10.2. What Makes Glutenin
Elastic?
Is it the primary structure of the subunits, the concatenation by
intermolecular disulfide bonds, the shape of the polymeric proteins? The
primary structure of the subunits is formed by a large repetitive domain
containing mainly the amino acids glutamine, proline and glycine. The sequence
of these amino acids is able to adopt a regular spiral configuration (Shewry et
al., 1998), which resembles a metal spring. It has therefore been admitted that
these domains might be rod-like and have elastic properties. Numerous springs
linked together by the covalent disulphide bonds might form an elastic
filament. But molecules of this shape are only conceivable at high shear
forces. Relaxed gliadin, glutenin subunits and glutenin polymers soluble in
acetic acid were made visible by transmission electron microscopy at a
magnification of 10,000x. At low concentrations filament-shaped particles were
not detected – only globular forms as can be seen in Fig. 88.
 |
Fig. 88: Transmission electron micrograph of high molecular weight
subunits (HMW; 2% in 0.05 mol/L acetic acid).
|
Because of the large number of possible intra-moleculare interactions,
e.g. by hydrogen bonds, the energetically most stable shape of monomeric and
polymeric single gluten molecules in water is a rather round one. The filament
structure of the polymers also has to be revised since the positions of the
sulfhydryl groups, which are able to form intermolecular bonds, have been
localized (Köhler et al., 1993). It is now clear that the polymers are highly
branched. There are three different types of glutenin subunits characterized by
two or more sulfhydryl groups able to form interchain bonds (Fig. 89). The
subunits with a high molecular weight (HMW) have more than two cysteine
residues and appear to form the backbone of the polymers to which branches of
low molecular weight subunits (LMW) are attached. These have two cysteine
residues capable of forming linear polymers only. The most probable structure
of a basic molecular unit of glutenin polymers is shown by the model by Wieser
(Wieser et al., 2005) (Fig. 89) which already has a molecular weight of about 2
million. A very high molecular weight is rapidly obtained by enlarging the
backbone of HMW units. The volume of subunits determined by microscopy is about
2 • 10-5 μm3, so glutenin particles of the size of a
B-starch kernel would contain about 70,000 molecular "Wieser units".
 |
| Fig. 89: Wieser model of a basic molecular unit of glutenin with the three types of subunits (LMW/HMW = low/high molecular weight; GS = glutenin subunit) |
Considering the gluten strands visible at low magnification in dough
(Fig. 90 and Fig. 91), a lot of molecules have to stick together in order to
produce the large extensible elastic structures. We do not know if this is done
only by secondary forces like hydrogen bonds or if molecules are entangled
(Singh and MacRitchie, 2001), nor if the latter is already done in the protein
bodies of the endosperm cells. These cellular gluten bodies already have a dimension
of up to 5 μm and spontaneously generate very long structures of glutenin when
flour particles are put on a water surface (Fig. 90). But it is not unusual for
globular proteins to unfold and spread on a water-to-air surface.
 |
| Fig. 90: Gluten structures issuing from a flour particle on a water surface. |
 |
Fig. 91: Glutenin filaments (stained green) separated from gliadin.
|
The filaments behave like the spokes of an umbrella. Between them
there is a surface film of gliadin which we were able to remove, using a
special technique with diluted ethanol. On the addition of water the glutenin
filaments contracted and took on an irregular shape (Fig. 91). We were able to
demonstrate the elastic character and high cohesiveness of the gluten in
filaments. When stirred with a glass capillary the filaments easily stick
together and form big lumps of gluten.
Different lumps or filaments spontaneously stick together when they
come into contact, forming large gluten structures. This must occur in the
unmixed flour system (Unbehend et al., 2004). Gluten also aggregates without an
input of mechanical energy and therefore without entanglements of glutenin.
This results in a poorly extensible dough.
To obtain a more extensible dough, the structures arising from the
endosperm cells of the flour particles have to be extended and disentangled in
a kneader or mixer by high shear forces. The aligned glutenin structures will
then interact at many more contact points and form bundles of filaments which
may form entanglements again on relaxation.
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