Agriculture Reference
In-Depth Information
vicinity of 10 μm. By weight, around 70%-80%
of the starch is in the large granules, greater than
10 μm, but these account for only about 20% of
the number of granules. In the washing isolation
of starch and gluten from wheat commercially,
separation of the large granules in a purifi ed state
requires much less energy and water than isolation
of the small granules (Rahman et al., 2000). Thus,
wheat with a reduced proportion of small
granules would provide a raw material better
suited to starch-and-gluten production, provided
its milling and dough characteristics are
satisfactory.
There is some confusion about the use of the
terms “A- starch” and “B-starch” In a commercial
setting, A-starch is the highly purifi ed form
comprising mostly large granules, with a very low
protein content (<0.3%), while B-starch has a
higher protein content. B-starch comprises
predominantly small granules but with some large
granules also present. In a biosynthetic view, A-
granules comprise the large granules each formed
in single amyloplasts (the starch synthetic cell)
(Briarty et al., 1979). These are initiated early in
the development of the wheat grain. B-granules
and C-granules (both <10 μm) are initiated later
in development and they are formed in an
amyloplast that has budded from an existing
amyloplast. The B-granules are initiated in a
second phase of starch synthesis, while the C-
granules are initiated at an even later phase
(Bechtel et al., 1990). There is no clear
morphological distinction between B- and C-
granules, and both are found in commercial
B-starch.
Small starch granules have a higher surface
area to mass ratio than do larger granules. Binding
of water by intact starch granules is proportional
to the surface area, so small granules bind more
water than the same mass of large granules
(Larsson and Eliasson 1997). Water absorption is
important in dough mixing and baking, and wheat
with an increased proportion of small granules
may show a higher processing quality for some
purposes, to the extent that starch properties
contribute to water absorption. There is also a
size effect of the granules contributing to dough
properties (Rasper and de Man 1980).
Gelatinization temperature
Gelatinization occurs when the internal crystalline
structure of the starch granule is lost (it “melts”)
during heating in the presence of water, which is
absorbed into the granule (Rahman et al., 2000).
In its native state, the amylopectin exists in
crystalline lamellae, and the ordered state of the
granule results in a structure which exhibits a
typical Maltese cross when viewed with transmitted
polarized light under a microscope (birefringence).
Gelatinized granules have lost this appearance.
Watching the disappearance of this birefringence
on a hot-stage microscope has been one of the
methods used to determine the gelatinization
temperature of starch. The other main method
has been differential scanning calorimetry (DSC).
Unlike pure organic crystalline materials, the
degree of crystallinity of a collection of starch
granules is not uniform. As a result, starch is
observed to gelatinize over a range of several
Celsius degrees. The point at which gelatinization
begins is known as the onset temperature, the
point at which most granules are melting is
the peak temperature, and the point at which
gelatinization is complete is known as the
completion temperature, abbreviated T
o
, T
p
, and
T
c
, respectively. The range of gelatinization
temperatures for starch from various grain species
is much wider than that for different naturally
occurring wheats. It is because wheat starch has a
relatively narrow range of gelatinization
temperatures that it is unique in its ability to
interact with gluten proteins in the formation of
bread. In reconstitution experiments, replacement
of wheat starch with starch of other origins yields
bread with reduced quality.
Viscosity of starch
Starch viscosity is usually measured with either
the Visco-amylograph (C.W. Brabender Instru-
ments, Inc., South Hackensack, New Jersey) or
the Rapid Visco-Analyser (Newport Scientifi c
Pty. Ltd., Warriewood, NSW, Australia).
Although some authors refer to determining the
gelatinization of starch with these instruments, it
is not gelatinization as such that is being measured
