A Review on Rheological Properties and Measurements of Dough and Gluten
D.N. Abang Zaidel,
The field of rheology has seen a wider application in the food industry recently although, it is a complex concept and that most food systems possess non-ideal characteristics. Nevertheless, the rheological behavior of foods are able to be determined using various techniques and equipment. Studies on rheological properties related to dough and gluten are often challenging due to its variance in nature and high dependence on many factors. This study attempts to give a review on the various types of experimental techniques and set-up used in quantifying rheological properties of dough and gluten. The rheological properties are defined and the behaviors are described by inducing stress and strains in small and large deformation studies.
Received: March 26, 2010;
Accepted: May 10, 2010;
Published: August 23, 2010
Rheology is defined as a study of the deformation and flow of matter (Bourne,
2002). The applications of rheology have expanded into food processing,
food acceptability and handling. Many researches have been conducted to understand
the rheology of various types of food such as food powder (Weert
et al., 2001; Grabowski et al., 2008),
liquid food (Sabato, 2004; Park,
2007), gels (Michon et al., 2004; Foegeding,
2007), emulsions (Robins et al., 2002; Corredig
and Alexander, 2008) and pastes (Abu-Jdayil et al.,
2002; Lim and Narsimhan, 2006). Vast food materials
show a rheological behavior that classifies them in between the liquid and solid
states; meaning that their characteristic varies in both viscous and elastic
behaviors. This behavior, known as viscoelasticity, is caused by the entanglement
of the long chain molecules with other molecules. Figure 1
shows the creep and recovery test on the ideal elastic, ideal viscous and viscoelastic
materials. The ideal elastic materials have the ability to recover to its original
shape upon the removal of stress while the stress acted on the ideal viscous
materials caused them to deform and it is non-recoverable. By combining both
the ideal elastic and viscous behaviors, the viscoelastic materials exhibit
behavior in recovering some of its original shape by storing the energy. They
show a permanent deformation less than the total deformation applied to the
||Creep and recovery curves for ideal elastic, ideal viscous
and viscoelastic materials (Steffe, 1996)
Dough and gluten consist of complex structures of protein and carbohydrate
cross links and due to this many studies had been reported on their rheological
properties. The focus of this study is to provide a description of rheological
properties of dough and gluten, to highlight the various types of experimental
techniques and set-up used in quantifying their rheological properties from
past and current studies initiated.
Rheological behavior and development of dough and gluten: Rheological
behavior of dough and gluten can be determined by two distinct measurements
that are fundamental and empirical. Studies on the fundamental rheology of dough
and gluten are usually carried out using small deformation while the empirical
measurements are measured using large deformation. Nonetheless, fundamental
dough and gluten rheological testings using large deformation are growing popularity
with the presence of newer techniques and equipment. Ferry
(1970) described that the rheological behavior of gluten is related to the
rheological properties of synthetic polymer where the fundamental rheological
properties of polymers reflect the degree and type of cross-linking of the polymers.
Thus, the rheological behavior of dough was predicted using molecular models
of gluten development during mixing by Belton (1999)
and Letang et al. (1999) as shown in Fig.
2 and 3. In these models, gluten development mainly involves
glutenin proteins interactions with each other in the loop by disulphide bonds.
At the early stage of mixing, the gluten fibrils are in contact with the mixer
blade, the sides of the bowl and other flour particles. The hydrated gluten
fibrils and starch granules are continuously dispersed throughout. Glutenins,
which are the long polymeric proteins, are folded and the chains are in random
orientation. As mixing proceeds, more protein becomes hydrated and the glutenins
tend to align because of the shear and stretching forces imposed.
||A model for the molecular structure of gluten. HMW subunits
are approximately by linear polymers, interchain disulphide links are not
shown. Other polymers are approximated by spheres (Belton,
||Molecular interpretation of gluten development (a) beginning
of mixing, (b) optimum development and (c) overmixing (Letang
et al., 1999)
At this stage, gluten networks are more developed by the cross-linking of
protein with disulphide bonds. At optimum dough development, the interactions
between the polymers cross-links are becoming stronger which leads to an increase
in dough strength, maximum resistance to extension and restoring force after
deformation. When the dough is mixed longer past its optimum development, the
cross-links begin to break due to the breaking of disulphide bonds. The glutenins
become depolymerised and the dough is overmixed. The presence of smaller chains
in the dough makes the dough stickier. The monomeric proteins, gliadins form
a matrix within the long polymer networks and contribute to resistance to extension
by forming viscous behavior. Increasing the interactions between protein polymers
increases gluten viscous resistance and resistance to extension. It was said
that gliadins acted like a plasticiser, promoting viscous behavior and extensibility
of gluten (Kuktaite, 2004).
Many works have been attempted on determining the rheological properties of
dough (Khatkar et al., 2002; Uthayakumaran
et al., 2002; Sliwinski et al., 2004a;
Chin and Campbell, 2005; Chi et al., 2005; Indrani
and Rao, 2007; Skendi et al., 2010) and gluten
(Amemiya and Menjivar, 1992; Janssen
et al., 1996a; Kieffer et al., 1998;
Khatkar et al., 2002; Tronsmo
et al., 2003; Song and Zheng, 2008). In application
studies, the rheological properties are related to the end-product quality such
as bread loaf volume (Janssen et al., 1996a; Kokelaar
et al., 1996; Kieffer et al., 1998;
Tronsmo et al., 2003; Sliwinski
et al., 2004b; Dobraszczyk and Salmanowicz, 2008),
texture (Uthayakumaran et al., 2002; Vetrimani
et al., 2005; Jacob and Leelavathi, 2007;
Sudha et al., 2007) and sensory attributes (Bhattacharya
et al., 2006; Lazaridou et al., 2007).
Factors affecting dough and gluten rheological properties: Rheological
properties of dough and gluten during mixing are affected greatly by the flour
composition (low or high protein content), processing parameters (mixing time,
energy, temperature) and ingredients (water, salt, yeast, fats and emulsifiers).
Studies were conducted to investigate the effect of protein content on the gluten
quality and rheological properties (Janssen et al.,
1996a; Tronsmo et al., 2003; Sliwinski
et al., 2004c), on bread making quality (Janssen
et al., 1996a; Sliwinski et al., 2004b)
and also on volume expansion resulted from frying (Chiang
et al., 2006). These works, conclusively suggested that the strong
flour produces a better gluten and dough quality than the weak flour in terms
of giving a higher response in extensibility, bread loaf volume and height and
also volume expansion.
Mixing is an important step in producing gluten with desired strength as to
produce a good quality end-product. Processing factors during flour-water mixing
include the mixing time, work input, mixer type and temperature. In order to
achieve optimum dough development, the mixing time and work input must be above
the minimum critical level (Angioloni and Dalla Rosa, 2005).
Different wheat flour has different optimum mixing time (Hoseney,
1985). A longer mixing time is expected for mixing dough from strong flour.
It is probably due to the dense particles of strong flour and slower water penetration
(Hoseney, 1985). Sliwinski et
al. (2004c) reported that a positive correlation was observed between
dough mixing time and the percentage of glutenin protein in flour. Dobraszczyk
and Morgenstern (2003) related optimum mixing time of dough with the development
of the glutens networks and monomers. Increasing mixing time and work input
above the optimum level during mixing induces the changes in mechanical properties
of dough (Cuq et al., 2002). Whilst mixing speed
influenced the development of gluten during dough mixing through the intensity
of mixing imparted on dough, insufficient mixing intensity would result in weak
gluten networks which bring failures in baking performance (MacRitchie,
Water is responsible in hydrating the protein fibrils and start the interactions
between the proteins cross links with the disulphide bonds during dough mixing.
Too much water addition to the flour will result in slurry and too little water
results in slightly cohesive powder (Faubion and Hoseney,
1989). Hence, an optimum water level is required to develop cohesive, viscoelastic
dough with optimum gluten strength. While the optimum water level differs from
flour to flour, the strong flours require higher water level than weak flours
largely due to the higher protein content and dense particles in the strong
flours. Protein content is known to be an important factor in determining the
water uptake of flour (Sliwinski et al., 2004c).
Mani et al. (1992) and Janssen
et al. (1996a) reported that the G and G decreased as
the water content of dough increased. Ablett et al.
(1985) explained the effect of water content on gluten networks in terms
of a rubber network such that its elongation reduced as water content increased
as if in rubber network. However, for dough, the elongation increased as water
content increased. It was suggested that the soft continuous phase of dough
will swell in direct proportion of free-water which is responsible in the increase
of the elongation (Ablett et al., 1985).
Sodium chloride or commonly known as salt is said to have a strengthening or
tightening effect on the gluten during mixing of dough (Niman,
1981). Salt must be added early in the dough-mixing to give maximum dissolution
time and accelerate gluten formation, tighten the dough and increase the mixing
time. Salt is used to overcome the low pH of dough since the effect of pH will
alter the mixing time; a low pH gives a shorter time and a high pH gives a longer
time (Hoseney, 1985). Roach et
al. (1992) suggested that the influences of salt on the protein solubility
affect the dough properties. Salt decreases the solubility of protein in the
wheat flour dough as its concentration increases. Salvador
et al. (2006) found that the elastic modulus (G) falls slightly
in the presence of salt. This reduction is probably due to the decrease in inter-protein
hydrophobic interactions which reduce the tendency of the proteins to aggregate
and thus reduce the elasticity. The amount of salt added into the dough mixing
can be varied from 1.8-2.1% on flour basis (Farahnaky and
Hill, 2007). However, due to increase concern in health related issues by
consumers in food intake, addition of lower amount of salt has become one of
the main focus in recent studies (Farahnaky and Hill, 2007;
Lynch et al., 2009). Omission of salt entirely leads to a significant
reduction in dough and bread quality and also the sensory attributes of bread,
where the bread was described as sour/acidic and having yeasty flavour (Lynch
et al., 2009).
RHEOLOGICAL MEASUREMENTS OF DOUGH AND GLUTEN
The rheological measurements used are dependent on foods types although in general, the small deformations are more meticulous than the large deformation testing. In small deformation testing, the rheological properties of foods are well-defined by exerting very small strain on the food. Large deformation testings on food material are easier to perform, the equipments are inexpensive comparatively and they are more commonly used in the food industry. Table 1 shows the various types of rheological testing methods available for obtaining different rheological parameters using different equipment.
Small deformation measurement: In small deformation measurement, the
tested material is assumed continuous, has regular shape and is exerted by small
strain (1-3% maximum) (Bourne, 2002). Tests performed
by various researchers to determine the rheological properties of dough and
gluten include the dynamic oscillation (Amemiya and Menjivar,
1992; Khatkar et al., 1995; Janssen
et al., 1996a, b; Uthayakumaran et al., 2002; Tronsmo et al., 2003; Sivaramakrishnan
et al., 2004), creep recovery (Janssen et al.,
1996a; Tronsmo et al., 2003; Sivaramakrishnan
et al., 2004; Onyango et al., 2009)
and stress relaxation tests (Rao et al., 2000;
Li et al., 2003; Song and
Zheng, 2008; Bhattacharya, 2010).
|| Rheological measurement for dough and gluten
Dynamic oscillation: The dynamic oscillation test is most suitable in
testing the rheological properties of viscoelastic material. The test material
is applied with sinusoidal oscillating stress or strain with time in a dynamic
oscillation shear measurement. When subjected to a sinusoidal strain (γ
= γo sin ωt), the viscoelastic material responds with a
sinusoidal stress (σ = σo sin ωt) which depends on
the properties of the material. The elastic component is accounted as the storage
modulus (G) and the viscous component is measured as the loss modulus
(G). The ratio of the viscous to elastic modulus (G/G) is
equal to the tangent of the phase angle (tan δ). A material having higher
degree cross-linking is expected to have a low tan δ. In the study of Tronsmo
et al. (2003), wet gluten was tested with a small strain of 2% and
frequency between 0.005-10 Hz. They reported that the elastic modulus (G)
was higher than the viscous modulus (G). This result agrees with studies
by Amemiyar and Menjivar (1992) who found that the storage
modulus (G) for all tested doughs are higher than the loss modulus (G).
They further described that the gluten network behaves like a cross-linked polymer
at the tested frequency. Uthayakumaran et al. (2002)
who conducted a study on rheological behavior of wheat gluten using dynamic
oscillation testing found that both the elastic and viscous modulus of flour
doughs were significantly higher than gluten doughs. This indicates that starch
content in the flour dough influence the viscoelasticity of the flour dough.
Other work which utilised this testing method on dough include studies on effect
of different protein content (Amemiya and Menjivar, 1992;
Janssen et al., 1996a; Tronsmo
et al., 2003), water level (Uthayakumaran et
al., 2002) and mixing time (Amemiya and Menjivar,
1992; Janssen et al., 1996a) on the rheological
properties of dough and gluten. Tronsmo et al. (2003)
found that dough with higher protein content gave lower G and G
but higher tan δ. Janssen et al. (1996a)
found that the resistance to small deformation was higher and more elastic for
gluten with higher protein content and as the angular frequency (ω) increased,
G increased more than G, indicating a viscous behavior of gluten
due to more bonds are involved in the response of stress or strain. Generally,
it can be concluded that gluten from poor quality wheats are rheologically characterised
as less elastic and more viscous than glutens from good quality wheats (Khatkar
et al., 1995; Janssen et al., 1996a;
Tronsmo et al., 2003).
Creep recovery: Creep recovery is performed by subjecting the material to a constant shear stress and the shear strain is monitored as a function of time.
||Creep analysis curves for (a) pure wheat (O), pure rice (lgrice,
sgrice) and composite flour (comp-lg, comp-sg) (Sivaramakrishnan
et al., 2004) and (b) gluten with different protein content (from
two types of wheat, i.e., Obelisk and Katepwa) (Janssen
et al., 1996a)
Sivaramakrishnan et al. (2004) performed creep
recovery test on pure wheat flour and combinations with long/short grain rice
flour found that the pure wheat flour dough showed high recovery of elastic
strain after removal of load (Fig. 4a) while the creep behavior
of the two composite flours with long and short grain rice flour showed considerable
variation with the pure rice flours. Janssen et al.
(1996a) conducted creep recovery test on two different wheat flours, weak
(Obelisk) and strong flour (Katepwa) found that Obelisk showed a higher recovery
of elastic strain after removal of load compared to Katepwa (Fig.
4b). Janssen et al. (1996a) suggested that
the apparent viscosity (ηapp) can be estimated from the slope
of the creep curve (as indicated by the arrow in Fig. 4b and
from their observation there was no clear strain hardening in creep tests since
the slope of the curve was nearly independent of time and strain at the end
of the load phase.
||Normalised stress relaxation of the different cultivars of
high protein flour-water doughs at applied shear strain of 0.05% and strain
rise time of 0.2 sec (Glenlea at 12.8% protein, Wildcat at 14.0% protein,
ES12 at 12.8% protein and ES20 at 12.4% protein) (Rao
et al., 2000)
Stress relaxation: In stress a relaxation test, the material is given
an instantaneous constant strain and the stress required to maintain the deformation
is observed as a function of time. This test is a convenient means to characterise
the linear viscoelastic properties of polymers which contain the information
on molecular weight. Rao et al. (2000) conducted
a test with 0.05% strain on dough for 200 sec at 25°C and relaxation spectrum
was calculated to characterise the rheological behavior. Figure
5 shows the stress relaxation curve for doughs plotted as G(t)/Go
versus time where G(t) is the relaxation modulus at any time and Go
is the initial relaxation modulus. The longest relaxation times are associated
with largest molecules. Dough and gluten obtained from strong flour (higher
protein content) had higher relaxation modulus (G(t)) and spectrum (H(τ))
over the whole relaxation time than those from weak flour (lower protein content)
(Li et al., 2003). It indicates that strong flour
dough and gluten has stronger network structure due to entanglements, physical
cross-links or combination of both.
Large deformation measurement: A material is applied to a large deformation
when the stress exceeds the yield value. Some of the common tests used in measuring
large deformation of dough and gluten are uniaxial extension and compression
(Janssen et al., 1996b; Kieffer
et al., 1998; Uthayakumaran et al., 2002; Tronsmo
et al., 2003; Dunnewind et al., 2004;
Sliwinski et al., 2004a; Song
and Zheng, 2008) and biaxial extension (Janssen et
al., 1996a, b; Kokelaar et
al., 1996; Dobraszczyk, 2004; Chin
and Campbell, 2005; Chi et al., 2005; Stojceska
et al., 2007;Tanner et al., 2008).
||Typical load-extension of gluten extension test from Kieffer
dough and gluten extensibility rig (Tronsmo et al.,
For the test of gluten quality used as food product, large deformation is more
suitable since it gives good correlations with breadmaking quality (Dobraszczyk
and Morgenstern, 2003; Tronsmo et al., 2003)
and can be related to its eating quality.
Uniaxial extension: The most commonly adapted large deformation test
of dough and gluten is the extension test where a material is clamped at two
ends and being pulled or extended by a hook at the centre of the sample at a
constant strain rate. During stretching, the material undergoes deformation
and break after the stress is beyond its limit or known as the tensile failure.
The main problem encountered in tensile test is to hold the material such a
way that it breaks within the material and not at the jaws holding the material.
Cutting the material in dumbbell-shaped and clamping the wide ends is often
done to solve the problem. Clamping the material in vertical plane is usually
performed for strong solid materials while for weak materials that cannot support
its own weight, such as dough, the test is usually performed on a horizontal
plane (Bourne, 2002). A typical curve of load-extension
obtained from the test is shown in Fig. 6. Figure
7, the stress-strain curves obtained shows that stress increases with increasing
strain and reaches a maximum at sample fracture point. The gradient of the curve
is related to the modulus of gluten and the curves displayed a curvature up
to fracture indicating that the modulus increased with extension. This behavior
is known as strain hardening in which the force that extend the material increases
in order for additional strain to occur. The phenomena of strain hardening occur
when the stress increases more than proportional with the strain. Sliwinski
et al. (2004a) reported that strong flour dough possesses higher
strain hardening than weak flour dough (Fig. 7) and thus prevents
premature fracture of dough and gluten.
||Typical stress-strain curve obtained from a large deformation
measurement of dough and gluten
Uthayakumaran et al. (2002) performed the uniaxial
extension of gluten dough by first compressing the dough sample in between two
parallel plates before pulling the dough apart by the moving upper plates at
a constant strain rate. Their results showed the strain hardening properties
exhibited during elongation was related to the baking performance. They also
suggested that gluten dough possessed larger elongational viscosities than flour
Biaxial extension: As oppose to uniaxial extension, a biaxial extension
is where a material is stretched at equal rates in two perpendicular directions
in one plane (Dobraszczyk and Morgenstern, 2003). Results
from this test are plotted as pressure versus drum distance trace of an inflating
bubble from dough sample. Chin and Campbell (2005) studied
the relationship of aeration and rheology of dough using biaxial extension and
found that dough from strong flour had higher peak pressure and further drum
distance before bubble rupture (Fig. 8). This suggests that
strong flour dough has stronger gluten network and needed higher pressure to
break them. The stress-strain curve obtained (Fig. 7) shows
considerable increase in stress with strain indicating increased shear modulus
and a clear strain hardening effect within the walls of the inflating dough
bubble. The advantage of this test is that it resembles practical conditions
experienced by the cell walls within the dough during proof and oven rise (Dobraszczyk
and Morgenstern, 2003). Sliwinski et al. (2004a)
studied the effect of water content, mixing time and resting time on the dough
rheology in biaxial extension. They reported that increasing the water content
led to a decrease of biaxial stress which supported the findings of Kokelaar
et al. (1996) while strain hardening was not significantly affected
by the water content. The biaxial stress and strain hardening are least affected
by the resting time but for mixing time, they both increased.
||Typical pressure-drum distance of inflating dough bubbles
The decrease of the fracture strain with increasing mixing time was reported.
In recent work, Song and Zheng (2008) studied the influence
of rest time on the structural development of gluten/glycerol mixtures for biodegradable
packaging material by equibiaxial deformations on a universal testing machine.
EQUIPMENT FOR RHEOLOGICAL MEASUREMENTS OF DOUGH AND GLUTEN
A wide range of equipment is available to determine rheological properties of dough and gluten. This section discusses the working principles of common instruments and their attachments used for measuring rheological properties of dough and gluten which include the rheometer for small deformation testing and the alveograph, extensograph, Kieffer rig and dough inflation system from the texture analyser and the universal testing machine for large deformation testing.
Rheometer: The rheometer is frequently used in determining the viscoelastic
properties of dough and gluten (Amemiya and Menjivar, 1992;
Uthayakumaran et al., 2002; Tronsmo et al.,
2003; Skendi et al., 2010). The parallel
plate configuration has the material loaded is between and while one plate is
rotating in a sinusoidal motion, the other plate is stationary. Surplus materials
between parallel plates are trimmed and coated with suitable fluid like silicon
oil to prevent it from drying. The common rheological parameters obtained using
the dynamic oscillatory, creep recovery and stress relaxation often related
to the behavior of dough and gluten at molecular level. Recent study on the
effect of water and β-glucan from two types of barley on the viscoelasticity
of wheat dough was performed on a rheometer equipped with a Paar Physica circulating
bath and a controlled peltier system (TEZ 150 P/MCR) that was maintained at
25±0.1°C throughout the experiment (Skendi et
al., 2010). Oscillatory and creep recovery tests were measured using
a 25 mm plate-plate geometry.
Extensograph and alveograph: The extensograph and alveograph are probably
the earliest instruments used for empirical dough testing. The extensograph
is essentially an extensional test where a cylindrical dough sample is clamped
horizontally in a cradle and stretched by a hook which is placed in the middle
of the sample and moves downwards until rupture after 45 min resting (Kokelaar
et al., 1996). Muller et al. (1961)
derived the equations of stress and strain from the extension test of dough
in Brabender extensograph and also reported that the maximum extensibility at
fracture is a better index of elasticity than the total extensibility.
The alveograph has been used to measure and evaluate wheat flours of breadmaking
(Khattak et al., 1974; Chen
and DAppolonia, 1985; Janssen et al., 1996b)
and cookie making quality (Rasper et al., 1986;
Bettge et al., 1989). The alveograph uses air
pressure to inflate a thin sheet of dough, simulating the bubbles that are present
in bread dough, that cause dough to stretch when rising. This instrument measures
the resistance to expansion and the extensibility of a dough by providing the
measurement for maximum over pressure, average abscissa at rupture, index of
swelling and deformation energy of dough (Indrani et
Texture analyser: The texture analyser has a robust measuring system due to the various attachments possible for a wide range of food types in different forms and giving reports on a long list of textural properties, such as hardness, brittleness, elasticity, cohesiveness, stickiness, gumminess, springiness, consistency, fracturability, etc. In the context of dough and gluten, most researchers have used the Kieffer dough and gluten extensibility rig and the dough inflation system.
Kieffer dough and gluten extensibility rig: The Kieffer dough and gluten
extensibility rig was developed with similar concept with the extensograph except
that the sample is pulled upwards. Figure 9 shows the extension
test of the gluten on Kieffer dough and gluten extensibility rig. A small amount
of sample in this system (Kieffer et al., 1998;
Tronsmo et al., 2003; Dunnewind
et al., 2004; Sliwinski et al., 2004a,
b). Kieffer et al. (1998),
who investigated the extension of wet gluten, used 10 g of flour in obtaining
dough during wet gluten preparation. Dunnewind et al.
(2004) used a 0.4 g sample with 5 cm length in their investigation of extension
of strong and weak flour dough using the Kieffer rig.
||The extension test of a strip of gluten on a Kieffer dough
and gluten extensibility rig fitted to a texture analyser (Wang,
The samples were clamped at a distance of 18 mm apart and the hooks used were
with 1.20 and 4.55 mm diameter. They concluded that the speed of the hook had
no influence on sample fracture and a thicker hook (4.55 mm) resulted in fracture
of dough occurring more often at the clamp. Dunnewind et
al. (2004) presented the formulas for calculating fundamental rheological
parameters namely the actual and measured force acting on gluten, length of
gluten at fracture, stress and strain from the Kieffer rig results. In comparing
rheological properties of dough and gluten, Tronsmo et
al. (2003), who performed a uniaxial extension on dough and gluten using
the Kieffer rig found that gluten showed higher maximum resistance to extension
(Rmax) and total extensibility (Ext) than dough.
Dough inflation system: The Dough Inflation System (DIS) was introduced
in the early 90s and was developed based on the concept of Alveograph
to provide fundamental rheological measurements. Traditionally, the DIS is used
for comparing flour quality (Dobraszczyk and Roberts, 1994;
Dobraszczyk, 1999; Dobraszczyk et
al., 2003) and measures the stress and strain relationships based on
the inflation of a sheet of dough through a biaxial extension test. It was designed
to operate at constant volumetric air flow rates which vary from 10 and 2000
mL min-1, corresponding to maximum strain rates of 0.001 to 0.2 sec-1,
unlike the Alveograph which operates at strain rates in the range of 0.1 to
1 sec-1, which are at least 100 fold higher than those occurring
in actual baking processes (Huang and Kokini, 1999; Chin
and Campbell, 2005). The deformations involved in biaxial extension tests
are preferred as they are more relevant to the type of deformation of the dough
around an expanding gas bubble during proving and baking.
Chin and Campbell (2005) and Chin
et al. (2005) used this instrument to measure and analyse rheological
properties of aerated doughs. In principle, the sheeted dough sample is cut
into circular sample of 55 mm diameter and 8 mm thickness, coated with paraffin
oil to prevent moisture loss and drying and placed securely in the sample holder
before inflation into dough bubble until a break or rupture was detected. Examples
of graphs of pressure versus drum distance and corresponding stress-strain produced
from the measurement using DIS are given in Fig. 7 and 8.
Universal testing machine: The Universal Testing Machine (UTM) is another
alternative equipment for rheological properties measurement of dough and gluten;
namely to measure tensile and compressive stress. Gujral
and Pathak (2002) studied the extensibility of chapatti dough by performing
tensile test using an attachment on an Instron UTM as shown in Fig.
10. They clamped the dough strip 50 mm apart and tighten the sample at the
two ends. The texture of the chapatti dough was reported in terms of its extensibility,
peak force to rupture, modulus of deformation and energy to rupture. Anderssen
et al. (2004) used a micro-extension tester with 19 mm gap and 6
mm hook diameter operating at 1 cm sec-1 to study the extension of
dough. Stojceska et al. (2007) conducted a biaxial
extensional measurement of dough on Instron UTM with cylindrical test pieces
of diameter 50 mm and thickness 12 mm and subjected it to biaxial compression
to a final height of 2 mm at constant displacement speed of the upper plate.
They found that at the given biaxial strain rate, the apparent biaxial viscosity
of dough was higher when compressed at lower cross-head speed and a weak negative
correlation was obtained between protein content and biaxial extensional viscosity.
||Tensile test set-up for gluten extensibility using two plastic
clips set at 40 mm distance and a hook attached to the Instron. Diagram
from (a) top and (b) side view (Abang Zaidel et al.,
Improvisation in dough and gluten rheological measurement: Improvised
attachments are developed from time to time for convenient measurements of dough
and gluten properties due to the inconsistency in shapes and sizes of samples.
Trevor et al. (2006) determined the extensibility
of a rectangular wheat flour dough sample mounted onto two cylindrical drums
by stretching it until a fracture occurs. They used the Sentmanat Extensional
Rheometer (SER) for measurement of rheological parameters including strain,
stress, strain rate and relaxation modulus. Abang Zaidel
et al. (2008) developed an attachment on Instron for gluten extensibility
studies (Abang Zaidel et al., 2009a, b)
(Fig. 11a, b). The set-up was built based
on the working principle of the Kieffer dough and gluten extensibility rig with
a dimension of 10x10x70 mm gluten strip. Rested gluten strip was clamped at
two ends using plastic clips placed at a distance of 40 mm, nailed to a wood
platform held tightly to the Instron platform. The tensile test set-up consists
of a hook bent into a V-shaped using metal rod of 3.2 mm diameter. The developed
tensile test attachment was successful in performing extensibility measurements
where the gluten does not fracture at the clamping areas. The extensibility
parameters obtained provided results which implied that the strong flour experienced
greater strain hardening effect and the extensibility of gluten from both strong
and weak flour dough increased as dough mixing time increased up to a peak point.
This demonstrated that the gluten development is at its optimum at the peak
and further mixing of dough passed this optimum time results in reduction in
extensibility of gluten.
Glutens and doughs are most unique from the point of material science as they have complex behaviors. Rheological studies are conducted to determine these properties using various suitable testing methods and equipments. These properties measured to accuracy, be it fundamental or empirical, are important as information and references for product development both in research and the industry. The correlation studies on rheological properties of dough and gluten with the baking performance of the end product is an example of such appliance. As such, the development and improvisation of new instruments and attachments for measuring dough and gluten rheology is evolving along the expanding knowledge of their unique behavior.
||Initial slope of extension curve
||Maximum extensibility at fracture
||Elastic (storage) modulus
||Viscous (loss) modulus
||Initial relaxation modulus
||Relaxation modulus at any time
||Final length after elongation
||Maximum resistance to extension
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