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34305-1 Applied Rheology
Volume 19 · Issue 3
Abstract:
The basic rheological properties of two Persian wheat flours - Tajan (11 % protein) and Back Cross Roshan (8 %
protein) and two Australian wheat flours-JANZ (12.9 % protein) and Rosella (8.6 % protein) have been charac-
terized. These properties have been interpreted via a damage function model. All samples could be reasonably
well described by the damage function model with a power-law relaxation spectrum. Although the shear stress-
es in the Australian samples were higher, the relaxation parameter G(1) and power-law exponent pfor the Aus-
tralian varieties were lower than those for the Persian samples and the damage functions were different. Since
protein contents were different, this indicates that the amount of protein is not the sole determinant of soft-
ness in the samples. The damage function fwas also calculated for all samples. This function gives a measure
of the softening due to working or kneading of the samples at a given strain level.
Zusammenfassung:
Die grundlegenden rheologischen Eigenschaften von zwei persischen Weizenmehlsorten, Tajan (11 % Eiweiß)
und Back Cross Roshan (8 % Eiweiß) sowie zwei australische Weizensorten, Flours-JANZ (12.9 % Eiweiß) und
Rosella (8.6 % Eiweiß) werden im Rahmen dieses Beitrags geschildert. Das damage-Funktionsmodell interpre-
tiert die Eigenschaften der Proben. Alle Proben konnten durch die damage-Funktion mit einem Potenzgesetz-
Relaxationszeitspektrum recht gut beschrieben werden. Obwohl die Schubspannungen in den australischen
Proben höher waren, waren die Entspannungsparameter G(1) und Potenzgesetz-Exponenten für die australi-
schen Sorten niedriger als die für die persischen Proben. Weiterhin wiesen die damage-Funktionen ebenfalls
Unterschiede auf. Die damage-Funktion fwurde ebenfalls für alle Proben berechnet. Diese Funktion gibt ein
Maß für die Anpassung der Verarbeitung sowie des Knetens der Proben bei einem bestimmten Belastungsni-
veau an.
Résumé:
Les propriétés rhéologiques de deux farines de blé persanes - Tajan (11 % de protéines) et Back Cross Roshan (8 %
de protéines) et de deux farines de blé australiennes-JANZ (12,9 % de protéines) et Rosella (8,6 % de protéines)
ont été caractérisées. Ces propriétés ont été interprétées par le biais d'un modèle d'endommagement. Tous les
échantillons ont pu être raisonnablement bien décrits par le modèle d'endommagement avec une loi puissan-
ce du spectre de relaxation. Bien que les contraintes de cisaillement soient plus élevées pour les échantillons
australiens, le paramètre de relaxation G(1) et l'exposant p de la loi de puissance des échantillons australiens
sont inférieurs à ceux des échantillons persans et les fonctions d'endommagement sont différentes. Puisque la
quantité de protéines est différente, cela indique que celle-ci n'est pas le seul critère déterminant le comporte-
ment des échantillons. La fonction d'endommagement fa également été calculée pour tous les échantillons.
Cette fonction donne une mesure de l'assouplissement dû au malaxage de l'échantillon pour une déformation
donnée.
Key words: dough rheology, wheat flour, relaxation, steady shear, elongation, damage function, power-law
A Comparison of the Rheology of Four Wheat Flour Doughs
via a Damage Function Model
Shiva Amirkaveei1*, ShaoCong Dai2, Marcus Newberry3, Fuzhong Qi2,
Mohammad Shahedi1and Roger I.Tanner2
1Department of Food Science, College of Agriculture, Isfahan University of Technology,
Isfahan, Iran
2Department of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney,
Sydney, NSW 2006, Australia
3CSIRO, Food Futures National Research Flagship and Division of Plant Industry, Canberra,
ACT 2601, Australia
* Email: sh_kavei@yahoo.com
Fax: x98.311.3912254
Received: 10.9.2008, Final version: 23.2.2009
© Appl. Rheol. 19 (2009) 34305
DOI: 10.1515/arh-2009-0012
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1 INTRODUCTION
Dough is made by combining flour, water and ener-
gy. The addition of sufficient mechanical energy
produces the distribution and hydration of flour
particles, allowing the formation of a unique vis-
coelastic material that exhibits both solid and liq-
uid properties [1, 2]. A small amount of other ingre-
dients such as yeast, salt, and preservatives is often
added. Dough is a complex system. It is basically a
cohesive three-dimensional cross-linked network
of gluten in which starch granules (of size ~ 10 mm)
are embedded [3]. Not only the hydrated protein
aggregates, but also the starch matrix and starch-
protein interaction give rise to viscoelastic proper-
ties and all these interactions affect the funda-
mental rheology [4]. In addition, dough rheology
plays an important role in the quality of the final
baking products and its understanding will lead to
progress in the food processing industries [3, 4].
Experimental rheological characterization
of wheat flour dough is desirable. It gives valu-
able information concerning the quality of raw
material, the textural characterization of fin-
ished products and properties needed for the
design and development of new equipment
[5 – 7]. During the past decade many attempts
have been made to apply fundamental rheo-
logical methods to dough. The early work
(1932 – 1937) of Schofield and Scott Blair estab-
lished the solid-like behaviour of dough and since
then there have been many investigations [2].
Bloksma (1962) was a pioneer in the measure-
ment of viscoelastic properties of wheat flour
doughs using creep tests [6].
Quantitative characterization of the rheolog-
ical properties of wheat flour and gluten doughs
has been difficult because of their complicated
strain dependent (non-linear) visco-elastic behav-
iour [6]. Bagley et al. [7] simulated dough rheo-
logical properties in uniaxial compression using
the upper convected Maxwell model [7, 8].
Leonard et al. [9] characterized the non-linear vis-
coelastic properties of hard and soft wheat flour
doughs using the Bird-Carreau constitutive mod-
el. Sweep strain experiments and stress relaxation
tests on Australian strong flour doughs indicate
that tests at higher shear strains can differentiate
flour types , and small and large strain rheology
of wheat gluten and comparison with the parent
flours have been investigated by Uthayakumaran
et al. [8]. Also it has been shown that stress relax-
ation tests were clearly useful in distinguishing
between long- and short-mixing flour [9]. Relax-
ation behaviour of dough and its gluten and
gluten fractions in both strong and wheat flours
has also been considered [10].
A striking feature of dough rheology is the
power-law behaviour of stress relaxation and
oscillatory response in the linear range. Gabriele
et al. [11] discussed the use of a power-law mem-
ory function for foods, including doughs, in the
linear range. Venkataraman and Winter [12] had
previously considered crosslinking polydi-
methylsiloxane (PDMS) near its gel point and
used a power-law memory function for the lin-
ear viscoelastic case and for the PDMS start-up
of simple shearing in finite strain. We note that
PDMS is much less shear-thinning than dough
and no “damping function” was used. Ng et al.
[13] used a Lodge-type model with a power-law
memory and a “damping function” of the KBKZ
type pioneered by Wagner [14], but this does not
equate to the “damage function” introduced in
[15] in 2007. Subsequent work by Ng et al. [16],
published in 2008, also uses the KBKZ damping
function. It does consider both shear and elon-
gation, but it is clear that the “damping function”
used is different to the damage function used in
[15] and [4]. Here we will use the damage func-
tion concept. We also note the recent work of
Berzin et al (2007) on In-line property measure-
ment of wheat starch during extrusion, which
will be of interest to industry [17].
Tanner et al. [15, 4] presented the “damage
function' model to describe multiple rheological
tests in 2008. To test this new model a new set
of experiments on bread dough included small
strain oscillatory behaviour, larger strain oscilla-
tory behaviour, simple shearing beginning from
rest, uniaxial elongation beginning from rest,
relaxation after sudden shear and recoil from
elongation. Wheat flour from various classes and
cultivars of wheat display great diversity in their
functional properties. The variation in function-
al properties of wheat cultivars are attributed
largely to its gluten quality and quantity [9]. Also
physical and chemical properties of wheat flours
are influenced by environmental factors and the
protein quality of hard wheat is more stable than
that of soft wheat and all of these factors affect
bread quality [18] so that bread making potential
is highly influenced by cultivar and environmen-
tal interactions.
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Two Persian wheat varieties have been cho-
sen; Tajan is known as a variety with good bread-
making quality while Back Cross Roshan flour
cannot be used for making good quality bread.
The objective of this study was rheological char-
acterization of Persian wheat flour doughs using
the damage function model (if applicable) and
comparison of their characteristics with some
Australian varieties. It seems that the damage
function model, for the tests made, is a reason-
able way to interpret and compare different flour
doughs from different wheat breeds, as will be
discussed below.
2 THE MATERIALS AND EXPERIMENTAL
METHODS USED
2.1 GRAIN SOURCES AND DOUGH
PREPARATION
Materials used for this study were two brands of
Australian commercial flour (JANZ and Rosella)
and two brands of Persian pure cultivars (Back
Cross Roshan and Tajan). Samples of Australian
wheat prepared by CSIRO plant industry, ACT and
samples of Persian wheat cultivars (Back Cross
Roshan and Tajan) were obtained from the Iran
Agriculture Research Center. All wheat samples
were tempered for 18 hours to 15.5 % moisture
and milled into straight grade flour using a Buh-
ler laboratory mill [19]. Table 1 shows some of the
chemical analyses and the mixograph values for
these four samples. Then the doughs were pro-
duced in a 10 g mixograph by mixing flour, dis-
tilled water and 200 mg of salt up to optimum
dough development time
2.2 RELAXATION, OSCILLATORY AND STEADY
SHEAR MEASUREMENTS
The dough used for relaxation, oscillatory and
steady shear measurements was stored in a
sealed bag after mixing. All of the shearing ex-
periments were carried out on a Paar Physica
MCR 301 rheometer. Here the parallel plates had
a diameter of 25 mm and the gap was set to 2 mm
for measurements. Slippage during testing was
prevented by two pieces of sandpaper that had
been glued to the parallel plates. Before testing
calibration of the rheometer was performed,
then the sample was mounted on the lower plate
and compressed between the plates by moving
down the upper plate to a set gap. Excess dough
was trimmed and the edge of the sample was
coated with petroleum jelly to prevent moisture
loss. After that the sample was allowed to relax
for 40 minutes.
Steady shear measurements have been
done at shear rates of 0.001, 0.01, 0.1 and 1.0 s-1.
For oscillatory shear measurements, frequency
sweeps in the frequency range of 0.01 – 30 Hz
were conducted at strain amplitudes of 0.1, 1, 5
and 10 %, respectively. After each test the sam-
ple was unloaded and a new sample was up-
loaded for the new frequency sweep. Stress
relaxation tests after application of a sudden
shear strain at time zero were done at four dif-
ferent initial shear strains: 0.1, 1, 5 and 10%.
2.3 ELONGATIONAL MEASUREMENTS
For the elongation measurements, the sample
after mixing was first formed in an aluminum
cylinder with an inside diameter of 30.6 mm and
stored in a sealed bag to relax for 40 minutes. The
sample then was transferred to an Instron 5564
rheometer to perform the elongation tests. The
sample was confined between two parallel
plates. One is a fixed lower plate with a diame-
ter of 31.0 mm, the other is a moving upper plate
with a diameter of 30.3 mm. The force in the sam-
ple was found using a sensitive load cell which
had a measuring range of 10 N. Before testing the
rheometer was calibrated without any loading.
The aluminum cylinder containing the dough
was then fitted to the upper plate. Simultane-
ously, the upper plate was brought down gently
until the sample was compressed properly
between the plates. Then the excess dough after
compressing was wiped upwards or downwards
evenly to wrap the upper and lower plates,
respectively, so that the sample would be well
attached to the plates during elongation. In addi-
tion, to prevent moisture loss, a thin layer of Shell
petroleum jelly was applied to the outside of the
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Volume 19 · Issue 3
Table 1:
Chemical analysis and mix-
ing properties of wheat
flour for four varieties.
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sample. Then the mounted sample was com-
pressed to the set gap of 9 mm and allowed to
relax for a further 10 minutes. The tests were con-
ducted at elongation rates of 0.001, 0.01 and 0.1
s-1, respectively. The samples were stretched until
they were broken. At the same time a digital cam-
era was used to capture the diameter of the cylin-
drical dough specimen at a rate of 25 fps and the
results were downloaded to a computer as a
movie. The circularity of the cross section was
generally excellent, as judged from specimen
after cutting in half. Hence, the elongation stress
s= F/Acould be calculated, where Fis the elon-
gation force measured by the load cell and Ais
the minimum cross sectional area of the sample.
The actual rate of the extension at the centre of
the sample was found from the variation of
diameter in the sample with time [4].
3 MODELLING OF BREAD DOUGH
A modified Lodge elastic fluid model with a dam-
age function has been used by Tanner et al. [4, 15]
to describe the mechanics and rheology of bread
dough. It has been shown that the rheology of
this soft, starch-filled solid could be described by
a constitutive equation of the Lodge form:
(1)
where s(t) is the stress tensor at time t, Pthe pres-
sure, Ithe unit tensor and C-1(t’) is the Finger strain
tensor at time t’ computed relative to the con-
figuration at the present time t. The memory
function m is assumed to be of the power-law
form [4, 15]:
(2)
where the constants pand G(1) can be found from
relaxation testing or by oscillatory testing at
small amplitudes [15]. Note that the constant G(1)
here signifies the numerical value of the stress
relaxation G(t) at t= 1 s – the constant does not
have the dimensions of stress. The “damage
function” ƒis assumed to be a function of the
Hencky strain at time t, computed relative to the
state of rest which is assumed to occur for times
less than zero.
In the relaxation test we also used the fol-
lowing power-law equation for the relaxation
function, which follows from the use of Eq. 2 [4]
(3)
Results for storage and loss moduli G’(w), G’’(w)
as functions of the applied frequency w(rad/s) are
also described by a power-law relation [15],G’(w)
= G’(w)w-p, with a similar relation for G’’. Also it is
known that the phase angle G’’ is given, for a
power-law material by the constant value
(4)
So the phase angle is defined by the exponent p;
only two constants, G(1) and pare needed to
describe the linear viscoelastic behaviour of
these solids, which are assumed to be incom-
pressible [20].
From Eqs. 1 and 2 the shear stress sis a func-
tion of p, G(1), g
·and t; G(1) has the dimension of
Pa·sp. It was shown [15] that with the model one
can test if it is reasonably concordant with exper-
iment by plotting s/g
·pas a function of strain. The
stress function can be evaluated for various pval-
ues. Instead of plotting against g, we use the
Hencky strain (see Figure 5) which is related to g
by [21]:
(5)
Elongational stress divided by e
·pwas also plot-
ted as a function of eH= e
·tfor our elongation mea-
surements (see Figures 7 - 8). The superposition
is reasonable and fracture usually occurs at eH~
3. We can again use Eqs. 1 and 2 to compute the
response to a steady elongation (e
·constant)
imposed at t= 0 and we find the following equa-
tion for calculating the stress (when f= 1.0)
(6)
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Thus the model predicts for a given strain eHthat
the tensile stress sis also proportional to e
·pas
indicated by the experimental data (Figures 7 –
8). For a shearing flow of shear rate gbeginning
at t= 0, one can evaluate the integrals explicitly,
finding the viscometric functions with the same
form of shear rate and strain dependence as in
elongation [15, 4].
(7)
where g= g
·t. To evaluate the damage function f
we divide the average measured stresses at a giv-
en strain with those calculated from Eqs. 6 and 7.
4 RESULTS
4.1 RHEOLOGICAL CHARACTERIZATION OF
PERSIAN WHEAT FLOUR
In the two Persian samples the rheological char-
acterizations of flour were conducted by using
different rheological tests including stress relax-
ation, steady shear, oscillation and elongation.
The elastic nature of dough is quite apparent in
a large deformation, such as simple shearing
flow. Since the total shear strain acquired in the
deformation is g= g
·t, an increase in stress with
gis consistent with the idea that a part of the
stress response is elastic in nature (stress ~
strain). It may be argued that the initial part of
the response is independent of the strain rate, up
to a strain of O(10-1) [22]. After a peak strain, the
shear stress quickly decreases, indicating a par-
tial failure of the elastic network. The peak strain
is an increasing function of the strain rate. In Fig-
ure 1, the shear stresses are plotted versus strain
for both Persian varieties and the Australian sam-
ples which will be considered in the following at
shear rates of 1 and 0.01 s-1. Values show that
Tajan has a higher shear stress and fracture point
than Back Cross. It seems that its protein network
is stronger than Back Cross. However, despite dif-
ferent protein contents, the shear values are very
close and there is no significant difference
between the two varieties.
The time dependence of the relaxation mod-
ulus in the linear limit (with strain magnitude of
0.1 %) for both Persian wheat flours and the Aus-
tralian samples is shown in Figure 2. The rheome-
34305-5 Applied Rheology
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Figure 1 (left above):
Shear stress as a function of
strain for Persian doughs
and Australian doughs at
strain rates 1 and 0.01 s-1.
Figure 2 (right above):
The stress relaxation of Per-
sian doughs and Australian
doughs at an applied strain
amplitude of 0.1 %.
Figure 3 (left below):
Load versus elongation for
Persian flour doughs in
elongation rates 0.1 and
0.001 s-1.
Figure 4 (right below):
Effect of variety and its
protein content on uniaxial
elongation and rupture
strain. Protein content was
11 % for Tajan and 8 % for
Back Cross Roshan. Elonga-
tion test was conducted at
elongation rate e
·= 0.01 s-1.
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ter in the stress relaxation mode gives the stress
relaxation modulus, G(t), as a function of time and
as the quota of the measured stress, s(t), to the
initial strain [4]. Despite different protein con-
tents, these two varieties showed very similar rhe-
ological behaviour as measured by the stress
relaxation modulus. The initial value for Tajan is a
little higher but after a while (t= 1 s), it becomes
lower than Back Cross (Figure 2). The linear relax-
ation function is assumed to be described by Eq. 3.
The slope of G(t) is - p, and the value of Gat t= 1 s
gives G(1). Relaxation data show that at higher
strains the rheological behaviors for both flours
are almost the same but in the linear region
(strain = 0.1 %) a difference of behaviour with a
difference in power law index has been observed
(pfor Tajan = 0.38, pfor Back Cross = 0.32).
The load in elongation versus Hencky strain
is shown in Figure 3. At an elongation rate of
0.001 s-1 the rupture strain for Tajan is nearly the
same as for Back Cross Roshan but the load is
higher, while at 0.1 s-1 the rupture strain for Tajan
is higher and it is around 2.8 since that of Back
Cross Roshan is 2.4. A possible reason may be the
difference in protein content, so that the sample
with the lower protein content, at higher strains,
has a lower resistance to rupture because of the
weak protein matrix.
Stress increases slowly at low strains or
extension level (Figure 4). The sharp increase in
stress with increasing strain level is known as
strain hardening, and then the dough sample rup-
tured. The strain and viscosity at which the dough
samples broke or ruptured were simple measures
of dough strain-hardening properties and corre-
spond to extensibility and maximum resistance to
extension measured in traditional dough testing.
Increasing protein content increases the strain
hardening properties of dough, as measured by
the elongational rupture and rupture strain; Tajan
(11 % protein content) has a higher stress value
than Back Cross (8 % protein content).
4.2. COMPARING RHEOLOGICAL CHARACTERIS-
TICS OF AUSTRALIAN WHEAT FLOUR AND PER-
SIAN WHEAT FLOUR
We considered differences in rheological behav-
iour of two Australian wheat flours (JANZ and
Rosella) in comparison to the Persian samples
(Tajan and Back Cross Roshan) in various rheo-
logical tests including steady shear, stress relax-
ation, shear oscillation and elongation. Going
back to Figure 1, the results of steady shear at two
shear rates 1 and 0.01 s-1 can be compared for all
samples. The results show that the fracture point
happens at higher stress values for both Aus-
tralian flours than for the Persian wheat flours.
It seems that elasticity in the protein matrix for
them is higher than for the Persian samples. Cal-
culating p for different samples shows that p val-
ues for Australian wheat samples are between
0.2 – 0.3 while they are higher for Persian wheat
flour - between 0.3 – 0.4 (Figure 2). Bearing in
mind that each of the shear rates used a new
sample, with some inevitable variability, the
agreement is reasonable. Estimates of errors in
the slop pare given; we compared different sam-
ples and relaxation oscillatory data (only the
mean dis given here).
Table 2 shows calculated values for these
four varieties, data show that power-law para-
meters for Australian wheat samples are lower
than for Persian wheat, however G(t) is lower for
the Australian samples. Uthayakumaran et al. [8,
23] have shown that the pvalue is not apprecia-
bly affected by changes in water content for a
strong flour dough, although G(t) changes. Also
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Figure 5 (left):
Shear stress data for Tajan
dough plotted t/g
·pas a
function of the Hencky
strain. Fracture occurs at
eH~ 2.9.
Figure 6:
Shear stress data for JANZ
dough plotted t/g
·pas a
function of the Hencky
strain. Fracture occurs at
eH~ 3.
Table 2:
Calculated constants for
wheat flour samples
(din degrees).
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with comparable protein contents and p values,
there is no correlation between protein content
and pvalue for Persian wheat samples while for
Australian wheat flour there is. Comparing pfor
Persian wheat shows that Tajan with the higher
protein content has a higher p. On the other
hand, for Australian samples, a lower protein
content sample shows a slightly higher value of
p. By using Eqs. 1 and 2 and plotting s/g
·pand s/e
·p
as a function of Hencky strain, it can be clearly
observed for the four varieties that the damage
function model correlation is reasonable (Figures
5 – 6 for shear and Figures 7 – 8 for elongation of
one Persian (Tajan) dough and one Australian
(JANZ) dough).
Strain hardening, a rapid increase in viscos-
ity at higher strain level, is thought to be respon-
sible for the ability of dough to expand and retain
the gas involved during fermentation and bak-
ing. Studies have shown that flours with good
baking quality tend to have much greater strain
hardening than flours that perform poorly in bak-
ing [24]. Figures 9 and 10 show plots of stress ver-
sus Hencky strain at elongation rates of 0.1 and
0.001 s-1. Result shows that at 0.1 s-1 the rupture
for Persian samples and Rosella happened at eH~
2.7 while for JANZ rupture happens at eH~ 3.1. In
0.001 s-1 for Rosella and Back Cross rupture hap-
pens in eH~ 2.5 but for Tajan and JANZ it happens
at a higher strain value eH~ 3, while rupture
'stress' for Persian samples is higher than that for
Australian samples. This increase in rupture
stress and rupture strain with increasing protein
content was confirmed by Uthayakumaran et al.
(1999) using a small scale extension tester. Also
increasing the glutenin to gliadin ratio increases
the elongational rupture viscosity but decreases
the rupture strain [23].
For steady elongation and shear discussed
above, we can then find f for elongation and
shear by using the result of Table 2 and Figures 5
- 8, plus Eqs. 1 and 2. The damage stress reduction
factor fcan be plotted as a function of Hencky
strain, Figure 11, for elongational data of four vari-
eties. (Formulas for fare given in the caption of
Figure 11). Note that the drop from f=1 at eH= 0
is very fast for all samples while the decrease pat-
tern in both sets of samples is seen to be differ-
ent. The damage function fmust be 1.0 at small
enough strains; however, by a strain magnitude
eHof only 0.01 fdrops abruptly to the values
shown in Figure 11, and the rapid rise for eH< 0.01
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Figure 7 (left above):
Elongational stress data for
Tajan plotted s/e
·pas a
function of the Hencky
strain (p = 0.38). Fracture
happens when eH~2.9.
Figure 8 (right above):
Elongational stress data for
JANZ plotted s/e
·pas a func-
tion of the Hencky strain
(p = 0.27). Fracture happens
when eH~3.2.
Figure 9 (left below):
Elongational stress versus
Hencky strain for all sam-
ples at an elongation rate
e
·= 0.1 s-1.
Figure 10 (right below):
Elongational stress versus
Hencky strain for all sam-
ples at an elongation rate
e
·= 0.001 s-1.
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is not shown in this figure. This rapid change is
discussed in [15]. The sudden drop is possibly due
to reduced starch particle-protein network inter-
action at quite low stresses. Figure 11 shows that
the Persian samples are softer at Hencky strains
less than unity.
We also conducted G' and G'’ measurements
at strain amplitudes of 0.001, 0.01, 0.05 and 0.1 for
all samples. As it has been shown in different stud-
ies the range of maximum strain that results in a
linear response is very small for dough [8]. For
wheat flour, the maximum shear strain permitted
in the linear range is around 0.001 (0.1 %). We used
this value in our (linear) oscillatory tests. Results
for storage module G'(w) as a function of applied
frequency w(rad/s) are shown in Figure 12. The
slope of the curves here is again pfor small ampli-
tudes (Eq. 3). Exponent pis close to the value of p
from relaxation as it has been shown in Figure 12.
It is in the range of 0.2 - 0.3 for Australian samples
and 0.3 - 0.4 for Persian samples and G' > G'’' indi-
cating soft solid behavior.
5 DISCUSSION
We have shown that it is possible to differenti-
ate between different flours by subjecting the
doughs to different rheological tests. It seems
that stress relaxation experiments at a strain of
0.1 % and determination of power-law pand G(1)
could be good indices for dough behaviour in that
stronger and more elastic flours show lower G(1)
and p, but it seems that these differences are not
solely related to protein content. The damage
function fis also clearly different for the various
flours. Another relatively high strain deforma-
tion that brings out subtle differences between
different flour types is the constant shear rate
flow between two parallel plates [22], where the
strain increases linearly with the time of defor-
mation. Typically Australian flours have higher
values of stress than Persian samples, so it seems
that they have stronger matrices. Of course in
shear flow, it should be mentioned that the val-
ue of the stress peaks are strain-rate dependent.
They are consistent with the assumption of a
finite elastic network in gluten.
In oscillation, with a strain amplitude of
0.1 %, the power-law index is very close to the
value in relaxation but we believe oscillation
tests are not as convenient as data from relax-
ation tests, so that they were not used as a crite-
rion for discriminating flours with similar protein
qualities. We note that stress relaxation tests at
various strain amplitudes are a convenient way
of finding the damage function [4]. Testing of the
samples by extensional testing seems to be a
good way for determination of sample differ-
ences since extensional properties of the flours
in constant elongation rate are quite different.
Elongation rate is very important in dough reac-
tions and baking so that at lower elongation
rates the reaction of dough is related to the pro-
tein matrix strength, protein quantity and sam-
ple behaviour.
Typically, the wheat breed has a significant
effect on dough rheology so that in all experi-
ments Australian flours showed stronger resis-
tance and a more elastic protein network. It
should be noticed that determination of opti-
mum water absorption and working are very
important factors for producing reliable and
reproducible data in experiments.
6 CONCLUSION
Comparing four rheological tests for these four
samples reveals that the breed has a remarkable
effect on the protein quality but differences in
protein quantity cannot be the sole reason for
having different rheological behaviour. It seems
that using chemical and analytical information
about the wheat protein besides rheological
characteristics can give a good perspective of the
protein efficiency for producing special products.
Finding a correlation between rheological and
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Applied Rheology
Volume 19 · Issue 3
Figure 11 (left):
Damage function f as a
function of Hencky strain
for elongation measure-
ments of four varieties.
The f function for JANZ:
f = - 0.0958logeH+ 0.0669,
Rosella: f = - 0.0841logeH+
0.0542, Tajan:
f = - 0.0144logeH+ 0.0561
and for Back Cross:
f = - 0.0695logeH+ 0.0025
(eH £0.1) and
f = - 0.0303logeH+ 0.0417
(eH> 0.1). The rise in f for eH
is not shown;
clearly f(0) = 1.0.
Figure 12:
G’ versus frequency at two
strain amplitudes (0.1 and
10 %) for Australian and Per-
sian samples. The slope of
power-law fitted curves has
been shown for each flour.
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analytical data may permit one to define some
indices for each variety as a control factor,
although it might be very hard to characterize
such flours. As data showed the damage model
seems quite useful for the description and com-
parison of the rheological behaviour of different
doughs. With an overall look at different experi-
ments, it seems that relaxation and elongation
and steady shear are reliable tests for ranking
flours but oscillatory tests also give useful com-
plementary information.
ACKNOWLEDGMENTS
The authors would like to thank you Iranian gov-
ernment for financial support and also Mr. Erwan
Bertevas, school of Aerospace, Mechanical and
Mechatronic Engineering at the University of
Sydney.
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