2009_Book_FoodChemistry

680 15 Cereals and Cereal Products
cereals, Triticeae prolamins contain significantly
higher levels of glutamic acid and proline. This
suggests that the difference in prolamin composition,
induced by these amino acids, may be responsible
for Celiac disease.
15.2.1.3 Protein Components
of Wheat Gluten
Wheat protein fractionation by the Osborne
method provides prolamins and glutelins in
a ratio of 1:1. Both fractions, in hydrated form,
have different effects on the rheological characteristics
of dough: prolamins are responsible,
preferentially, for viscosity, and glutelins for
dough strength and elasticity.
The genes for the gluten proteins occur at nine
different complex loci in the wheat genome.
The high molecular weight glutenin subunits
are coded by the loci Glu-A1, Glu-B1 and
Glu-D1, which are carried on the long arms
of the chromosomes 1A, 1B and 1D. The
low molecular weight glutenin subunits, the
ω-gliadins and the γ-gliadins are coded by the
loci Gli-A1, Gli-B1 and Gli-D1, which occur
on the short arms of the chromosomes 1A, 1B
and 1D. The α-gliadins are coded by the loci
Gli-A2, Gli-B2 and Gli-D2 on the short arms of
the group G chromosomes. It is presumed that
the variation seen in different varieties is due
to the presence of allelic genes at each of the
nine storage protein loci. The relative importance
of different alleles for gluten quality seems to
be Glu-1 > Gli-1 > Gli-2.
A fractionation of gluten proteins is possible by
two-dimensional electrophoresis. Figure 15.3
provides a schematic overview of the position
of the most important protein groups in a twodimensional
electropherogram. The pattern of
glutenins of two wheat cultivars are shown in
Fig. 15.4.
Gluten proteins can be separated into their components
on an analytical and micropreparative
scale by using RP-HPLC. In general, this separation
starts with the Osborne fractions or subfractions.
In this way, the prolamines of wheat can be separated
into ω-, α-, γ-gliadins (Fig. 15.5), different
varieties of wheat giving different patterns,
e. g., the cultivars Clement and Maris Huntsman
Fig. 15.3. Wheat endosperm proteins (cultivar “Chinese
Spring”). Simplified schematic representation of a twodimensional
electrophoretic separation.
1st. Dimension: isoelectric focussing (IEF) and nonequilibrium
pH gradient electrophoresis (NEPHGE).
The electropherograms obtained by both methods are
put together at the broken line in such a way that a continuous
pH gradient is formed.
2nd. Dimension: polyacrylamide gel electrophoresis in
the presence of sodium dodecyl sulfate and mercaptoethanol
(SDS-PAGE).
The following protein fractions can be recognized:
high-molecular glutenin subunits (1); basic (2) and
acidic (3) low-molecular glutenin subunits; γ- (4)and
ω-gliadins (5); subunits of the triplet band (6, 10); highmolecular
albumins (7); globulins (8) and nonreserve
proteins (9). (according to Payne et al., 1985)
known to produce sticky dough have a characteristically
high ω-gliadin content.
The prolamin patterns of other cereals (Fig. 15.6)
differ greatly from that of wheat. In rye, the
hydrophilic ω-secalins are followed by the hydrophobic
γ-secalins. And unlike wheat (α-gliadins),
the area of moderate hydrophobicity is
not occupied. In barley, a hydrophilic fraction is
missing: the C-hordeins eluted in the middle area
are followed by the hydrophobic B-hordeins. The
chromatogram of oats is characterized by two
hydrophobic fractions that are close to each other.
The low-molecular subunits of wheat glutelins
also give a chromatogram rich in components