1.2. Viscosity and Elasticity
of Dough
When mixed with water the flour becomes dough, a cohesive mass in
which the gluten forms a three-dimensional network made up of strands and
membranes in which the starch granules are embedded (Amend, 1996 and Bloksma,
1990). The viscosity or consistency of the dough depends on the amount of water
and other ingredients added, but also on the intensity of mixing. The expansion
and volume of the baked products as a quality attribute of the flour is the
result of the production and retention of gas. In this context the viscosity or
consistency of the dough is initially the main characteristic that determines
the gas retention necessary for making up a flour into bread and other
products. The baker will strive to achieve an optimum consistency which is
thick enough to make the dough workable (kneading, moulding) and ensure that it
keeps its shape and on the other hand thin enough to allow the carbon dioxide
generated by the yeast to cause the expansion that results in the desired
leavening of the dough and its baked volume. It was established only recently
that a dough can come about as a "hydrated, unmixed flour system"
through aggregation of the gluten proteins without an input of energy from
mixing. Its physical, rheological properties are somewhat different from those
of doughs produced by mixing; it is firmer and less extensible, has a high
initial viscosity and elasticity but low stability (Unbehend, 2002).
Nevertheless, mixing is likely to remain indispensable, for the air bubbles
introduced into the dough by mixing and in which the carbon dioxide collects
during fermentation are the "starting point" for the pores that
result in the baked volume of the products (Hoseney, 1986).
Doughs without Mixing
Efforts to minimize the energy input necessary for making up a dough
have led to the development of the new Rapidojet technique. On the basis of
observations by Amend (1996), Noll (2002) came to the same conclusion as Unbehend
(2002), namely that far less energy is needed for preparing dough than is
normally used in bakeries. With the Rapidojet, Noll developed a fast method of
dough preparation that saves space, time and above all energy. Air and water are
introduced at high pressure into a stream of flour running into a pipe. Within
seconds this results in a dough that has viscoelastic properties similar to
those of doughs produced by mixing and is just as easy to make up into bread.
The energy introduced by the pressure of the added water is much less than that
normally required by bakeries (Noll, 2002).
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| Fig. 53: Baked volume as a function of viscosity and viscoelastic
dough properties |
If we view the bread-making process as an interaction of gas
production and gas retention it may be said that gas production can be adjusted,
controlled and kept constant with the amount of yeast in the formulation, the quantity
of the fermentable sugars maltose and glucose added or present and other
technical measures such as fermentation temperature and time. There remains gas
retention as a factor that demands the baker's attention and technical skill (Fig.
53). If we assume that gas retention depends directly on the consistency of the
dough, we may expect gas retention to increase in proportion to consistency.
This holds true in practice: firm doughs with a high gas retention capacity
combined with good gas production result in a high volume yield. Conversely it
is logical that the low gas retention capacity of doughs with low viscosity (consistency)
results in a low volume yield. Because of their gluten structure the soft and sometimes
weak doughs are permeable to gas. It is also logical that very firm, short or "bucky"
doughs are too strong to be stretched by the developing carbon dioxide because
of their firmness and stability. The result is a low volume yield. This means
that an optimum viscosity or consistency of the dough is desirable in order to
ensure the highest possible volume yield. Since this factor can be adjusted and
controlled by determining water absorption with the Farinograph, it may also be
regarded as constant. The viscoelastic properties of the dough, generally known
by bakers and cereal processors as "dough characteristics", vary from
soft and weak to firm and short; it is the "normal" dough
characteristics that bring the best results in each case. The volume of the baked
products is therefore a function of viscosity and elasticity. If the viscosity
can be adjusted (by adding water) and may therefore be regarded as a constant
factor, it is ultimately elasticity or the rheological balance between extensibility
and elasticity that determines the value of a wheat flour for baking. Too much elasticity
results in short, bucky doughs; too little makes the doughs soft and weak.
Dough rheology makes it possible to identify these variety-related properties –
viscosity and elasticity – quickly and reliably. We also know ways and means by
which the viscoelastic properties can be altered and optimized to a certain
extent; here, especially, rheometry helps with dosing and control.
The objective and purpose of rheology is to identify the basic
rheological properties of substances and interpret the changes in these under
defined measurement conditions.
Basic rheological properties are:
• strength (solidity),
• viscosity,
• elasticity, and
• plasticity.
To establish these properties, rheometry – a sub-discipline of
rheology – uses a deformation force and measures the effect of this force on
the specimen (in this case the dough) as its deformation. The deformation force
may be great or small; the measurement will vary accordingly.
Strength as a further property of a material is easy to determine. A
body, as rheologists call the substance to be tested, is a viscous mass or a
fluid if it has no yield point, flows by its own gravity and is therefore not
dimensionally stable. By contrast a solid (solid body) keeps its shape, can
only be made to flow by the effect of a deformation force and has a yield point. A solid can also
be a plastic, elastic or viscoelastic body, depending on its structure. Viscosity
is an important characteristic of any material; it is made up of the components
elasticity and "pure viscosity" or plasticity. In the case of
liquids, viscosity may be described as the internal friction between the molecules and molecular aggregates;
in the case of solids it is the cohesion resulting from their structure. When
determining basic properties it is possible to measure viscosity (more precisely
complex viscosity) and its component elasticity, whereas "plasticity"
has to be calculated as an imaginary part of viscosity and the difference
between the measured viscosity and elasticity. A body is termed elastic if it
is difficult to deform and regains its original shape when the deformation
force has ceased to act on it. Deformation was then reversible and the
deformation energy applied was stored. If, on the other hand, a body is easy to
deform and remains deformed after the deformation force has ceased to act, the
body is irreversibly plastic and the deformation energy has been lost.
The directly measurable and determinable basic properties
"viscosity" and "elasticity" would therefore seem to be the
most important characteristics for describing a material and for its behaviour
as a raw material, in the process itself and finally as an end product. That is
why dough rheology gives special attention to these two properties. The materials
known to us, including foods, have mainly viscoelastic properties, and the characteristics
elasticity and plasticity occur in different ratios to each other.
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