1.4.6. Stretching Methods –
Extensograph versus Alveograph
The other difference in dough preparation between the two methods
(nature and duration of mixing) is not so fundamentally important. In the
Alveograph the measurement itself is performed by bi-axial stretching, carried
out by inflating a piece of dough into a bubble with an air pump until it
bursts. In the Extensograph it is done by uni-axial, linear stretching of a strip
of dough with a hook until it breaks. The speed of deformation is similar for
the two methods; in the Extensograph it is 1.5 cm/s. But although the resulting
measurements, the recorded curves, are supposed to provide the same
information, they have come about differently. The Alveogram shows the pressure
curve of the air trapped in the dough bubble, whereas the Extensogram is a
deformation curve from which the resistance to extension (a measure of strength
or even the elastic component) and extensibility (as the compliant, plastic
component of the dough properties) can be read off. The maximum pressure in the
Alveogram, the P value, that denotes strength, actually shows the yield point
of the dough, i.e. the force that has to be exerted in order to start stretching
the gluten fibrils in a dough. This P value serves to estimate the dough yield
or the amount of water to be added. But a pressure curve is very different from a
deformation curve. A deformation curve can be obtained by recording the increase
in volume of the expanding dough bubble in a vertical direction (Fig. 62).
A further difference between the two methods which is often neglected
lies in the time factor, or the duration of the test. An Alveogram is recorded
28 minutes after the start of mixing; for technical reasons only one
measurement can be performed on each dough specimen (Faridi and Rasper, 1987).
An Extensograph test usually consists of three Extensograms made at intervals
of 45 minutes during the dough resting time. This time factor is important for
two reasons and must not be ignored. Kneading and moulding for the test cause a
"structural activation" of the dough during which the mechanical
energy of the mixing and moulding is "stored" in the elastic component
and greatly influences the result of the measurement (Rasper and Preston, 1991 and
Weipert, 1981). In this state, resistance to extension is higher and
extensibility lower. The stored energy subsides after about 45 – 60 minutes;
the taut "springs" of elasticity relax during this time and the dough
undergoes a structural relaxation or structural recovery so that its "real,
uninfluenced" rheological properties can be measured. The stretching of a
dough resulting from inflation and an increase in volume during fermentation
and in the early part of the oven phase take place in a relaxed state.
Moreover, the effect of the ascorbic acid, enzymes and emulsifiers added as
flour improvers or baking ingredients can naturally be identified better after
a longer time of action than after a short one. This effect is therefore only
visible to a certain extent in Alveograms (Weipert, 1981 and 1992).
During fermentation, the dough undergoes a process of inflation in
which the carbon dioxide enlarges the pores and gives the dough greater volume.
The gas retention capacity of a dough is therefore considered a quality characteristic
and shown in the form of extension curves. As a displacement/time function the
stretching of the fermenting dough may be regarded as a slight deformation, but
for technical reasons the stretching tests in laboratories are carried out with
greater deformation forces. For this reason such tests are rightly classified
as empirical methods.
The principle of the stretching tests is that a dough made according
to the standard method and prepared for extension is stretched and an extension
curve recorded from which characteristics such as viscosity can be read
directly and viscoelasticity indirectly. At present two stretching methods are
in common use, carried out with fundamentally different measuring instruments
and procedures. The methods were developed at the same time but independently
of each other in regions with different wheat qualities and types of bread: the
Chopin Alveograph in France and the Brabender Extensograph in Germany. Their
predecessor was probably the Aleurograph and Laboragraph after Muller (1964 and
1966).
The extension curves in the Extensograph method (Extensograms) and in
the Alveograph method (Alveograms) describe the extensional work (energy in the
case of the Extensogram and W value in the case of the Alveogram) which is
understood to be gas retention capacity (Faridi and Rasper, 1987, Rasper and Preston,
1991 and Weipert, 1993). In the further interpretation the height of the curve (R
with the Extensogram and P value with the Alveogram) is understood as
resistance to extension and the length of the curve read on the X axis (E with
the Extensogram and L with the Alveogram) is taken to be extensibility. If resistance
is now viewed in relation to extensibility, the quotients R/E = ratio and P/L describe
the viscoelastic properties of the dough.
 |
Fig. 58: Extensograms (135 min) of dough with different recipes and
mixing processes (with and without salt, sugar, fat, malt flour, ascorbic acid;
mixing in the Farinograph or Stephan mixer).
ASC - ascorbic acid
RMT - Rapid Mix Test
FAR - Farinograph
|
The question as to the usefulness of Extensograms and the reliability
of the information they yield as a means of describing the visco-elastic properties
of doughs has been answered by making Extensograms of unblended flours with
different dough properties in various different formulations and using
differentmethods of preparation (Bolling and Weipert, 1984). The Extensograms
reacted very sensitively to the changes in the formulation, the individual ingredients
added to the flour in the RMT standard baking test (salt, ascorbic acid, fat,
malt flour, sugar) having a characteristic effect on the curve of the
Extensograms(Fig. 58). Even the ascorbic acid alone had a very strong effect. The
interaction of all the ingredients in the RMT formulation with the flour showed
itself in the Extensogram with the largest area; preparation of the dough in the
Farinograph or in the Stephan mixer during the RMT standard baking test made no
appreciable difference to the curves of the Extensograms. The most important
result was that the Extensogram made with salt and ascorbic acid according to
the standard method was found to be practically identical to the Extensogram of
the RMT dough (complete formulation but without yeast). This confirms and justifies the Extensogram method as a practical and
informative procedure.
 |
Fig. 59: Alveograms and Extensograms of wheat flour with different
dough properties and protein content.
WA = Water Absorption by Farinograph
DP = Dough Property Index (1 = soft, slack, sticky, 25 = short and
dry; about 9 is normal and desirable)
VY = Volume Yield in RMT baking trial mL/100 g flour
|
Extensograms are indeed capable of expressing the quality of a flour
and its suitability for making different bakery items. Using flours from three
different wheat varieties with extremely different dough properties (short, normal,
soft), each with two different protein levels (low and high), it was possible
to demonstrate that Extensograms show both the variety-related quality of the
wheat flours and the influence of nitrogen fertilizers (Fig. 59). The
Extensograms differentiated clearly between the flours at both protein levels.
This could be seen both from the energy values (area below the curve) and from
the ratios (R/E). At a low protein content the Extensograms of all three
varieties showed higher resistance and lower extensibility, thus indicating
flours with shorter dough properties. This was especially evident in the
variety with genetically short dough properties. At high protein levels, all
three varieties produced Extensograms with lower resistance but higher extensibility,
indicating softer dough properties; again this was especially evident in the
variety with genetically short dough properties. In the variety with the
"normal" dough properties an increase in the protein content of the
flour resulted in slightly reduced resistance and increased extensibility, but
in both cases the Extensogram data – including energy and the ratio – indicated
good quality which was enhanced further by the protein increase. The energy
values and ratios of all the Extensograms were in line with the baked volumes
achieved with these flours. The low energy values in conjunction with low
ratios (0.6) that indicated soft and weak doughs and the high ratios (7 and
above) in conjunction with low energy values that stand for short doughs were confirmed
by low baked volumes. High energy values and ratios in the optimum range (about
1.5 to 3.0) in the Extensograms indicated a flour of good quality and high baked
volumes (Weipert, 1981, 1992 and 1993).
 |
| Fig. 63: Creep recovery / stress relaxation curves of a gluten or a dough |
The Alveograms recorded at the same time and with the same flours did
not distinguish so clearly between the various flour qualities. Although some
differences were found in the W values, the P/L ratio was virtually identical in
all the Alveograms (0.42 - 0.56); this made it impossible to read off
differences in the dough properties. A recommended procedure for determining the elastic
properties of a dough directly with the Alveograph is to carry out a second test,
a pressure relaxation test, in which the air pressure suddenly stops after the
formation of the dough bubble and the relaxation of the dough is read off from
the resulting curve (Faridi and Rasper, 1987). This measurement procedure was
developed on the lines of the creep recovery or stress relaxation graphs used
in fundamental rheometry and recorded with a rotating viscometer or rheometer (Fig.
63). However, this measurement method has not established itself in practical testing
with the Alveograph.
The reasons why the extension curves of the Extensogram and the
Alveogram yield different information lie in the way the tests are carried out.
The most important, most fundamental and decisive difference between the two
ICC standard methods is already to be found in the preparation of the dough.
The Alveograph method uses a constant amount of water, which naturally results
in doughs of different consistency; the Extensograph method assumes that the
doughs are of constant consistency following determination of optimum water absorption
in the Farinograph. If the two methods are assumed to describe the rheological properties
of the dough for processing in the bakehouse, the Alveograph method records a condition of the dough that is far removed from its actual
rheological condition at the time of processing into bread or other products
because of the addition of a constant amount of water irrespective of the
quality of the flour; this amount is in any case far too small for bakers'
doughs. The constant amount of water added to the Alveograph doughs corresponds
to a water absorption of 50% for all flours irrespective of their quality, whereas
today's wheat flours have a water absorption capacity between 54% and over 60%
and are processed into bread at these water absorptions, or at the
corresponding dough yields. We should not forget that water is a
"plasticizer" that makes the dough softer but optimizes its consistency if properly dosed and ensures good baked
results when combined with flour improvers or other ingredients. With the
addition of 50% or 58% water, for example, depending on its water absorption,
one and the same flour yields dough with greatly differing rheological
properties, viscosity and elasticity (Fig. 54).
 |
| Fig. 54: Baked volume as a function of dough viscosity and water
addition (the arrows indicate optimum water absorption as determined with the
Farinograph). |
 |
| Fig. 62: Comparison of recorded curves: Extensogram, Alveogram and
stress-strain curve |
When evaluating the extension curves of Alveograms and Extensograms it
is necessary to take all these factors into consideration. Only then can the
right conclusions be drawn concerning the properties of the flours and their
suitability for certain baking purposes. Besides determining the viscosity of a
dough it is also extremely important to establish its viscoelastic properties.
An Extensogram reveals both the viscosity and the viscoelasticity of a flour as
a genetic characteristic of the variety and as the influence of the environment
– chiefly the supply of nutrients and the use of fertilizers (Fig. 59). It was
evident that the Extensograms had clearly recognized and expressed the dough
properties of the wheat varieties, described as short, normal or soft (Weipert,
1992 and 1993). This was especially apparent in flours with a low protein
content. Protein levels in the flour that had been raised by nitrogen
fertilizers increased the extensibility of the dough; the Extensograms of the
wheat variety with genetically short dough properties therefore showed normal
dough properties with balanced viscoelasticity at higher protein levels. The
variety with normal dough properties retained these properties even at a higher
protein level, but its energy value (area below the curve) was greater; the
soft dough properties of the soft variety became softer still. The softening of
the dough properties, known by bakers as suppleness or pliancy, is explained by the increase in the reserve protein component of the gluten, the gliadin.
Nitrogen fertilization causes more of this component than of the glutenin component
to be formed and stored. But in a dry, warm climate, more glutenin is stored in
the wheat grain, and this results in wheat with dry, short dough properties. Unlike
glutenin, that determines the strength and therefore the elastic behaviour of
the gluten and the dough by forming strands and membranes as well as binding
large amounts of water, the gliadin component of the gluten only contributes to
the viscosity (consistency, water binding capacity) of the gluten and the dough.
Besides nitrogen fertilization, cooler and wetter environmental conditions
favour the formation of gliadin and result in softer, pliant doughs.
 |
| Fig. 56: Schematic representation and demonstration of the structure of gluten and its fractions, gliadin and glutenin |
 |
| Fig. 57: Viscoelastic behaviour of rehydrated vital wheat gluten,
gliadin and glutenin. (Photographs by Mühlenchemie GmbH & Co. KG.
Commercial vital wheat gluten was suspended in 70% v/v ethanol and then centrifuged.
The liquid phase was filtered through a filter paper. Solid and liquid phase
were then dried at 40°C under vacuum, and rehydrated prior to the rheological
demonstration.) |
The functional properties and interaction of the two components,
gliadin and glutenin, have been explained very clearly by Hoseney (1986; Fig.
56). As the photograph shows (Fig. 57), the pure gliadin obtained by washing
out and isolation is sticky and highly extensible; the pure glutenin is firm,
elastic and difficult to deform. It is the ratio and functional properties of
these two components of the gluten that determine the latter's viscoelastic
properties and thus the rheological properties of the dough. These properties
can be deduced from the Farinogram, but they are more apparent in an extension
curve like the Extensogram.
Without wishing to question the usefulness of the Alveograph method we
have to admit, on the basis of these examples, that the pattern and individual
characteristic data of the Alveograms do not reveal the dough properties of the
varieties and the ways in which they are changed by higher protein levels in
the flour – i.e. their current quality. The reasons for this have already been
discussed. For the sake of completeness we should mention that the necessity of
determining optimum water absorption has been recognized even by the supporters
of the Alveograph, and that a method of determining water absorption with the
Alveograph mixer was recently presented (see chapter on Modern Cereal
Analysis). Unfortunately it is still not possible to apply the water absorption
determined in this way as the amount of water needed to prepare the dough for
the Alveogram recording and thus to indicate the rheological properties of the dough
with dough consistencies close to those used in practice. The biaxial
stretching test is not fundamentally unsuitable as a measurement method, as
Dobraszcyk has shown (Dobraszcyk, 2002). At the time of its development and use
in France the Alveograph method was a suitable means of characterizing flour:
the flours obtained from wheat varieties with a soft grain structure and with a
low protein content and water absorption could be described and compared well
from one lot to the next by means of Alveograms. But now that even in France
the trend in wheat breeding is towards varieties with a hard grain structure
(which may result in mechanical damage to the starch grains during grinding)
and flours with higher protein levels and thus greater water absorption,
efforts are being made to adjust the Alveograph method to the new wheat
qualities.
The advantages of the Extensograph method in showing the
"rheological" behaviour of doughs at a consistency such as is used in
the production of very different types of baked goods have been used to define
the term "rheological optimum" (Schäfer, 1972). Schäfer suggested
taking this to mean the state of the dough most suitable for producing a bakery
item, which would naturally ensure the best results during baking and an end
product of the desired quality. The requirement for this state is doubtless
optimum quality of the flour, but it can be influenced and controlled by flour
improvers and ingredients that act on the properties of the dough. For this
purpose there are product ranges offering a choice of emulsifiers and enzyme
preparations designed to achieve the rheological optimum and enhance the final
result of baking. A further practical application of the rheological optimum
lies in the controlled treatment of flours with ascorbic acid at the mill and
with enzyme preparations (amylase, proteases, pentosanases, xylanases) and
other flourimproving ingredients based on lecithin, cystine, cysteine and
emulsifiers, which result in better inflation of the dough, increased water
absorption and ultimately better flavour and prolonged shelf life of the baked
products.
In practice, a flour can be optimized in respect of its baking properties
at a mill by blending flours with different dough properties. In a flour blend
the energy values of the Extensograms of the two flours making up the blend are
combined. The energy value of the blend lies between the values for the
components in accordance with their ratio in the mixture. But the volume yield
as a quality characteristic of the baked product is higher than that calculated
from the individual volume yields of the blended components (Bolling, 1980). This
effect is due to optimization of the viscoelastic properties of the flour blend
and is therefore recognizable from the ratio R/E, which is within the optimum
and desired range of the Extensogram for the blend. This value increases with
the extent of the difference between the dough properties "short" and
"soft" of the components of the blend, which ultimately result in
"normal" dough properties and achievement of a rheological optimum (Schäfer,
1972). But this does not mean that any arbitrary flour blend with two or more components
achieves the desired quality of a normal commercial flour: the components must
suit each other and have a high energy value as well as a sufficiently high
ratio.
To increase the protein content of a flour and improve its baking
properties it is usual to add 2 - 3% vital wheat gluten (dried gluten). Rehydrated
wheat glutens have different physical and rheological properties according to
the initial quality of the flour, the method of drying the gluten and the
temperature at which it was dried during its production at the starch factory.
When the glutens are added to the flour, these properties are clearly visible from
the viscoelastic properties of the dough and thus ultimately from its baking
performance. Even when dried gently, every wheat gluten suffers heat damage
which manifests itself in different degrees of reduction of the water-binding
capacity and extensibility and in an increase in the elasticity of the
rehydrated gluten or in its shortness. The properties of the rehydrated wheat
gluten can be tested sensorily, by hand, or by conducting extension and shear tests,
but an Extensogram of the flour mixture shows most plainly the effect of the
wheat gluten in conjunction with the proteins of the flour (Weipert and Zwingelberg,
1992). A flour with soft, weak dough properties requires a firm wheat gluten that
is not very extensible; a flour with short dough properties can be improved
with a soft, extensible gluten. It is really very surprising that the usual
addition of about 2% wheat gluten has such a decisive influence on the dough
properties of the flour.
All in all it may be said that Extensograms make it possible to
describe the quality of a flour clearly and with sufficient reliability. They describe
the viscosity or consistency of the dough, which can be checked by the water absorption
determined in the Farinograph. But what is even more important for processing the flour is that they describe the viscoelastic properties of the
dough and make a considerable contribution to the quality of the final baked
product.
The rheological properties of the freshly washed out wet gluten –
called "gluten structure" by the cereal processors – have long been
described by means of stretching by hand in a sensory test. This sensory rating
has been made more objective by mechanical, automatic washing and the use of
simpler instruments. A measurement of this kind carried out with a Glutograph or
texture analyzer or determined as a gluten index can doubtless be taken as a
guide. But it cannot completely describe the properties of the dough (Bloksma,
1990, Weipert, 1998a and Weipert and Zwingelberg, 1992). The behaviour of isolated
wet gluten and rehydrated dried gluten alone is quite different from their behaviour
when they are combined with starch, pentosans, lipids and other ingredients of
dough.
In the case of a wheat flour for bread making, the proteins are
expected to form a gluten as quickly as possible; the gluten must bind water
and thus determine the consistency of the dough. On the other hand a flour for making
wafers is expected to form gluten late or preferably not at all, so that the
mass retains a low viscosity. The suitability of wafer flours is determined
with the aggregation test and the viscosity test using a flow pipette (Gluzynski
et al., 2002). Both are ultimately a measurement of the viscosity and
viscoelasticity of the mass.
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