayakawa et al., 1997). Hayakawa et al.
(1997) reported that the onset gelatinization tem-
perature of waxy wheat starch was 1-3 ºC higher
than that of normal wheat starch, whereas Yasui
et al. (1996) did not detect differences in onset
and fi nal temperatures. The endothermic
enthalpy (Δ
H
) of gelatinization of waxy wheat
starch was higher than for normal wheat starch
(Yasui et al., 1996; Hayakawa et al., 1997; Sasaki
et al., 2000). However, when ΔH was based
on amylopectin content, little difference was
observed between waxy wheat starch and normal
wheat starch (Yasui et al., 1996; Hayakawa et al.,
1997). Gelatinization temperatures of waxy wheat
starch were about 9 ºC lower than that of waxy
maize starch, whereas the endothermic values
(ΔH) of waxy wheat and waxy maize starches
were identical (Hayakawa et al., 1997).
Because waxy wheat starch contains no or little
amylose, no peak due to the dissociation of the
lipid-amylose complex was observed by DSC
when waxy wheat starch was heated in water.
Starch lipid content of waxy starch, expressed as
