Passing the Test on Wheat End-Use Quality - Wheat Science and Trade - page 457

other types of empirical rheometers quite diffi -
cult, and it is also no easy task to derive funda-
mental rheological data.
That said there are many disadvantages to
standard rheometers for characterizing starch
or starchy fl our. Starch granules are dense
(~1.5 g cm −3 ) and thus sink like stones if not kept
suspended in a turbulent fl ow. Hence clumping
and settling out of granules before gelatinization
can be problematic in concentric cylinder geom-
etries. However, strategies such as gelatinizing
part of the sample can be used to prevent sedi-
mentation of granules (see Lagarrigue and Alvarez
2001).
Basic descriptors from a RVA pasting curve
(Batey 2007; Batey and Bason 2007) are shown in
Fig. 20.3. These are mostly consistent with
descriptors for VAG curves described by Rasper
(1980) and are largely valid for both fl our and
starch analyses. The fi rst parameter that can be
observed is pasting temperature, or the fi rst
detectable increase in apparent viscosity. This is
not the gelatinization temperature. Many already
irreversible structural changes have occurred in
starch granules before swelling has progressed
enough to be detected as a viscosity increase
(Batey 2007). Further heating leads to a peak in
the viscosity-vs.-time curve that is related in both
height and timing to competing phenomena:
granule swelling and the breakdown of the swollen
granules under shear. Continued mixing of the
paste at 95 ºC leads to a decrease in viscosity,
which is commonly more evident under the shear
conditions in the RVA. On cooling, the paste
again increases in viscosity. The extent of this
increase in viscosity is related often to starch
amylose content. Flours containing starch higher
in amylose have higher cooled paste viscosities as
a result of the ability of amylose molecules to
reassociate on cooling (Batey 2007).
Much valuable information has been created
using paste viscosity analysis. The characteristic
early pasting and high paste viscosity of reduced-
amylose wheat were fi rst detected with the VAG
(Moss 1980). These characteristics are now
known to be also associated with a relatively large
decrease in paste viscosity with continued stir-
ring. These diagnostic indicators from paste vis-

cosity curves were subsequently exploited using
both the RVA and VAG to identify wheat culti-
vars suitable for soft-bite noodles. High peak
paste viscosity is associated with high FSV, and
high FSV is also used to identify reduced-amylose
wheats suitable for soft-bite noodles. The con-
verse pasting characteristics—low peak viscosity
in the absence of amylase and relatively low
breakdown—have been identifi ed as potentially
benefi cial for hard-bite noodles (Moss 1980).
Crosbie et al. (2002) cautioned that the associa-
tion between high RVA peak viscosity and cooked
noodle softness could be masked if a method to
inactivate the low levels of α-amylase present
even in sound wheat was not applied, such as the
use of a dilute solution of silver nitrate. In con-
trast the standard FSV test is relatively insensi-
tive to amylase activity (Crosbie and Lambe
1993) and needs no such intervention. Higher
paste peak viscosities have also been associated
with increased cooking losses in durum pasta
(Sissons and Batey 2003).
Given the multiple parameters that are avail-
able in a paste viscosity analysis of fl our or starch,
the rotating viscometers have an advantage over
swelling power tests with regard to information
value. However, the RVA, and in particular, the
VAG, are limited by low throughput potential,
even with the application of a rapid RVA profi le
(Crosbie et al., 2002). The FSV test may remain
the method of choice in assessing breeding mate-
rial for the presence of reduced-amylose pheno-
types. However, for early-generation screening,
NIRS calibrations for FSV (Crosbie et al., 2007)
have been reported as well as an antibody test
specifi c only for the Wx-B1b null allele (“Null-
4A” gene) that is common in Australian wheat
(Gale et al., 2004). This particular antibody test
is blind to the Wx-A1b or Wx-D1b null alleles,
which may limit its widespread use. There are
reports of PCR-based assays that can identify all
of the main Wx null alleles ( Wx-A1b , Wx-B1b ,
and Wx-D1b ) that could be used in breeding pro-
grams (e.g., Nakamura et al., 2002). However,
this type of genetic screening does not indicate
the complete response of the starch to thermal
processing, which is also affected by variation in
amylopectin structure (Konik-Rose et al., 2007)