Agriculture Reference
In-Depth Information
than the viscoamylograph that heats at 1.5 ºC s
−1
(see Chang et al., 1999, for discussion of FN).
Temperature profi les are simple to customize and
thereby alter the sensitivity of the RVA test, but
this capability is very restricted in the FN test,
where only small changes are available by chang-
ing the volume of material used (Chang et al.,
1999).
More recently an immunoassay called the
WheatRite test has been developed as a rapid test
using antibodies specifi c to α-amylase molecules
(Skerritt and Heywood 2000). The assay is
directed at on-farm testing with the potential
benefi t that growers could avoid binning sprouted
with sound grain, thus maximizing the grower's
income potential in a rain-affected harvest. Alter-
natively one can use a laboratory based test to
measure α-amylase directly using an enzymatic
assay with an exogenous chromogenic substrate
(e.g., AACC approved method 22-05, AACC
2000).
Warriewood, NSW, Australia) (AACC approved
method 22-08, AACC 2000), and an assessment
using the viscoamylograph (VAG) (AACC
approved method 22-10, AACC 2000). These
tests can be applied to both fl our and wholemeal,
but the VAG is more commonly applied to fl our
alone. Operational principles of the latter two
instruments are detailed in subsequent sections.
Each of the three methods relies on the thermal
and enzymatic breakdown of starch to thin the
paste viscosity after the starch has been gelati-
nized
in situ
. Short FN times and low RVA stir-
ring numbers or VAG viscosities are indicative of
the presence of α-amylase.
Specifi cations for FN are widely quoted in
grain classifi cation schemes, as the FN test was
the fi rst one widely used for this application. Its
entrenchment as a standard test and the use of FN
specifi cations in buying and selling wheat speaks
to both the usefulness of the test and the inertia
of the wheat enterprise in taking up new methods.
Wheat with FN greater than 350 s is generally
considered to be sound. Wheat with FN below
250 s can be considered to contain enough α-
amylase to have some detrimental effects in pro-
cessing. Threshold values can change depending
on the local environment and local product char-
acteristics. In Italy, for example, the Synthetic
Index of Quality (
Indice Sintetico di Qualità
) gives
a FN threshold for the highest quality category of
improver wheat (
frumento di forza
) of only 250 s
(Foca et al., 2007).
The RVA has not been widely used for its
original intent as a screening tool for preharvest
sprouting (Ross et al., 1986, 1987). However, in
recent times the Canadian Grain Commission has
been investigating the RVA as a tool for assessing
harvest receivals. Using over 4,400 hard red
spring samples, Hatcher (2007) observed a poly-
nomial relationship between RVA viscosity and
FN with
r
2
of 0.93 and a standard error of predic-
tion of 14 s. Under the standard conditions delin-
eated in the approved methods, the FN and the
RVA-stirring number tests are not as sensitive to
low levels of α-amylase as the viscoamylograph.
This is because the stirring number and FN tests
both use ballistic heating to high temperatures,
and so have smaller windows of amylase activity
Polyphenol oxidase
Polyphenol oxidase is mostly regarded as a
problem in noodlemaking, but Demeke and
Morris (2002) and references therein also
mentioned PPO-induced darkening in chapattis,
Middle Eastern fl at bread, and steamed bread. As
noted earlier, PPO is associated with the bran
layers in mature kernels (Kruger and Reed 1988),
and total wheat PPO can be an order of magni-
tude greater than fl our PPO depending on the
level of fl our refi nement (Hatcher and Kruger
1993). Reducing the impact of PPO on wheat
foods has been primarily addressed by selection
of low-PPO lines in breeding programs; hence
most effort has gone into developing whole-
kernel screening methods to detect and reduce
total kernel PPO, and then by inference fl our
PPO. Whole-kernel assays have used a variety of
phenolic substrates, including catechol, phenol,
tyrosine, and
L
-DOPA (Bettge 2004). AACC-
approved method 22-85 (Bettge 2004) uses
L
-
DOPA and is based on the work of Anderson and
Morris (2001). In that study
L
-DOPA and cate-
chol were equally good PPO substrates. However,
L
-DOPA has the advantage of not being toxic to
