The Role of Gluten Elasticity in the Baking Quality of Wheat

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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