8.2. Instruments and Parameters
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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)
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In so-called dynamic measurements with parallel-plate rheometers the
material is subjected to very small oscillating movements – in our experiments
(Tab. 67) less than 3 μm – so the properties are determined under conditions
where the structure of dough and gluten is presumably not destroyed. The
elastic and viscous resistance at a given deformation can be calculated. The
relative contributions of the viscous and elastic components to the rheological
properties of the sample are expressed by tan δ (See Fundamental Rheometry).
in which G'' is called the loss modulus, a measure of viscosity, and
G' the storage modulus, a measure of elasticity. Both are derived from δ and from
the complex shear modulus G* which represents the total resistance of dough or
gluten to imposed deformation. Larger deformations between 3 and 9 mm can be
used with creep and creep-relaxation tests. Here the sample is loaded with a
sudden stress that is kept constant for a certain time. Then the stress is
removed and the sample is allowed to relax. This test gives information on the
total deformation Δ (%) or the ease of deformation under constant stress, and
also the magnitude of the elastic deformation (E) and the irreversible, viscous
deformation (V). A third easy rheological test is the microextension test
(Kieffer, 1981). It is similar to the Brabender test, but uses samples of
constant diameter and a Texture Analyser (See Empirical Rheometry). In this way,
gluten and dough properties can be compared by the same method. This test is a
rupture test showing the behaviour of the material at large and rapid
deformations. Elastic properties can tentatively be evaluated from the initial
slope of the Extensograms. The force at the breaking point of the dough, the
maximum resistance (Re) and the extensibility (Ex) are recorded.
9. Comparative Tests with
Various Rheological Methods to Reveal the Function of Elasticity
It is a well established fact that rheological properties largely
govern dough development and baking (See Empirical Rheometry). Tests made at different shear velocities
show that the elasticity of the dough is linked to the elasticity of the gluten
and that elasticity can influence the baked volume.
9.1. Materials
Seven wheat cultivars differing in protein content (N = 1.44 - 2.04%)
and protein quality (gliadin / glutenin ratio = 1.72 - 2.61) from the quality
classes E (extra high) to C (not bread quality) were analysed by the methods
described above. Rounded and non-rounded doughs were used. Both have the same
composition but the mechanical input is different. Non-rounded dough is shaped
into test pieces within 20 s after kneading in Farinograph, rounded dough is
left to relax for 10 min then rounded as in a baking test. These two types of
dough were compared because non-rounded doughs are normally used in dough
testing, but rounded dough is used for most bread-making processes. Rounding
after a short relaxation time changes the gluten and starch structures as shown
in Fig. 86.
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Fig. 86: Sections of dough (40 μm); proteins stained blue. Dough not
rounded (left) and rounded (right) with the corresponding micro-baking results
(bottom).
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This makes the form ratio of bread better at a comparable fermentation
time. This gentle action is said to cause hardening of the gluten structures,
but in fact more cohesive and thicker structures are built up (Kieffer and
Stein, 1999). The small and feeble gluten filaments separate from starch and
starch kernels are packed more closely. This also helps to stabilize the dough.
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