1.4.5. Farinograph
The importance and benefit of determining water absorption in
practical bread baking can be demonstrated by tests in which the amount of
water added is increased or reduced by 5% or 10% as compared to the water
absorption determined at 500 FU for four flours with different baking
properties (Fig. 54). With all four flours a reduction of the amount of water
added caused a noticeable thickening of the consistency of the dough in the
Farinograph and resulted in a considerable fall in the volume yield of the
baked product (in this case the bread rolls in the RMT standard baking test).
As expected, the addition of more water resulted in a softer dough consistency,
but the effect was initially a slight rise in the volume yield at a 5% increase in water absorption followed by a slight fall
in the volume yield at 10% additional water. The flours showed differences in
water absorption according to their quality, and the degree of their reactions
to the different amounts of added water varied also. The fact that good wheat
flours responded with increased baked volume to a higher dough yield or a
softer dough is a sign that they have quality reserves. It also explains why
some bread formulations require a larger amount of water, which would result in
a Farinogram of 450 or even 400 FU. In terms of volume yield, one and the same
amount of water led to different results in the products baked with the four
flours; this again confirms the proposition that the viscoelastic properties of
the dough are more important than its consistency.
Wheat flours described by bakers as "weak" reach the 500 FU
mark quickly and show no stability worth the name before undergoing a
considerable decline in viscosity (Fig. 55). The "strong" flours take
longer to develop before reaching the 500 FU line, where they remain for some
time at good stability and then show only a minor decline in viscosity. The
width of the curve for the two flours differs correspondingly. After reading
off the dough development time and stability it is possible to decide how much
mechanical development and energy input is needed. Such measurements support
the theory of the specific energy input requirement of flours, which makes it
possible to produce goodquality bread from weak flour if the latter's mixing
requirements are taken into account (Frazier et al., 1979).
The recording, reading and analysis of a Farinogram, the curve of
measurements obtained with a Farinograph, is described by the recommendations
of the manufacturer Brabender, the Farinograph manual (D'Appolonia and Kunerth,
1984) – a study of the use of the Farinograph – and finally stipulated by the
standard methods (ICC; AOAC).
In the bowl of a Farinograph the flour is mixed with the water to form
a dough; the dough is then developed mechanically and weakened mechanically by
over-mixing until its structure is destroyed. This procedure is measured and
recorded as kneading resistance in the form of torque by means of a
dynamometer; the recorded curve is therefore a force/time diagram from which
the work or energy input can be read off and calculated. The kneading
resistance is assumed to be the viscosity of the dough, although the remaining
properties of the dough such as its surface stickiness and adherence to the
walls of the mixer and the paddles contribute quite considerably to the
measured kneading resistance. In such tests this was most apparent with the
wheat varieties that produced doughs with a very sticky surface; the water
absorption capacity of these flours, which was high already, was increased even
further, which made the dough softer and more sticky still. In cereal
laboratories the viscosity of dough is often termed consistency. The viscosity
or consistency of the dough is stated in the Farinogram in relative units (FU)
specific to the Farinograph, on a scale from 0 to 1,000 FU.
In practical baking, determination of viscosity in the Farinograph
serves chiefly to establish the water absorption of a flour. This is the term
for the amount of water that has to be added to a flour to achieve a viscosity
of 500 FU. The water absorption of a flour depends on the latter's
water-binding capacity and thus determines the yield of the dough and the
amount of water to be added in the preparation of the dough. Besides the
swelling substances in the wheat (proteins and pentosans), the mechanically
damaged starch granules also contribute to the water-binding capacity of a
flour. The dough consistency of 500 FU is an empirical value felt to guarantee
the best possible processing properties; it has been adopted in the RMT
standard baking test for determining the amount of water to be added. Different
dough consistencies have proved most suitable for some other types of baked
products that require doughs of a soft or firm consistency.
 |
| Fig. 54: Baked volume as a function of dough viscosity and
water addition (the arrows indicate optimum water absorption as determined with
the Farinograph). |
A method has recently been developed which also makes it possible to
determine the water absorption of rye flours (Brümmer, 1987). Since rye doughs
react differently to mixing and rye flours result in a higher dough yield than
wheat, the water absorption of rye flours or their optimum dough yield is read
as viscosity after a mixing time of 10 min.
An analysis of a Farinogram shows the development time of the dough
(up to reaching the 500 FU line), stability (unchanged structure of the dough
without a fall in viscosity) and softening (fall in viscosity) at the end of
the mixing time. Whereas the readings on the Y axis of the Farinogram, expressed
in Farinograph units, denote viscosity and changes in viscosity during the mixing
process, the width of the Farinogram curve is read as the elastic properties of
the dough. This empirically based opinion of the cereal processors is correct
with the reservation that the width of the curve can be adjusted on the
Farinograph itself and thus influenced; it is not an absolute value comparable
from one instrument to another. The viscosity curve of the Farinogram gives
information on the structure of the dough, its tolerance to kneading or the
required input of mechanical energy and permits conclusions as to the intensity
of mixing that is tolerable or necessary.
 |
| Fig. 55: Farinograms of weak and strong flour |
There have always been "strong" flours whose Farinograms
show a second peak after the dough development time; such cases have recently
become more common, especially with unblended flours from certain newly bred
wheat varieties. The standard method recommends reading this second peak as
dough development time, but does not explain the reason for it. A glance at the
structure of wheat gluten shows that it consists mainly of the fractions
gliadin and glutenin (Hoseney, 1986). These two fractions differ considerably
in respect of their molecular structure and functional properties (Fig. 56 and
Fig. 57).
 |
| 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.) |
Whereas the insoluble fraction glutenin is known to form strand-like
shapes called fibrils that give the gluten its firmness and elasticity, the
gliadin fraction, which is soluble in alcohol, appears as a sticky mass and
filler between the fibrils and only contributes to the viscosity of the gluten.
Consequently, the viscoelastic behaviour of the gluten and the dough is closely
bound up with the ratio of these two components. A weak flour (a C wheat18
variety or a wheat lot with weak gluten) in which the functional properties of
the gliadin fraction prevail will bind water quickly but in smaller amounts; it
will form the dough faster but show a rapid fall in viscosity. A strong flour
(an E or A variety or a wheat lot with strong gluten) in which the functional
properties of the glutenin fraction prevail is characterized by a longer
development time and longer stability. In a flour rich in protein or gluten the
gliadin fraction present is initially responsible for the development of the
dough (measured by achievement of the dough viscosity of 500 FU) together with
part of the glutenin fraction, whereas a further part of the glutenin fraction requires
more mechanical energy input and produces a second peak. This behaviour of doughs
made from strong flour can be demonstrated by applying more mechanical energy (through
faster mixing) in the Farinograph (Fig. 55). In this case the gliadin component
of the dough is "developed" first but is soon weakened, whereas the
glutenin component requires more energy for development and resists the mechanical
energy during mixing. Such behaviour, known as stiffening, has already been
observed under the standard conditions of the Farinograph method with some
wheat varieties of American parentage. But similar behaviour is also found when
flours of greatly differing quality are blended (as was observed years ago with the weak Maris Huntsman variety and very strong
Canadian wheat of the CWRS class). On the basis of their Farinograms such
blends have been rated poor, although such a combination of flours with greatly
differing properties in the dough may result in a blend with very positive
effects, as Extensograms show. Here too, the reason for such behaviour is the
nature of the gluten fractions gliadin and glutenin, which depends on the
variety, and their ratio in the gluten. And here too, the gliadin of the weak
flour component results in early dough development and the glutenin of the
strong flour component leads to stiffening. Although the ratio of the two
gluten fractions is of genetic origin and thus a characteristic of the
particular variety, it may be influenced by the environment; besides climatic
conditions, such influences are chiefly the result of fertilizers. The properties
of the gluten and the dough that are characteristic of the variety and may be
influenced by the environment can be shown even more clearly with the Extensograph.
Note :
18 For German wheat classes see number 3
Besides water absorption, a Farinogram shows other quality
characteristics of the dough such as development time, stability and softening;
each of these provides important information in itself, but together they
represent a multitude of data. To simplify the measurements the Valorigraph value
was suggested at an early stage; it integrates these Farinogram characteristics
in a single number. Read from the Farinogram by means of a special template, this
value may lie between the theoretical figures 0 (for extremely weak flours) and
100 (for extremely strong flours). But these values can scarcely be achieved in
practice; as a result, the method did not meet with acceptance in spite of some
positive aspects. On the other hand the suggestion of reading a quality number (QN)
off the Farinogram as the time taken for the viscosity (consistency) of the
dough to fall by 30 FU after stability met with a positive response and has
been introduced into the ICC standard method as one of the quality characteristics.
This value integrates the development time and stability of the dough and
indicates its softening; determination of the QN permits a faster but no less
reliable evaluation of the Farinograms. For various reasons the Mixograph has
scarcely been used in Europe. A new measuring instrument with a number of uses
in the field of food rheology has recently been introduced: the Rheotec
Multigraph. Like the Farinograph, the instrument works on the principle of a
recording mixer but with controlled heating of the dough. It records the
changes in the viscosity (consistency) of the dough in the course of mixing and
heating which reflect the effect of the proteins, starches and enzymes in the
flour on the binding of water and theviscous properties of the dough. It might
be said that such measurement is a kind of "recording baking test"
(Sinaeve et al., 2001).6tygv The method is based on the tests for the effect of
additives and baking improvers on dough carried out by Nagao with a modified Farinograph
(Tanaka et al., 1980).
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