Agriculture Reference
In-Depth Information
but rather “pasting.” The important viscosity
parameters measured during pasting are peak
viscosity, trough viscosity or holding strength,
and fi nal viscosity. Both instruments utilize a
controlled temperature profi le that heats and then
cools the starch in the presence of excess water,
from an initial temperature (usually 50 ºC) to
about 95 ºC and then back to 50 ºC. During the
heating phase, the starch does gelatinize, but the
observed viscosity change does not occur until the
gelatinized starch granules rapidly absorb water
and swell. Swelling of the granules requires their
gelatinization, and it may occur immediately
when the gelatinization temperature is reached
or it may occur later at a higher temperature
(Morrison et al., 1993).
Amylose content affects the granules' ability to
absorb water; waxy starch absorbs water and
swells at a lower temperature than starch
containing amylose (Miura and Tanii 1993). After
reaching a peak viscosity on heating, the viscosity
reduces as the swollen granules are ruptured
and degraded. The minimum viscosity reached
(holding strength) is an indication of the stability
of the starch gel at a high temperature. When the
gel is cooled, its viscosity increases. This increase
is almost entirely due to the amylose in the starch,
as waxy starch shows very little viscosity increase
in this stage of the process. The fi nal viscosity
reached is an indication of gel strength in products
at lower temperatures, such as custards or even
gravies.
The viscosity of starch is often related to
quality, as it affects the processing and fi nal gel
structure of many products. For those products
processed or utilized at high temperature (e.g.,
hot sizing in paper manufacture), the holding
strength of the starch may be of more importance
than the fi nal viscosity.
Swelling power is particularly important in
products where the granules are gelatinized but
remain essentially intact. One such example is the
quality of Japanese udon noodles. For these
noodles, the softness of the cooked product is an
important attribute, and this is imparted by the
starch granule's ability to swell but maintain its
integrity. In the selection of lines suitable for this
end use, swelling power is used by the breeder
as a screening tool (Crosbie 1991; Crosbie and
Lambe 1993). In wheat, swelling power is
determined by the GBSS alleles referred to above.
A single-null allele confers a greater swelling
power than the wild type with all three forms of
GBSS. Double-null genotypes confer more
swelling, while triple-null genotypes (waxy) have
even greater swelling power. Peak viscosity in a
viscogram is also a measure of swelling power, but
it is less useful as a screening tool due to the
requirements of sample size and time.
NONSTARCH POLYSACCHARIDE
COMPOSITION AND WHEAT QUALITY
While starch predominates in the endosperm, the
nonstarch polysaccharides are important compo-
nents of the outer bran layers of the grain. They
play an important role in the milling process,
during which this material must be removed
cleanly and effi ciently from the endosperm
(Posner and Hibbs 2005). The value of fl our is
greater than that of the nonendosperm parts of
the grain. Thus, a high extraction rate for white
fl our is a top priority for the miller. As milling
quality is a signifi cant economic consideration,
some pressure rests on the breeder to consider it
as a quality objective, involving such aspects
as grain hardness, bran thickness and germ
size, as well as kernel size and shape (Fincher
and Stone 1986). An obvious breeding goal
is to aim for a high ratio of endosperm to
whole-grain mass, but this goal confl icts with a
yield-oriented goal of breeding for large germ
size to provide early seedling vigor (Rebetzke et
al., 2001). A molecular marker for milling yield
was found on chromosome 5B in two distinct
Swelling power
The swelling power of starch is a measure that
indicates its ability to absorb water and increase
in size. As stated previously, amylose content is a
major factor determining the ability of the granule
to swell on heating in water (Crosbie 1991).
