9.2. Results
Tab. 67 shows the best coefficients of correlation between the
different rheological parameters monitored for gluten, rounded and nonrounded
dough and baking volume.
 |
Tab. 67: Highest correlation coefficients (r) obtained by comparison
between baking results and rheological properties measured on gluten and dough
in dynamic, creep and creep recovery tests and with micro-Extensograms; n = 7;
95.0% confidence level at r > 0.727 (correlated); 99.0% at r > 0.837
(high correlation); 99.9% at r > 0.924 (very high correlation)
|
A very good correlation of r=0.922 was found between gluten and
rounded dough or the elastic deformation E but not between gluten or dough and
baked volume. Neither the dynamic moduli G' and G'' nor tan δ were useful in
comparing gluten and dough properties or predicting baked volume. The sole
value of G* for gluten and dough was highly but negatively correlated to the
volume. G* is a measure of the resistance of the material to deformation. That
means that when the gluten or dough is too strong, the volume will be low, a
fact well known in practice.
The very high coefficient between E of gluten and E of rounded dough
shows that in this case the gluten properties are more apparent than in
non-rounded dough. As we said above, this results from better aggregation of
gluten through rounding and the formation of a more continuous network.
9.3. Correlations between
Rheological Properties and Baked Volume Show that Cohesiveness is the Key
From the high negative correlation of G*, the high positive
correlation of Re and the nonexistent correlation of any elastic modulus or
deformation to volume, the conclusion can be drawn that elasticity is not
involved in volume generation. The rupture tests mainly monitor the
cohesiveness of dough. The better this is, the greater will be the expansion of
dough during proofing and oven rise.
10. The Mechanism of Elastic
Deformation
10.1. The Role of Gliadin and
Glutenin
Although elasticity does not play the major role we expected, it is
interesting to look at the origins of elasticity. Chemical properties and
aggregates with high molecular weight will be of importance. But cereal
chemists are far from having a full explanation.
Gluten proteins can be divided into two fractions according to their
solubility: the gliadins account for 65 to 75% of the gluten and the glutenins
for the rest. Gliadins are mainly monomer with molecular weights ranging from
30,000 to 50,000 and soluble in 50% ethanol. Glutenins are polymeric with
molecular weights from 400,000 to more than 1 million, presumably up to 10
million. They are insoluble in ethanol. So far it has not been possible to
determine the real molecular weight because of big, insoluble particles
sometimes called glutenin macro polymers (GMP), which cannot be dissolved or
separated but are necessary for molecular weight determination. In an aqueous
milieu gliadin is a honey-like viscous fluid, whereas pure glutenin is hardly
extensible, very strong and mainly elastic as can be seen from Tab. 68.
 |
Tab. 68: Rheological properties of gliadin, glutenin and whole gluten.
Results of dynamic testing at 1 Hz and 0.15% deformation and as shown in
micro-Extensograms.
|
The proper mixture of gliadin and glutenin is responsible for the
viscoelastic properties and ease of extensibility under the low pressure
generated by yeast. That is why gliadin is sometimes regarded as a plasticizer
or solvent for glutenin. But this would only be true if the two fractions were
homogenously mixed. This is not the case, as I will show by following the
hydration process of the proteins and their mechanism of elongation and
rupture. Besides the quality of protein fractions their quantity is an
important factor for functionality and for the resulting dough properties and
baking results (Wieser and Kieffer, 2001). To a large extent it is the gliadin
/ glutenin ratio that determines the firmness and elasticity of gluten (Fig.
87; Kim et al., 1988). The amount of glutenin and GMP particles in flour
increases dough firmness and kneading time (Weegels et al., 1996).
 |
Fig. 87: Micro-Extensograms of gluten with different gliadin levels (%
of total gluten)
|
إرسال تعليق