7. The Role of the Insolubility
of Gluten
Cohesiveness is lost as the proteins become more soluble. Their native
insolubility is due to a small and well-balanced amount of anionic and cationic
amino acid side chains and of the end-groups of the subunits (Belitz et al.,
1982). Changing the number of one sort of ion e.g. by derivatisation of amino
groups to carboxylic groups (Dirndorfer et al., 1986) solubilizes gluten
without changing the molecular weights, with the result that elasticity is lost
completely.
When experiments in which the solubility of gluten is changed are used
to deduce the importance of elasticity for wheat bread making, it must be kept
in mind that in fact the composition of the dough is changed. In the case of
reduced disulfide bonds, or when gluten fractions are made water-soluble, the
nature of the gluten is changed completely, in other words there is no longer
any gluten in the dough capable of generating cohesive structures. It is like
cutting a rubber band in small pieces. The elastic behaviour vanishes because
of a lack of cohesiveness of the pieces. There is still rubber, but no longer a
rubber band.
A better way to prove the correlation between elasticity and baking
quality is to compare flours from different wheat cultivars by the following
rheological methods.
8. Methods for the Evaluation
of Elasticity
8.1. Prerequisites for Physical
Testing
Elasticity is only one component of the bulk property of gluten or
dough, so methods must be used which show this property isolated from the
others.
A second problem is that when gluten and dough are compared it is
essential for both samples to have exactly the same dimensions. It is rather
difficult to shape gluten. But this can be done by using parallel plate
geometries with stress rheometers or the SMS/Kieffer gluten and dough
extensibility method (Kieffer et al., 1998 and Mann et al., 2004) or
highpressure capillary viscosimetry (Kieffer et al., 1982). The latter will not
be considered as it is very time-consuming.
The third and most important question is what we want to see. Cereal
chemists want to clarify the mechanism of elasticity on a molecular basis, and
determinations in the field of linear viscoelasticity can be useful. If, on the
other hand, the function of elasticity during dough preparation and for the
resulting product is of interest, we first have to find the range of stress or
deformation velocity; that means we have to imitate very closely what happens
in dough. Because this is not so simple we have to vary test conditions until
they are correlated e.g. to the baked volume. Using this approach, very useful
empirical methods such as the Brabender methods have been developed.
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