4. Elasticity of Gluten and
Dough
For most real solid materials elasticity is one of three physical
parameters contributing to firmness. The materials exhibit elasticity,
viscosity and cohesiveness simultaneously, which makes it difficult to measure
elasticity independently of viscosity and cohesiveness. Gluten and dough are
normally said to be viscoelastic. Both exhibit solid-like properties
(cohesiveness and elasticity) and liquid-like properties (viscous or
unrecoverable deformability).
Hook's law is not, therefore, applicable to dough or gluten.
Elasticity has to be determined from the resulting rheological property. There
are in fact methods for separating elastic and viscous behaviour, but a
comparison of different samples is only permissible if cohesiveness is
comparable. However, cohesiveness cannot be determined since there is no method
for doing so with samples like dough.
5. The Role of Molecular Weight
in Elasticity
In addition to rheological tests to quantify elasticity, chemical
modifications of gluten proteins have been used to demonstrate the importance
of elasticity for baking. These methods
alter elastic behaviour by changing the molecular weight of glutenin or its
solubility:
The partial or total reduction of intermolecular disulphide bonds
lowers both molecular weight and elasticity (Belitz et al., 1986) and is
detrimental to baking. High-pressure treatment (Kieffer and Wieser, 2004) or
enzymes like transglutaminase (Bauer et al., 2003) create new covalent bonds
and raise the molecular weight of gluten. This can lead to less viscous and
more elastic gluten and higher dough stability.
6. The Role of Non-Covalent
Interactions
But the high molecular weight of glutenin is not the only reason for
the occurrence of elasticity. It is the fact that gluten proteins are
exceptionally cohesive, mainly because of their ability to form hydrogen bonds
between the amide side chains of the amino acid glutamine, which accounts for
about 35% of all residues (Fig. 84).
 |
Fig. 84: Two amide groups of glutamine of adjacent gluten chains (P
and P') linked by two hydrogen bonds (dotted) sharing hydrogen atoms
|
Hydrogen bonds interchange easily, which means that unlike the
covalent disulphide bonds they can be separated and fixed again during
deformation of the material. Nevertheless, their total binding energy in gluten
may be considerable: 10 hydrogen bonds equal the strength of one disulfide
bond, and over 30 times more hydrogen bonds can be formed than disulfide bonds
because of the great number of glutamine residues in gluten. The function of
these bonds can be observed when dough is cooled or heated. At low temperatures
hydrogen bonds are strengthened, whereas at high temperatures the opposite is
the case. The firmness and elasticity of gluten (Fig. 85) and dough are changed
in the same manner. Up to 60 °C resistance decreases, then heat denaturizing
begins and extensibility decreases too.
 |
| Fig. 85: Resistance and extensibility of gluten at various temperatures (micro-Extensograph) |
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