Agriculture Reference
In-Depth Information
sets of doubled-haploid populations by Smith et
al. (2001). They were unable to identify any asso-
ciated aspect of grain chemistry. A quantitative
trait locus has been found accounting for about
one-third of the variation in arabino-xylan ratio
on the long arm of chromosome 1B (Fincher and
Stone 2004).
The nonstarch polysaccharides of the endo-
sperm play a lesser but signifi cant role in deter-
mining milling quality. Largely the remnants of
the endosperm cell walls, these nonstarch poly-
saccharides may cause problems in milling by
clogging the sieves. This fault may be another
aspect of milling quality that can be resolved in
the breeding process, given that there is consider-
able variation in the level of nonstarch polysac-
charides in the endosperm. This variation covers
the range from 0.5% to 2.3% for β-glucans in
whole grain, according to Fincher and Stone
(2004). However, there is much yet to be learned
about the extent of heritability of this chemical
trait.
Noncellulosic polysaccharides, especially pen-
tosans (heteroxylans) and β-glucans, constitute a
high proportion of the walls of the aleurone
and endosperm cells (Fincher and Stone 2004).
The beta-glucans are linear polysaccharides,
unbranched, and polymerized through both (1→
4) and (1→3) linkages. They range in molecular
weight from about 50,000 to 3 million Daltons.
They are poorly extractable into water at room
temperature. Strong alkali (e.g., 4 M sodium
hydroxide) is required for complete extraction.
Although the nonstarch polysaccharides are
minor white-fl our components (about 2%), their
high water-binding capacity makes them signifi -
cant in relation to dough properties. Despite the
role of nonstarch polysaccharides in determining
the extent of water absorption by a fl our sample,
they share this function with the protein and
starch fractions, the latter's role depending greatly
on the degree of starch damage. The extended
conformation of water-soluble pentosans means
that they contribute signifi cantly to viscosity, pos-
sibly over 15 times more than the viscosity con-
tribution of globular proteins of similar molecular
weight (Fincher and Stone 1986). Their water-
holding capacity has been reported to affect dough
extensibility and loaf texture (D'Appolonia and
Rayas-Duarte 1994). During mixing, the soluble
pentosans become less extractable, presumably
due to their interaction with proteins. Various
oxidative bread “improvers” may accelerate this
cross-linking reaction, thereby making a positive
contribution to dough rheology.
The nonstarch polysaccharides are also signifi -
cant in relation to dietary fi ber and wellness for
humans. This positive contribution to human
nutrition is contrasted to the negative role of the
nonstarch polysaccharides in the feed value of
grain for nonruminants, for which they contrib-
ute antinutritive effects. Nonstarch polysaccha-
rides create special feeding problems for poultry,
giving the health diffi culty of “sticky droppings”
and lower available metabolizable-energy results
than expected by the energy content of the grain.
The soluble nonstarch polysaccharides (arabino-
xylans, xylans, and β-glucans), in particular, have
a negative effect as their content is inversely pro-
portional to the energy availability of grain in
poultry (Black 2004).
FLOUR COLOR AND WHEAT QUALITY
The color of fl our (or of semolina in the case of
durum wheat) and end-products represents
important criteria of wheat quality, playing a sig-
nifi cant role in determining the suitability of grain
for particular products and markets. Color clearly
has an immediate visual impact. Considerable
effort has gone into the development of refl ec-
tance technologies and algorithms that can provide
a reproducible, quantitative assessment of color
that is related to color perception by the human
eye. The result is a range of refl ectance instru-
ments that defi ne the color of a sample in a
three-dimensional color space using the coordi-
nates L* (brightness), b* (yellow-blue), and a*
(red-green).
Flour or semolina color is infl uenced by two
major components: (i) bran speck size and number
(speckiness), a function of the milling and sieving
processes; (ii) the inherent color of the starchy
