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Effect of extrusion cooking of sorghum flour on rheology, morphology and heating rate of sorghum–wheat composite dough
Accepted Manuscript Effect of extrusion cooking of sorghum flour on rheology, morphology and heating rate of sorghum–wheat composite dough Morteza Jafari, Arash Koocheki, Elnaz Milani PII:
S0733-5210(17)30286-2
DOI:
10.1016/j.jcs.2017.07.011
Reference:
YJCRS 2406
To appear in:
Journal of Cereal Science
Received Date: 3 April 2017 Revised Date:
13 July 2017
Accepted Date: 18 July 2017
Please cite this article as: Jafari, M., Koocheki, A., Milani, E., Effect of extrusion cooking of sorghum flour on rheology, morphology and heating rate of sorghum–wheat composite dough, Journal of Cereal Science (2017), doi: 10.1016/j.jcs.2017.07.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of extrusion cooking of sorghum flour on rheology,
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morphology and heating rate of sorghum–wheat composite dough
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Morteza Jafari1, Arash Koocheki1*, Elnaz Milani2
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(FUM), PO Box: 91775-1163, Mashhad, Iran
2. Iranian Academic Center for Education Culture and Research (ACECR), Mashhad, Iran
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1. Department of Food Science and Technology, Ferdowsi University of Mashhad
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*Corresponding author: Koocheki, A. Tel: +98 915 313 9459; Fax: +98 511 8787430
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e-mail: [email protected] 1
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Abstract
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Sorghum flour was extruded at 110°C and 160°C die temperature with 10%, 14% and 18% feed
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moisture. The effect of extruded sorghum flour incorporation (10%) on rheological (farinography
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and stress relaxation behavior), morphological and temperature profile of sorghum-wheat
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composite dough were evaluated. The addition of extruded sorghum flour increased the water
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absorption and dough development time but it decreased the dough stability. Native sorghum-
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wheat composite dough showed viscoelastic liquid-like behavior whereas addition of sorghum
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flour extrudate changed dough to a more viscoelastic solid-like structure. Elastic nature of
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extruded sorghum-wheat composite dough was not fully formed and represent high initial
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compressive force. Maxwell models recognition in difference among samples were more
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appropriate than Peleg model. Extrusion cooking decreased composite dough elasticity and
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viscosity. Sorghum extrudate increased the heating rate of composite dough crumb during
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baking.
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Keywords: Sorghum flour; Extrusion cooking; Stress-relaxation; Heating rate.
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1. Introduction
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Sorghum (Sorghum bicolor L. Moench) is world’s fifth cereal crop which is drought-tolerant,
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resistant to water-logging and grows in various soil conditions (Taylor and Shewry, 2006).
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Sorghum flour is used in many food products such as gluten-free (Marston et al., 2016; Schober
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et al., 2007; Schober et al., 2005) and composite breads (Abdelghafor et al., 2011; Onyango et
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al., 2011).
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The characterization of dough is frequently determined using traditional empirical instruments
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like Farinograph and Extensograph. On the contrary, fundamental rheological measurements can
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provide detailed information regarding the quality of the final product (Bhattacharya, 2010). In
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addition, viscoelasticity is one of the most important rheological properties which have a direct
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influence on the sensory properties of foods. Stress relaxation test is one of the fundamental,
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common and convenient tests to study the viscoelastic nature of food (Andrés et al., 2008;
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Bhattacharya, 2010; Bhattacharya and Narasimha, 1997; Sozer et al., 2008). Low-strain Stress
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relaxation test can generate data to understand the behavior of dough and the aspects of structural
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features while large-strain non-linear deformation data are suitable for practical aspects of dough
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handling (Bhattacharya, 2010).
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Gluten forms a viscoelastic network which is responsible for the gas retention, high volume, and
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low hardness in bread. Therefore, incorporation of gluten-free flour such as sorghum lowers the
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bread quality (Demirkesen et al., 2010; Lazaridou et al., 2007; Schober et al., 2005). In order to
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enhance this defect, researchers has been used several technics such as gum incorporation
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(Lazaridou et al., 2007; Rosell et al., 2001), heat treatment (Marston et al., 2016), heat-moisture
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treatment (Miyazaki and Morita, 2005) and extrusion cooking (Gómez et al., 2011; Martínez et
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al., 2013; Martínez et al., 2014; Wang et al., 2013) could be used.
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Extrusion cooking is a high temperature short time (HTST) process which modifies flour
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properties through starch gelatinization, protein denaturation, complex formation between
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amylose and lipids, degradation of pigments and improvement of sensory characteristics
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(Martínez et al., 2013; Martínez et al., 2014; Wang et al., 2013).
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During extrusion cooking, the nature of raw material, flour particle sizes, screw speed, feed
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moisture and die temperature affect the physicochemical properties of the extrudate. However,
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feed moisture and die temperature have the highest impact on flour extrudate (Hagenimana et al.,
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2006; Martínez et al., 2013; Martínez et al., 2014). Extrusion cooking alters dough and bread
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properties through partial gelatinization of starch granules. The influence of extrusion cooking
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on sorghum flour and its effect on sorghum-wheat composite dough is not fully understood.
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Therefore, in order to enhance the composite sorghum-wheat dough, the effect of feed moisture
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and die temperature on the properties of extruded sorghum-wheat composite dough
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(farinography, stress relaxation behavior, morphological and heating rate properties) was
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evaluated in the present study.
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2. Material and methods
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2.1. Materials
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Sorghum flour (Sorghum bicolour L.) with 9% moisture (db.), 10.5% proteins (db.), 3.25% fat,
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1.64% ash and 2.15% fiber and wheat flour with 10.2% moisture (db.), 10.1% proteins (db.),
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0.81% fat and 0.31% ash were purchased from a local supplier.
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2.2. Extrusion cooking
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Calculated amount of distilled water was added to sorghum flour to obtain desired moisture
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levels (10, 14, and 18% db.) and then vacuum sealed in polyethylene bags and allowed to
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equilibrate for 24 h before extrusion process. Extrusion cooking was performed in a co-rotating
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twin screw extruder (DS56, Jinan Saxin, China) with L/D ratio of 10:1 and die diameter of 4
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mm. The feed rate of flour and the screw speed were set at a constant 40 kg/h and 150 rpm,
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respectively. Sorghum flour was extruded at 110°C and 160°C. The dried extruded products
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were grounded using a blender (IKA, Model A11, Germany) and passed through a 70 mesh sieve
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screen.
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2.3. Dough preparation
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Sorghum flour (10%), water (based on flour water absorption), sunflower oil (4%), salt (2.5%),
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sugar (3%), and instant yeast (1.5%) were used to prepare sorghum-wheat dough. Dried yeast
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was allowed to reactivate and hydrate in 30°C water prior to mixing. The remaining ingredients 5
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were blended together to break up any clumps and then added to the hydrated yeast mixture.
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Dough was kneaded for 8 min using a Huger mixer (model HG550TMEM, China) on its low
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mixing rate.
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2.4. Farinographic characteristics
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Farinograph properties were determined using Brabender farinograph (Farinograph-E,
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Germany). Dough water absorption (percentage of water required to yield dough consistency of
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500 BU), development time (time to reach maximum consistency) and stability (time during
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dough consistency is at 500 BU) were determined according to AACC (2000).
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2.5. Dough stress relaxation tests
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Stress relaxation characteristics of dough were measured according to the method proposed by
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Bhattacharya, (2010) and Sozer et al., (2008) with some modifications. The prepared dough
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samples placed immediately in a plastic container to avoid dehydration and left to rest for 20 min
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before testing in order to relieve the residual stresses produced during sample preparation
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(Rodriguez-Sandoval, Fernandez-Quintero, Sandoval-Aldana, & Quicazan, 2008). The stress
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relaxation behavior of the lubricated (using paraffin oil) cylindrical samples (35 mm diameter 20
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mm height) at engineering strain of 0.05 for low-deformation stress relaxation test and 0.25 for
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high-deformation stress relaxation were determined using the Texture analyzer (AMETEK lloyd,
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TA Plus instruments Ltd, USA) with a load cell of 5 kgs, a P/50 cylindrical probe and crosshead
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speed of 200 mm/min. When the desired extent of compression was attained the crosshead
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surface (50 mm in diameter) was stopped and the dough allowed to relax for 480 s at room
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temperature (25°C). Stress relaxation tests were done with three replications.
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Dough usually exhibits a non-linear viscoelastic behavior when subjected to a large-deformation
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test and the stress relaxation data can be normalized Peleg and Normand, (1983) using the
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following model (Eq. 1): /(
− )=
+
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where, σ is the stress at time t, σ0 is the initial stress, t is the time of relaxation testing, K1 and K2
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are empirical constants.
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For low-deformation test, the decay curves (stress as a function of time) were obtained and fitted
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to a Maxwell model (Eq. 2). Maxwell model is comprised of an elastic (spring) and a viscous
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(dashpot) element in series. Data were fitted to Maxwell model by MATLAB 14 software (The
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MathWorks, Inc., USA), using curve fitting toolbox and Trust-Region algorithm. The degree of
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fit was judged by R2 coefficient computed by MATLAB software. +
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where, n is the number of Maxwell bodies, Ei is the modulus of the spring(s) in Maxwell body, λi
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is relaxation time of the Maxwell body and Ee is the modulus of the lone spring. The stress
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relaxes exponentially and the rate constant is called the relaxation time, λ as shown in Eq. (3)
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(Steffe, 1996).
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2.6. Scanning electron microscopy (SEM)
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The morphology of dough was studied using a scanning electron microscope (VP 1450, LEO
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Company, Germany). The samples were instantly frozen in a liquid nitrogen and dried in a
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freeze-dryer (XO-12N model Top press Freezing Dryer, China). Doughs contained no yeast to
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avoid any alteration in image visualization. Small pieces were mounted on an aluminum stub
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using a double-sided stick tape and coated with a thin film of gold. Pictures were then taken at an
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accelerating voltage of 20 kV.
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2.7. Temperature profile of dough during baking
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Dough temperature profile and oven temperature during the baking were recorded using k-type
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thermocouples (Omega, USA) connected to a data logger (Lutron 801, model SW-U801-WIN,
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Taiwan). Three thermocouples were inserted inside the dough. For measuring center
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temperature, Thermocouples were installed at 3.5 cm deep. Others were introduced at 1 cm deep
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to measure the temperature of the crumb near the top crust and the bottom crust. Then, time-
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temperature curve were determined at 200 °C. Two replicates were performed for each sample.
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3. Results and discussion
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3.1. Farinograph
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Addition of extruded sorghum increased the water absorption, dough development time (DDT)
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and decreased the stability of wheat-sorghum composite dough (Table 1). Extrusion cooking
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increases the water absorption index of sorghum flour through starch gelatinization (Jafari et al.,
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2016). Therefore, the addition of extruded sorghum flour increases the water absorption of
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sorghum-wheat composite dough. Increasing the water absorption might decrease the water-
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gluten interactions and hence, the DDT of such composite dough increases. According to Gómez
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et al. (2011) decreasing the gluten hydration due to pre-gelatinization of starch is the main reason
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for the high DDT.
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In contrast with our results, Cai et al. (2016) observed DDT reduction with increasing rice flour
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water absorption index for gluten-free dough. This may be related to the different nature of rice
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dough compared to the wheat-sorghum composite dough. For gluten-free dough, starch is the
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main factor for dough viscoelastic properties while for sorghum-wheat composite dough gluten
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is still responsible for such property.
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Extrusion cooking increases the sorghum flour amylose-lipid complex formation and decreases
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its swelling power (Jafari et al., 2016). At this condition, the remaining starch granules absorb
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water slowly due to their compact structure (Cai et al., 2016; Miyazaki and Morita, 2005) which
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can affect on reducing sorghum-wheat composite dough.
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Increasing the die temperature and feed moisture increased the water absorption but decreased
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the DDT and stability of dough (Table 1). Similar results have also been reported for dough
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containing extruded wheat bran (Gómez et al., 2011), extruded rice flour (Wang et al., 2013) and
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extruded wheat flour (Martínez et al., 2013). The lost in granule integrity during extrusion
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cooking of sorghum flour (Jafari et al., 2016) could influence the water holding capacity and as a
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result the stability of composite dough. For dough with lower water holding capability,
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pregelatinized starch granules quickly lose their water after reaching to a maximum torque (Cai
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et al., 2016). As a result, this high free water drops the torque intensely and reduces the dough
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stability.
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3.2. Dough viscoelasticity using high-strain relaxation test
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Stress relaxation test is one of the fundamental tests to study the viscoelastic nature of food
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(Bhattacharya and Narasimha, 1997). The level at which the stress begins to decay during
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relaxation is represented by the constant K1. K1 is a measure of how easily the material deforms
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(Rodriguez‐Sandoval et al., 2008). The reciprocal of the constant K1 indicates the initial decay
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rate. Thus, low K1 means that dough possesses a high decay rate indicating less elastic behavior
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(Bhattacharya, 2010). Addition of sorghum flour extrudate did not significantly change the K1 of
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composite dough (Table 2).
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The composite dough containing raw and extruded sorghum flour offered a high extent of
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relaxation (89% and 80–87%, respectively) (Table 2), meaning that these doughs possessed high
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viscous characteristics. Extruded sorghum-wheat composite dough had less viscous characteristic
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due to the presence of amylose-lipid complexes formed during extrusion of sorghum flour. On
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the other hand, decreasing the feed moisture and die temperature decreased the viscous
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characteristic of composite dough (Table 2).
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The stress relaxation rate (K2) values represent the degree of solidity (Karaman et al., 2016).
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Therefore, high K2 values of extruded sorghum-wheat composite dough (Table 2) indicates that
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the dough had more viscoelastic solid structure. As a result, more normalized force is needed to
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achieve the same deformation than the composite dough prepared by native sorghum. It can also
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be suggested that the amount of such force requirement would be higher by decreasing the feed
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moisture and die temperature.
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3.3. Dough viscoelasticity using low-strain relaxation test
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For interpret stress–relaxation data of a viscoelastic material, the generalized Maxwell model
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with a small number of parallel simple elements (a spring and a dashpot arranged in a series) is
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used. The constant values for the generalized (two-element) Maxwell model (Eq. 2) are shown in
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Table 3. Extrusion cooking significantly decreased the E1 and increased the E2. Decreasing the
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feed moisture and die temperature significantly increased both E1 and E2 (Table 3 and Fig. 1).
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The decay forces represented by E1 and E2 are the elastic components of the Maxwell element
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which indirectly show the elasticity of the material being tested (Sozer et al., 2008).
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Bhattacharya and Narasimha, (1997) reported that the first term of Maxwell model (E1 and λ1)
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had a major contribution (90%) to the total modulus and so, the first terms (E1 and λ1) could be
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used to describe the structure behavior. High E1 indicates that the dough is more elastic
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(Karaman et al., 2016). Therefore, low E1 of the composite dough containing extruded sorghum
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flour indicates its less elastic consistency. On the other hand, increasing the E1 with decreasing
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the feed moisture and die temperature demonstrated the high elastic consistency of these doughs
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(Table 3 and Fig. 1). Wang et al. (2013) expressed that the dough elasticity of the extruded
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hemp/ rice flours decreased with increasing the die temperature.
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Low elasticity could be related to the less water holding capability of extruded flour. Fu et al.
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(2016) also suggested that the partially gelatinized starch provides more elastic and firmer
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consistency due to its greater water holding capacity. For extruded sorghum-wheat composite
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dough, elastic nature of composite dough was not fully formed due to the reduction of gluten
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hydration.
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On the other hand, Shiau and Chang, (2013) expressed that the decay forces (E1 and E2)
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indirectly measures the rigidity of the material being tested. Therefore, decreasing the decay
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force as a result of extrusion cooking supports that composite dough containing extruded
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sorghum were soft. Singh and Singh, (2003) also reported that the gelatinized starch forms
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smoother texture.
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Decreasing the elasticity with increasing the feed moisture and die temperature could also be
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related to the high moisture in these doughs. Similar results were reported by Lazaridou et al.
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(2007) who also determined that the elasticity of rice flour, corn starch and sodium caseinate
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dough decreased with increasing dough moisture content.
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Extrusion cooking of sorghum flour significantly increased the relaxation time (λ1) of composite
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dough. Dough relaxation time increased with decreasing the feed moisture and die temperature
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(Table 3). Relaxation time (λ) is the time that the viscoelastic material dissipates its force to
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about 36.8% of the originally applied force. High λ indicates slow decay during relaxation,
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which is a typical behavior for a viscoelastic matrix with more solid-like structure (Bhattacharya,
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2010). Therefore, extrusion cooking of sorghum flour formed a more solid-like composite
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dough.
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Extrusion cooking significantly decreased the composite dough viscosity. Dough viscosity
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increased with decreasing the feed moisture and die temperature (Table 3). Partially gelatinized
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starch adhere to one another in the dough (Fu et al., 2016). As a result, the mobility of starch
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granules decreases and the dough viscosity increases. However, our finding did not follow this
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fact probably due to the formation of amylose-lipid complexes during extrusion cooking of
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sorghum flour. Because of amylose-lipid complex formation in extruded sorghum flour (Jafari et
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al., 2016), starch granules hardly interact with other compounds (Jafari et al., 2016; Miyazaki et
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al., 2005). Therefore, the mobility of starch granules increases and lowers the dough viscosity.
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On the other hand, dough water absorption increases through extrusion cooking. According to
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Bhattacharya and Narasimha, (1997), increasing the moisture content decreases the dough
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viscosity.
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Initial compressive force for a dough reflects the resistance per unit area of the sample towards
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compression prior to relaxation; the higher initial and end compressive force values, the more
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solid-like behavior of samples. Extrusion cooking increased the initial compressive force,
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representing a more solid-like structure for extruded sorghum-wheat composite dough (Table 3
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and Fig. 1). On the other hand, increasing this parameter with decreasing the feed moisture and
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die temperature shows that the samples were more solid-like.
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Increasing dough moisture decreases the initial force (Bhattacharya, 2010). At low moisture, the
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dough composition absorbs less water and forms a firm dough. Therefore, the initial compressive
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force and resistance to compression increases. According to what mentioned above, because of
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high moisture in extruded sorghum-wheat dough, the initial force reduces in these samples.
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However, initial force increased in the composite dough containing extruded sorghum flour. This
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might be due to the high water solubility index of extruded sorghum flour (Jafari et al., 2016).
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Flour with higher water solubility index forms sticky dough (Spies, 1990) which increases the
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initial compressive force and develop a solid-like behavior. On the other hand, increasing dough
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moisture enhances the stickiness of dough (Bhattacharya, 2010).
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Starch gelatinization and water absorption index of extruded sorghum flour increases with
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increasing the feed moisture and die temperature (Jafari et al., 2016). The samples with high
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water binding capacity may inhibit the development of gluten which lowers the consistency of
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the gluten network and hence the initial force and resistance to strains decrease (Fustier et al.,
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2008). Therefore, initial force decreased with increasing the feed moisture and die temperature.
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3.4. SEM of composite dough
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The morphology of native and extruded sorghum-wheat composite dough are presented in Fig 2.
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Starch granules were more intact in native sortghum-wheat dough compared to extruded
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sorghum-wheat dough (Fig. 2 (1)). The intact starch granules structures decreased with
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increasing the feed moisture and die temperature (Fig. 2 (2, 3, 4, 5, 6 and 7)). Jafari et al. (2016)
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reported that the degree of sorghum starch gelatinization increased with increasing the feed
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moisture and die temperature which as a result decreases the intact granules in the extruded
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sorghum-wheat composite dough.
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It is well known that the large and small starch granules in native sorghum-wheat composite
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dough were more embedded in gluten matrix compared to those of extruded sorghum-wheat
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composite dough (Fig. 2 (1)). However, composite doughs containing sorghum extruded at 110
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and 160°C with 10% moisture had similar structure to composite dough containing native
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sorghum (Fig. 2 (2 and 3)). For the native sorghum-wheat composite dough, starch granules were
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close to each other and formed a uniform and compact structure compared to those of extruded
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sorghum-wheat composite dough. Increasing the feed moisture and die temperature destructed
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the dough structure uniformity (Fig. 2 (7)). Martínez et al. (2013) also reported that the wheat
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dough had a compact structure with starch granules embedded in protein matrix whereas
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incorporation of extruded wheat flour produced an open and less compact structure with a
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smaller number of starch granules. On the other hand, extruded sorghum-wheat composite dough
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had voids (Fig. 2 (2, 3, 4, 5, 6 and 7-a)) probably due to the inability of such structure to
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maintain the fermented gas. Composite dough containing sorghum extruded at 18% moisture and
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160°C had larger voids (Fig. 2 (7-a)) giving the bread the highest hardness.
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3.5. Temperature profile of dough during baking
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Understanding the temperature profile of oven and dough during baking helps us to achieve the
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desired product quality. On the other hand, thermal profile affects the starch gelatinization,
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volume expansion and browning reactions (Therdthai et al., 2002).
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The temperature profiles for the center, bottom and upper section (Fig. 3) of native and extruded
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sorghum-wheat composite dough crumbs were evaluated. The central thermal profile of crumb
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could be divided into three phases (Fig. 3); phase I, characterized by a very slow temperature
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increase to up to 38±3°C followed by phase II where a rapid increase in temperature was
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observed. The end of the thermal profile is characterized by an asymptotic increase in
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temperature (89±2°C) called the plateau.
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The heating rate of dough was calculated in the linear region between 45 and 85 °C where a
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rapid increase in temperature occur (phase II). Extrusion cooking increased the dough heating
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rate (Table 1). High heating rate value in extruded sorghum-wheat dough might be due to the
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high free water in this dough. These results were in accordance with the crumb moisture (data
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not shown) in which the crumb moisture increased by extrusion cooking. The heating rate
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increased with increasing the feed moisture and decreasing the die temperature due to the
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different amount of free water in these doughs.
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The bottom and upper section had higher temperature compared to the center of dough (Fig. 3).
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This might be due to the water condensing phenomena in the center section of dough (Besbes et
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al., 2013). At temperatures up to 90±3°C, the upper section had higher temperature than the
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bottom part. In the final stages, the temperature of center part of extruded sorghum-wheat
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composite dough was even higher than those of upper part (except for dough containing sorghum
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flour extruded at 110°C with 10% moisture (Fig. 3 (2))). For native sorghum-wheat composite
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dough, the upper part during all three stages had higher temperature than the center part. This
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could be related to the high water content of extruded sorghum-wheat dough. Thus, with
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increasing the temperature during baking, more water moves from the center to the upper part
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and hence the upper part temperature decreases.
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Conclusion
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The effect of extruded sorghum flour (10%) on rheological (farinography and stress relaxation
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behavior), morphological and temperature profile of sorghum-wheat composite dough was
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investigated. The extrusion cooking of sorghum altered the sorghum-wheat composite dough
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properties. Addition of extruded sorghum flour increased the water absorption and dough
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development time but it decreased the dough stability. Composite doughs containing both native
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and extruded sorghum flour showed high viscose property. However, extrusion cooking
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decreased the viscose characteristic and hence the extruded sorghum-wheat dough showed more
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solid-like structure. Maxwell models was more appropriate than Peleg model for evaluation of
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viscoelastic properties of dough. In general, extrusion cooking decreased the composite dough
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elasticity. Sorghum extrudate increased the heating rate of composite dough crumb. For the
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native sorghum-wheat composite dough, starch granules were close to each other and formed a
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uniform and compact structure compared to those of extruded sorghum-wheat composite dough.
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Increasing the feed moisture and die temperature destructed the dough structure uniformity. The
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intact starch granules structures decreased with increasing the feed moisture and die temperature.
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Extruded wheat flour produced an open and less compact structure with smaller starch granules.
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362
AACC., 2000. Approved methods of the American association of cereal chemists. 10th ed. St.
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Paul, Minnesota: American Association of Cereal Chemists.
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Abdelghafor, R., Mustafa, A., Ibrahim, A., Krishnan, P., 2011. Quality of Bread from Composite
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Besbes, E., Jury, V., Monteau, J.-Y., Le Bail, A., 2013. Characterizing the cellular structure of
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Bhattacharya, S., 2010. Stress relaxation behaviour of moth bean flour dough: Product
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characteristics and suitability of model. Journal of Food Engineering 97, 539-546.
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Bhattacharya, S., Narasimha, H., 1997. Puncture and stress relaxation behavior of blackgram
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(phaseolus mungo) flour‐based papad dough. Journal of food process engineering 20, 301-316.
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Cai, J., Chiang, J.H., Tan, M.Y.P., Saw, L.K., Xu, Y., Ngan-Loong, M.N., 2016.
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Physicochemical properties of hydrothermally treated glutinous rice flour and xanthan gum
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mixture and its application in gluten-free noodles. Journal of Food Engineering 186, 1-9.
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Demirkesen, I., Mert, B., Sumnu, G., Sahin, S., 2010. Rheological properties of gluten-free bread
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formulations. Journal of Food Engineering 96, 295-303.
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Fu, Z.-q., Che, L.-m., Li, D., Wang, L.-j., Adhikari, B., 2016. Effect of partially gelatinized corn
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starch on the rheological properties of wheat dough. LWT-Food Science and Technology 66,
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324-331.
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Fustier, P., Castaigne, F., Turgeon, S., Biliaderis, C., 2008. Flour constituent interactions and
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their influence on dough rheology and quality of semi-sweet biscuits: A mixture design approach
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with reconstituted blends of gluten, water-solubles and starch fractions. Journal of Cereal
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Gómez, M., Jiménez, S., Ruiz, E., Oliete, B., 2011. Effect of extruded wheat bran on dough
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rheology and bread quality. LWT-Food Science and Technology 44, 2231-2237.
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Hagenimana, A., Ding, X., Fang, T., 2006. Evaluation of rice flour modified by extrusion
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cooking. Journal of Cereal Science 43, 38-46.
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Jafari, M., koocheki, A., & Millani, E., 2016. Effect of extrusion cooking on chemical structure,
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morphology, crystallinity and thermal properties of sorghum flour extrudates. Academic press.
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Karaman, S., Yilmaz, M., Toker, Ö., Yuksel, F., Kayacier, A., Dogan, M., 2016. Effect of apple
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fibre on textural and relaxation properties of wheat chips dough. Quality Assurance and Safety of
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Crops & Foods 8, 457-472.
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Lazaridou, A., Duta, D., Papageorgiou, M., Belc, N., Biliaderis, C., 2007. Effects of
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hydrocolloids on dough rheology and bread quality parameters in gluten-free formulations.
399
Journal of Food Engineering 79, 1033-1047.
400
Marston, K., Khouryieh, H., Aramouni, F., 2016. Effect of heat treatment of sorghum flour on
401
the functional properties of gluten-free bread and cake. LWT-Food Science and Technology 65,
402
637-644.
403
Martínez, M., Oliete, B., Gómez, M., 2013. Effect of the addition of extruded wheat flours on
404
dough rheology and bread quality. Journal of Cereal Science 57, 424-429.
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Martínez, M.M., Oliete, B., Román, L., Gómez, M., 2014. Influence of the addition of extruded
406
flours on rice bread quality. Journal of Food Quality 37, 83-94.
407
Miyazaki, M., Morita, N., 2005. Effect of heat-moisture treated maize starch on the properties of
408
dough and bread. Food Research International 38, 369-376.
409
Onyango, C., Mutungi, C., Unbehend, G., Lindhauer, M.G., 2011. Rheological and textural
410
properties of sorghum-based formulations modified with variable amounts of native or
411
pregelatinised cassava starch. LWT-Food Science and Technology 44, 687-693.
412
Peleg, M., Normand, M., 1983. Comparison of two methods for stress relaxation data
413
presentation of solid foods. Rheologica Acta 22, 108-113.
414
Rodriguev‐Sandaval, E., Fernandez‐Quintero, A., Sandoval‐Aldana, A., Quicazan, M.C., 2008.
415
Effect of cooking time and storage temperature on the textural properties of cassava dough.
416
Journal of Texture Studies 39, 68-82.
417
Rosell, C., Rojas, J., De Barber, C.B., 2001. Influence of hydrocolloids on dough rheology and
418
bread quality. Food hydrocolloids 15, 75-81.
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Schober, T.J., Bean, S.R., Boyle, D.L., 2007. Gluten-free sorghum bread improved by sourdough
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fermentation: biochemical, rheological, and microstructural background. Journal of agricultural
421
and food chemistry 55, 5137-5146.
422
Schober, T.J., Messerschmidt, M., Bean, S.R., Park, S.-H., Arendt, E.K., 2005. Gluten-free bread
423
from sorghum: quality differences among hybrids. Cereal chemistry 82, 394-404.
424
Shiau, S.-Y., Chang, Y.-H., 2013. Instrumental textural and rheological properties of raw, dried,
425
and cooked noodles with transglutaminase. International Journal of Food Properties 16, 1429-
426
1441.
427
Singh, J., Singh, N., 2003. Studies on the morphological and rheological properties of granular
428
cold water soluble corn and potato starches. Food hydrocolloids 17, 63-72.
429
Sozer, N., Kaya, A., Dalgic, A.C., 2008. The effect of resistant starch addition on viscoelastic
430
properties of cooked spaghetti. Journal of Texture Studies 39, 1-16.
431
Spies, R., 1990. Application of rheology in the bread industry, Dough rheology and baked
432
product texture. Springer, pp. 343-361.
433
Steffe, J.F., 1996. Rheological methods in food process engineering. Freeman press.
434
Taylor, J., Shewry, P., 2006. Preface to sorghum and millets reviews. Journal of Cereal Science
435
44, 223.
436
Therdthai, N., Zhou, W., Adamczak, T., 2002. Optimisation of the temperature profile in bread
437
baking. Journal of Food Engineering 55, 41-48.
438
Wang, Y.-Y., Norajit, K., Kim, M.-H., Kim, Y.-H., Ryu, G.-H., 2013. Influence of extrusion
439
condition and hemp addition on wheat dough and bread properties. Food Science and
440
Biotechnology 22, 89-97.
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Figures caption
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Figure 1. Stress relaxation curves for native and extruded sorghum flour dough at low
452
strain.
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Figure 2. Scanning electron micrographs of native (1) and extruded sorghum flour (2: 10%-
454
110°C; 3: 10%-160°C; 4: 14%-110°C; 5: 14%-160°C; 6: 18%-110°C; 7: 18%-160°C)-
455
wheat composite dough at 500× (a) and 2000× (b).
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Figure 3. Temperature profiles of native (1) and extruded sorghum flour (2: 10%-110°C; 3:
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10%-160°C; 4: 14%-110°C; 5: 14%-160°C; 6: 18%-110°C; 7: 18%-160°C)-wheat
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composite dough in these three zones of dough baked at 200°C during 21 min.
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M (%)
Water absorption (%)
DDT (min)
Stability (min)
Heating rate (°C/min)
Native
-
60.57±0.2e
2.34±0.2c
4.93±0.1a
8.73±0.9d
110
10
62.50±0.4d
4.99±0.3a
3.66±0.3b
9.63±0.5b
14
62.46±0.1d
4.36±0.1a
3.56±0.3b
10.48±0.9a
18
63.52±0.6c
3.5±0.2b
3.51±0.2b
10.64±0.8a
10
63.82±0.4bc
3.42±0.1b
2.89±0.2c
8.94±0.6cd
14
64.18±0.1ab
3.21±0.1b
2.74±0.3c
9.04±0.5cd
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64.57±.3a
3.12±0.2b
2.31±0.1c
9.24±0.8bc
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Table 1. Farinograph and heating rate properties of native and extruded sorghum-wheat dough
Extruded sorghum (T: die temperature, M: feed moisture)-wheat dough and bread, Native: native sorghum-wheat dough and bread. DDT: dough development time.
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Table 2. High-strain relaxation parameters of native and extruded sorghum-wheat dough End force (N)
K1 (s)
K2 (-)
%SR
R2
0.82±0.10a
0.03±0.002bc
1.16±0.10a
24.84±0.2e
89.2±2.1a
99.29
10
0.82±0.06a
0.06±0.005a
1.25±0.09a
39.79±0.5a
80.1±1.8e
99.54
14
0.76±0.10b
0.05±0.004ab
1.19±0.20a
36.34±0.6b
83.3±1.2cd
99.39
18
0.68±0.10dc 0.03±0.002bc
1.14±0.08a
27.59±0.4d
85.3±1bc
99.71
10
0.81±0.08a
0.05±0.007ab
1.15±0.06a
32.51±0.1c
82.9±1.1de
99.38
14
0.71±0.09c
0.03±0.004bc
1.12±0.10a
27.99±0.3d
86.2±1.3b
99.52
18
0.67±0.10d
0.01±0.002c
1.09±0.10a
18.21±0.3f
87.2±1.9ab
99.47
110
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Initial force (N)
M (%)
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T (°C)
Extruded sorghum (T: die temperature, M: feed moisture)-wheat dough, Native: native sorghumwheat dough. %SR: % stress relaxation.
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Table 3. Low-strain relaxation parameters of native and extruded sorghum-wheat dough
0.13±0.09g
1.70±0.08f
28.40±0.40a 55.45±0.2a
3.75±0.4g
99.63
10
0.142±0.04b
42.27±0.10a
31.57±0.10b
1.90±0.08bc
4.50±0.4b
80.53±0.3a
99.52
14
0.115±0.08cd
30.90±0.10b
25.87±0.05d
1.74±0.07bc
2.98±0.1cd
53.9±0.4c
99.76
18
0.109±0.10d
22.82±0.08d
24.17±0.07e
1.62±0.04c
2.64±0.3d
37.13±0.2d
99.81
10
0.121±0.09c
28.37±0.20c
35.95±0.10a
2.07±0.08b
4.38±0.2b
58.8±0.2b
99.75
14
0.114±0.10cd
18.11±0.20e
28.17±0.20c
1.89±0.07bc
3.21±0.3c
34.32±0.1e
99.42
18
0.098±0.04d
15.57±0.09f 25.08±0.30de
1.66±0.09c
2.46±0.1d
25.97±0.2f
99.58
SC
Extruded sorghum (T: die temperature, M: feed moisture)-wheat dough, Native: native sorghumwheat dough. %SR: % stress relaxation.
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Reported values correspond to the mean±standard deviation. Different letters in the same column indicate significant differences (P<0.05).
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110
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32.510±0.20a
Native
E2
η1 (KPas)
R2
(KPa)
E1 (KPa)
λ2 (s)
η2 (KPas)
λ1 (s)
M (%)
M AN U
T (°C)
M AN U
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Figure 1.
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Figure 2.
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Figure 3.
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- Using sorghum extrudate increased the water absorption and dough development time while it decreased the stability of sorghum-wheat composite dough. - Extrusion cooking decreased sorghum-wheat composite dough elasticity and viscosity.
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- Maxwell models was well used to describe the viscoelasticity of the sorghum-wheat composite dough. - Sorghum extrudate increased the heating rate of sorghum-wheat composite dough during baking.
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- Sorghum extruded at higher feed moisture and die temperature had negative effect on the uniformity of sorghum-wheat composite dough structure.
S0733-5210(17)30286-2
DOI:
10.1016/j.jcs.2017.07.011
Reference:
YJCRS 2406
To appear in:
Journal of Cereal Science
Received Date: 3 April 2017 Revised Date:
13 July 2017
Accepted Date: 18 July 2017
Please cite this article as: Jafari, M., Koocheki, A., Milani, E., Effect of extrusion cooking of sorghum flour on rheology, morphology and heating rate of sorghum–wheat composite dough, Journal of Cereal Science (2017), doi: 10.1016/j.jcs.2017.07.011. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of extrusion cooking of sorghum flour on rheology,
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morphology and heating rate of sorghum–wheat composite dough
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Morteza Jafari1, Arash Koocheki1*, Elnaz Milani2
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(FUM), PO Box: 91775-1163, Mashhad, Iran
2. Iranian Academic Center for Education Culture and Research (ACECR), Mashhad, Iran
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1. Department of Food Science and Technology, Ferdowsi University of Mashhad
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*Corresponding author: Koocheki, A. Tel: +98 915 313 9459; Fax: +98 511 8787430
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e-mail: [email protected] 1
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Abstract
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Sorghum flour was extruded at 110°C and 160°C die temperature with 10%, 14% and 18% feed
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moisture. The effect of extruded sorghum flour incorporation (10%) on rheological (farinography
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and stress relaxation behavior), morphological and temperature profile of sorghum-wheat
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composite dough were evaluated. The addition of extruded sorghum flour increased the water
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absorption and dough development time but it decreased the dough stability. Native sorghum-
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wheat composite dough showed viscoelastic liquid-like behavior whereas addition of sorghum
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flour extrudate changed dough to a more viscoelastic solid-like structure. Elastic nature of
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extruded sorghum-wheat composite dough was not fully formed and represent high initial
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compressive force. Maxwell models recognition in difference among samples were more
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appropriate than Peleg model. Extrusion cooking decreased composite dough elasticity and
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viscosity. Sorghum extrudate increased the heating rate of composite dough crumb during
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baking.
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Keywords: Sorghum flour; Extrusion cooking; Stress-relaxation; Heating rate.
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1. Introduction
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Sorghum (Sorghum bicolor L. Moench) is world’s fifth cereal crop which is drought-tolerant,
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resistant to water-logging and grows in various soil conditions (Taylor and Shewry, 2006).
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Sorghum flour is used in many food products such as gluten-free (Marston et al., 2016; Schober
46
et al., 2007; Schober et al., 2005) and composite breads (Abdelghafor et al., 2011; Onyango et
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al., 2011).
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The characterization of dough is frequently determined using traditional empirical instruments
49
like Farinograph and Extensograph. On the contrary, fundamental rheological measurements can
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provide detailed information regarding the quality of the final product (Bhattacharya, 2010). In
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addition, viscoelasticity is one of the most important rheological properties which have a direct
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influence on the sensory properties of foods. Stress relaxation test is one of the fundamental,
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common and convenient tests to study the viscoelastic nature of food (Andrés et al., 2008;
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Bhattacharya, 2010; Bhattacharya and Narasimha, 1997; Sozer et al., 2008). Low-strain Stress
55
relaxation test can generate data to understand the behavior of dough and the aspects of structural
56
features while large-strain non-linear deformation data are suitable for practical aspects of dough
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handling (Bhattacharya, 2010).
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Gluten forms a viscoelastic network which is responsible for the gas retention, high volume, and
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low hardness in bread. Therefore, incorporation of gluten-free flour such as sorghum lowers the
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bread quality (Demirkesen et al., 2010; Lazaridou et al., 2007; Schober et al., 2005). In order to
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enhance this defect, researchers has been used several technics such as gum incorporation
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(Lazaridou et al., 2007; Rosell et al., 2001), heat treatment (Marston et al., 2016), heat-moisture
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treatment (Miyazaki and Morita, 2005) and extrusion cooking (Gómez et al., 2011; Martínez et
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al., 2013; Martínez et al., 2014; Wang et al., 2013) could be used.
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Extrusion cooking is a high temperature short time (HTST) process which modifies flour
66
properties through starch gelatinization, protein denaturation, complex formation between
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amylose and lipids, degradation of pigments and improvement of sensory characteristics
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(Martínez et al., 2013; Martínez et al., 2014; Wang et al., 2013).
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During extrusion cooking, the nature of raw material, flour particle sizes, screw speed, feed
70
moisture and die temperature affect the physicochemical properties of the extrudate. However,
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feed moisture and die temperature have the highest impact on flour extrudate (Hagenimana et al.,
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2006; Martínez et al., 2013; Martínez et al., 2014). Extrusion cooking alters dough and bread
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properties through partial gelatinization of starch granules. The influence of extrusion cooking
74
on sorghum flour and its effect on sorghum-wheat composite dough is not fully understood.
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Therefore, in order to enhance the composite sorghum-wheat dough, the effect of feed moisture
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and die temperature on the properties of extruded sorghum-wheat composite dough
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(farinography, stress relaxation behavior, morphological and heating rate properties) was
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evaluated in the present study.
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2. Material and methods
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2.1. Materials
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Sorghum flour (Sorghum bicolour L.) with 9% moisture (db.), 10.5% proteins (db.), 3.25% fat,
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1.64% ash and 2.15% fiber and wheat flour with 10.2% moisture (db.), 10.1% proteins (db.),
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0.81% fat and 0.31% ash were purchased from a local supplier.
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2.2. Extrusion cooking
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Calculated amount of distilled water was added to sorghum flour to obtain desired moisture
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levels (10, 14, and 18% db.) and then vacuum sealed in polyethylene bags and allowed to
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equilibrate for 24 h before extrusion process. Extrusion cooking was performed in a co-rotating
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twin screw extruder (DS56, Jinan Saxin, China) with L/D ratio of 10:1 and die diameter of 4
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mm. The feed rate of flour and the screw speed were set at a constant 40 kg/h and 150 rpm,
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respectively. Sorghum flour was extruded at 110°C and 160°C. The dried extruded products
98
were grounded using a blender (IKA, Model A11, Germany) and passed through a 70 mesh sieve
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screen.
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2.3. Dough preparation
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Sorghum flour (10%), water (based on flour water absorption), sunflower oil (4%), salt (2.5%),
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sugar (3%), and instant yeast (1.5%) were used to prepare sorghum-wheat dough. Dried yeast
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was allowed to reactivate and hydrate in 30°C water prior to mixing. The remaining ingredients 5
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were blended together to break up any clumps and then added to the hydrated yeast mixture.
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Dough was kneaded for 8 min using a Huger mixer (model HG550TMEM, China) on its low
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mixing rate.
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2.4. Farinographic characteristics
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Farinograph properties were determined using Brabender farinograph (Farinograph-E,
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Germany). Dough water absorption (percentage of water required to yield dough consistency of
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500 BU), development time (time to reach maximum consistency) and stability (time during
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dough consistency is at 500 BU) were determined according to AACC (2000).
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2.5. Dough stress relaxation tests
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Stress relaxation characteristics of dough were measured according to the method proposed by
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Bhattacharya, (2010) and Sozer et al., (2008) with some modifications. The prepared dough
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samples placed immediately in a plastic container to avoid dehydration and left to rest for 20 min
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before testing in order to relieve the residual stresses produced during sample preparation
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(Rodriguez-Sandoval, Fernandez-Quintero, Sandoval-Aldana, & Quicazan, 2008). The stress
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relaxation behavior of the lubricated (using paraffin oil) cylindrical samples (35 mm diameter 20
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mm height) at engineering strain of 0.05 for low-deformation stress relaxation test and 0.25 for
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high-deformation stress relaxation were determined using the Texture analyzer (AMETEK lloyd,
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TA Plus instruments Ltd, USA) with a load cell of 5 kgs, a P/50 cylindrical probe and crosshead
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speed of 200 mm/min. When the desired extent of compression was attained the crosshead
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surface (50 mm in diameter) was stopped and the dough allowed to relax for 480 s at room
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temperature (25°C). Stress relaxation tests were done with three replications.
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Dough usually exhibits a non-linear viscoelastic behavior when subjected to a large-deformation
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test and the stress relaxation data can be normalized Peleg and Normand, (1983) using the
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following model (Eq. 1): /(
− )=
+
(1)
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where, σ is the stress at time t, σ0 is the initial stress, t is the time of relaxation testing, K1 and K2
133
are empirical constants.
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For low-deformation test, the decay curves (stress as a function of time) were obtained and fitted
135
to a Maxwell model (Eq. 2). Maxwell model is comprised of an elastic (spring) and a viscous
136
(dashpot) element in series. Data were fitted to Maxwell model by MATLAB 14 software (The
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MathWorks, Inc., USA), using curve fitting toolbox and Trust-Region algorithm. The degree of
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fit was judged by R2 coefficient computed by MATLAB software. +
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where, n is the number of Maxwell bodies, Ei is the modulus of the spring(s) in Maxwell body, λi
140
is relaxation time of the Maxwell body and Ee is the modulus of the lone spring. The stress
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relaxes exponentially and the rate constant is called the relaxation time, λ as shown in Eq. (3)
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(Steffe, 1996).
(3)
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2.6. Scanning electron microscopy (SEM)
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The morphology of dough was studied using a scanning electron microscope (VP 1450, LEO
145
Company, Germany). The samples were instantly frozen in a liquid nitrogen and dried in a
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freeze-dryer (XO-12N model Top press Freezing Dryer, China). Doughs contained no yeast to
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avoid any alteration in image visualization. Small pieces were mounted on an aluminum stub
148
using a double-sided stick tape and coated with a thin film of gold. Pictures were then taken at an
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accelerating voltage of 20 kV.
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2.7. Temperature profile of dough during baking
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Dough temperature profile and oven temperature during the baking were recorded using k-type
153
thermocouples (Omega, USA) connected to a data logger (Lutron 801, model SW-U801-WIN,
154
Taiwan). Three thermocouples were inserted inside the dough. For measuring center
155
temperature, Thermocouples were installed at 3.5 cm deep. Others were introduced at 1 cm deep
156
to measure the temperature of the crumb near the top crust and the bottom crust. Then, time-
157
temperature curve were determined at 200 °C. Two replicates were performed for each sample.
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3. Results and discussion
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3.1. Farinograph
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Addition of extruded sorghum increased the water absorption, dough development time (DDT)
177
and decreased the stability of wheat-sorghum composite dough (Table 1). Extrusion cooking
178
increases the water absorption index of sorghum flour through starch gelatinization (Jafari et al.,
179
2016). Therefore, the addition of extruded sorghum flour increases the water absorption of
180
sorghum-wheat composite dough. Increasing the water absorption might decrease the water-
181
gluten interactions and hence, the DDT of such composite dough increases. According to Gómez
182
et al. (2011) decreasing the gluten hydration due to pre-gelatinization of starch is the main reason
183
for the high DDT.
184
In contrast with our results, Cai et al. (2016) observed DDT reduction with increasing rice flour
185
water absorption index for gluten-free dough. This may be related to the different nature of rice
186
dough compared to the wheat-sorghum composite dough. For gluten-free dough, starch is the
187
main factor for dough viscoelastic properties while for sorghum-wheat composite dough gluten
188
is still responsible for such property.
189
Extrusion cooking increases the sorghum flour amylose-lipid complex formation and decreases
190
its swelling power (Jafari et al., 2016). At this condition, the remaining starch granules absorb
191
water slowly due to their compact structure (Cai et al., 2016; Miyazaki and Morita, 2005) which
192
can affect on reducing sorghum-wheat composite dough.
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Increasing the die temperature and feed moisture increased the water absorption but decreased
194
the DDT and stability of dough (Table 1). Similar results have also been reported for dough
195
containing extruded wheat bran (Gómez et al., 2011), extruded rice flour (Wang et al., 2013) and
196
extruded wheat flour (Martínez et al., 2013). The lost in granule integrity during extrusion
197
cooking of sorghum flour (Jafari et al., 2016) could influence the water holding capacity and as a
198
result the stability of composite dough. For dough with lower water holding capability,
199
pregelatinized starch granules quickly lose their water after reaching to a maximum torque (Cai
200
et al., 2016). As a result, this high free water drops the torque intensely and reduces the dough
201
stability.
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3.2. Dough viscoelasticity using high-strain relaxation test
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Stress relaxation test is one of the fundamental tests to study the viscoelastic nature of food
205
(Bhattacharya and Narasimha, 1997). The level at which the stress begins to decay during
206
relaxation is represented by the constant K1. K1 is a measure of how easily the material deforms
207
(Rodriguez‐Sandoval et al., 2008). The reciprocal of the constant K1 indicates the initial decay
208
rate. Thus, low K1 means that dough possesses a high decay rate indicating less elastic behavior
209
(Bhattacharya, 2010). Addition of sorghum flour extrudate did not significantly change the K1 of
210
composite dough (Table 2).
211
The composite dough containing raw and extruded sorghum flour offered a high extent of
212
relaxation (89% and 80–87%, respectively) (Table 2), meaning that these doughs possessed high
213
viscous characteristics. Extruded sorghum-wheat composite dough had less viscous characteristic
214
due to the presence of amylose-lipid complexes formed during extrusion of sorghum flour. On
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the other hand, decreasing the feed moisture and die temperature decreased the viscous
216
characteristic of composite dough (Table 2).
217
The stress relaxation rate (K2) values represent the degree of solidity (Karaman et al., 2016).
218
Therefore, high K2 values of extruded sorghum-wheat composite dough (Table 2) indicates that
219
the dough had more viscoelastic solid structure. As a result, more normalized force is needed to
220
achieve the same deformation than the composite dough prepared by native sorghum. It can also
221
be suggested that the amount of such force requirement would be higher by decreasing the feed
222
moisture and die temperature.
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3.3. Dough viscoelasticity using low-strain relaxation test
225
For interpret stress–relaxation data of a viscoelastic material, the generalized Maxwell model
226
with a small number of parallel simple elements (a spring and a dashpot arranged in a series) is
227
used. The constant values for the generalized (two-element) Maxwell model (Eq. 2) are shown in
228
Table 3. Extrusion cooking significantly decreased the E1 and increased the E2. Decreasing the
229
feed moisture and die temperature significantly increased both E1 and E2 (Table 3 and Fig. 1).
230
The decay forces represented by E1 and E2 are the elastic components of the Maxwell element
231
which indirectly show the elasticity of the material being tested (Sozer et al., 2008).
232
Bhattacharya and Narasimha, (1997) reported that the first term of Maxwell model (E1 and λ1)
233
had a major contribution (90%) to the total modulus and so, the first terms (E1 and λ1) could be
234
used to describe the structure behavior. High E1 indicates that the dough is more elastic
235
(Karaman et al., 2016). Therefore, low E1 of the composite dough containing extruded sorghum
236
flour indicates its less elastic consistency. On the other hand, increasing the E1 with decreasing
237
the feed moisture and die temperature demonstrated the high elastic consistency of these doughs
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(Table 3 and Fig. 1). Wang et al. (2013) expressed that the dough elasticity of the extruded
239
hemp/ rice flours decreased with increasing the die temperature.
240
Low elasticity could be related to the less water holding capability of extruded flour. Fu et al.
241
(2016) also suggested that the partially gelatinized starch provides more elastic and firmer
242
consistency due to its greater water holding capacity. For extruded sorghum-wheat composite
243
dough, elastic nature of composite dough was not fully formed due to the reduction of gluten
244
hydration.
245
On the other hand, Shiau and Chang, (2013) expressed that the decay forces (E1 and E2)
246
indirectly measures the rigidity of the material being tested. Therefore, decreasing the decay
247
force as a result of extrusion cooking supports that composite dough containing extruded
248
sorghum were soft. Singh and Singh, (2003) also reported that the gelatinized starch forms
249
smoother texture.
250
Decreasing the elasticity with increasing the feed moisture and die temperature could also be
251
related to the high moisture in these doughs. Similar results were reported by Lazaridou et al.
252
(2007) who also determined that the elasticity of rice flour, corn starch and sodium caseinate
253
dough decreased with increasing dough moisture content.
254
Extrusion cooking of sorghum flour significantly increased the relaxation time (λ1) of composite
255
dough. Dough relaxation time increased with decreasing the feed moisture and die temperature
256
(Table 3). Relaxation time (λ) is the time that the viscoelastic material dissipates its force to
257
about 36.8% of the originally applied force. High λ indicates slow decay during relaxation,
258
which is a typical behavior for a viscoelastic matrix with more solid-like structure (Bhattacharya,
259
2010). Therefore, extrusion cooking of sorghum flour formed a more solid-like composite
260
dough.
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Extrusion cooking significantly decreased the composite dough viscosity. Dough viscosity
262
increased with decreasing the feed moisture and die temperature (Table 3). Partially gelatinized
263
starch adhere to one another in the dough (Fu et al., 2016). As a result, the mobility of starch
264
granules decreases and the dough viscosity increases. However, our finding did not follow this
265
fact probably due to the formation of amylose-lipid complexes during extrusion cooking of
266
sorghum flour. Because of amylose-lipid complex formation in extruded sorghum flour (Jafari et
267
al., 2016), starch granules hardly interact with other compounds (Jafari et al., 2016; Miyazaki et
268
al., 2005). Therefore, the mobility of starch granules increases and lowers the dough viscosity.
269
On the other hand, dough water absorption increases through extrusion cooking. According to
270
Bhattacharya and Narasimha, (1997), increasing the moisture content decreases the dough
271
viscosity.
272
Initial compressive force for a dough reflects the resistance per unit area of the sample towards
273
compression prior to relaxation; the higher initial and end compressive force values, the more
274
solid-like behavior of samples. Extrusion cooking increased the initial compressive force,
275
representing a more solid-like structure for extruded sorghum-wheat composite dough (Table 3
276
and Fig. 1). On the other hand, increasing this parameter with decreasing the feed moisture and
277
die temperature shows that the samples were more solid-like.
278
Increasing dough moisture decreases the initial force (Bhattacharya, 2010). At low moisture, the
279
dough composition absorbs less water and forms a firm dough. Therefore, the initial compressive
280
force and resistance to compression increases. According to what mentioned above, because of
281
high moisture in extruded sorghum-wheat dough, the initial force reduces in these samples.
282
However, initial force increased in the composite dough containing extruded sorghum flour. This
283
might be due to the high water solubility index of extruded sorghum flour (Jafari et al., 2016).
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Flour with higher water solubility index forms sticky dough (Spies, 1990) which increases the
285
initial compressive force and develop a solid-like behavior. On the other hand, increasing dough
286
moisture enhances the stickiness of dough (Bhattacharya, 2010).
287
Starch gelatinization and water absorption index of extruded sorghum flour increases with
288
increasing the feed moisture and die temperature (Jafari et al., 2016). The samples with high
289
water binding capacity may inhibit the development of gluten which lowers the consistency of
290
the gluten network and hence the initial force and resistance to strains decrease (Fustier et al.,
291
2008). Therefore, initial force decreased with increasing the feed moisture and die temperature.
292
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3.4. SEM of composite dough
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The morphology of native and extruded sorghum-wheat composite dough are presented in Fig 2.
295
Starch granules were more intact in native sortghum-wheat dough compared to extruded
296
sorghum-wheat dough (Fig. 2 (1)). The intact starch granules structures decreased with
297
increasing the feed moisture and die temperature (Fig. 2 (2, 3, 4, 5, 6 and 7)). Jafari et al. (2016)
298
reported that the degree of sorghum starch gelatinization increased with increasing the feed
299
moisture and die temperature which as a result decreases the intact granules in the extruded
300
sorghum-wheat composite dough.
301
It is well known that the large and small starch granules in native sorghum-wheat composite
302
dough were more embedded in gluten matrix compared to those of extruded sorghum-wheat
303
composite dough (Fig. 2 (1)). However, composite doughs containing sorghum extruded at 110
304
and 160°C with 10% moisture had similar structure to composite dough containing native
305
sorghum (Fig. 2 (2 and 3)). For the native sorghum-wheat composite dough, starch granules were
306
close to each other and formed a uniform and compact structure compared to those of extruded
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sorghum-wheat composite dough. Increasing the feed moisture and die temperature destructed
308
the dough structure uniformity (Fig. 2 (7)). Martínez et al. (2013) also reported that the wheat
309
dough had a compact structure with starch granules embedded in protein matrix whereas
310
incorporation of extruded wheat flour produced an open and less compact structure with a
311
smaller number of starch granules. On the other hand, extruded sorghum-wheat composite dough
312
had voids (Fig. 2 (2, 3, 4, 5, 6 and 7-a)) probably due to the inability of such structure to
313
maintain the fermented gas. Composite dough containing sorghum extruded at 18% moisture and
314
160°C had larger voids (Fig. 2 (7-a)) giving the bread the highest hardness.
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3.5. Temperature profile of dough during baking
317
Understanding the temperature profile of oven and dough during baking helps us to achieve the
318
desired product quality. On the other hand, thermal profile affects the starch gelatinization,
319
volume expansion and browning reactions (Therdthai et al., 2002).
320
The temperature profiles for the center, bottom and upper section (Fig. 3) of native and extruded
321
sorghum-wheat composite dough crumbs were evaluated. The central thermal profile of crumb
322
could be divided into three phases (Fig. 3); phase I, characterized by a very slow temperature
323
increase to up to 38±3°C followed by phase II where a rapid increase in temperature was
324
observed. The end of the thermal profile is characterized by an asymptotic increase in
325
temperature (89±2°C) called the plateau.
326
The heating rate of dough was calculated in the linear region between 45 and 85 °C where a
327
rapid increase in temperature occur (phase II). Extrusion cooking increased the dough heating
328
rate (Table 1). High heating rate value in extruded sorghum-wheat dough might be due to the
329
high free water in this dough. These results were in accordance with the crumb moisture (data
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not shown) in which the crumb moisture increased by extrusion cooking. The heating rate
331
increased with increasing the feed moisture and decreasing the die temperature due to the
332
different amount of free water in these doughs.
333
The bottom and upper section had higher temperature compared to the center of dough (Fig. 3).
334
This might be due to the water condensing phenomena in the center section of dough (Besbes et
335
al., 2013). At temperatures up to 90±3°C, the upper section had higher temperature than the
336
bottom part. In the final stages, the temperature of center part of extruded sorghum-wheat
337
composite dough was even higher than those of upper part (except for dough containing sorghum
338
flour extruded at 110°C with 10% moisture (Fig. 3 (2))). For native sorghum-wheat composite
339
dough, the upper part during all three stages had higher temperature than the center part. This
340
could be related to the high water content of extruded sorghum-wheat dough. Thus, with
341
increasing the temperature during baking, more water moves from the center to the upper part
342
and hence the upper part temperature decreases.
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Conclusion
345
The effect of extruded sorghum flour (10%) on rheological (farinography and stress relaxation
346
behavior), morphological and temperature profile of sorghum-wheat composite dough was
347
investigated. The extrusion cooking of sorghum altered the sorghum-wheat composite dough
348
properties. Addition of extruded sorghum flour increased the water absorption and dough
349
development time but it decreased the dough stability. Composite doughs containing both native
350
and extruded sorghum flour showed high viscose property. However, extrusion cooking
351
decreased the viscose characteristic and hence the extruded sorghum-wheat dough showed more
352
solid-like structure. Maxwell models was more appropriate than Peleg model for evaluation of
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viscoelastic properties of dough. In general, extrusion cooking decreased the composite dough
354
elasticity. Sorghum extrudate increased the heating rate of composite dough crumb. For the
355
native sorghum-wheat composite dough, starch granules were close to each other and formed a
356
uniform and compact structure compared to those of extruded sorghum-wheat composite dough.
357
Increasing the feed moisture and die temperature destructed the dough structure uniformity. The
358
intact starch granules structures decreased with increasing the feed moisture and die temperature.
359
Extruded wheat flour produced an open and less compact structure with smaller starch granules.
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362
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Therdthai, N., Zhou, W., Adamczak, T., 2002. Optimisation of the temperature profile in bread
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442
443
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Figures caption
451
Figure 1. Stress relaxation curves for native and extruded sorghum flour dough at low
452
strain.
453
Figure 2. Scanning electron micrographs of native (1) and extruded sorghum flour (2: 10%-
454
110°C; 3: 10%-160°C; 4: 14%-110°C; 5: 14%-160°C; 6: 18%-110°C; 7: 18%-160°C)-
455
wheat composite dough at 500× (a) and 2000× (b).
456
Figure 3. Temperature profiles of native (1) and extruded sorghum flour (2: 10%-110°C; 3:
457
10%-160°C; 4: 14%-110°C; 5: 14%-160°C; 6: 18%-110°C; 7: 18%-160°C)-wheat
458
composite dough in these three zones of dough baked at 200°C during 21 min.
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M (%)
Water absorption (%)
DDT (min)
Stability (min)
Heating rate (°C/min)
Native
-
60.57±0.2e
2.34±0.2c
4.93±0.1a
8.73±0.9d
110
10
62.50±0.4d
4.99±0.3a
3.66±0.3b
9.63±0.5b
14
62.46±0.1d
4.36±0.1a
3.56±0.3b
10.48±0.9a
18
63.52±0.6c
3.5±0.2b
3.51±0.2b
10.64±0.8a
10
63.82±0.4bc
3.42±0.1b
2.89±0.2c
8.94±0.6cd
14
64.18±0.1ab
3.21±0.1b
2.74±0.3c
9.04±0.5cd
18
64.57±.3a
3.12±0.2b
2.31±0.1c
9.24±0.8bc
M AN U
160
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T (°C)
SC
Table 1. Farinograph and heating rate properties of native and extruded sorghum-wheat dough
Extruded sorghum (T: die temperature, M: feed moisture)-wheat dough and bread, Native: native sorghum-wheat dough and bread. DDT: dough development time.
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Reported values correspond to the mean±standard deviation. Different letters in the same column indicate significant differences (P<0.05).
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Table 2. High-strain relaxation parameters of native and extruded sorghum-wheat dough End force (N)
K1 (s)
K2 (-)
%SR
R2
0.82±0.10a
0.03±0.002bc
1.16±0.10a
24.84±0.2e
89.2±2.1a
99.29
10
0.82±0.06a
0.06±0.005a
1.25±0.09a
39.79±0.5a
80.1±1.8e
99.54
14
0.76±0.10b
0.05±0.004ab
1.19±0.20a
36.34±0.6b
83.3±1.2cd
99.39
18
0.68±0.10dc 0.03±0.002bc
1.14±0.08a
27.59±0.4d
85.3±1bc
99.71
10
0.81±0.08a
0.05±0.007ab
1.15±0.06a
32.51±0.1c
82.9±1.1de
99.38
14
0.71±0.09c
0.03±0.004bc
1.12±0.10a
27.99±0.3d
86.2±1.3b
99.52
18
0.67±0.10d
0.01±0.002c
1.09±0.10a
18.21±0.3f
87.2±1.9ab
99.47
110
160
SC
Native
RI PT
Initial force (N)
M (%)
M AN U
T (°C)
Extruded sorghum (T: die temperature, M: feed moisture)-wheat dough, Native: native sorghumwheat dough. %SR: % stress relaxation.
AC C
EP
TE D
Reported values correspond to the mean±standard deviation. Different letters in the same column indicate significant differences (P<0.05).
ACCEPTED MANUSCRIPT
Table 3. Low-strain relaxation parameters of native and extruded sorghum-wheat dough
0.13±0.09g
1.70±0.08f
28.40±0.40a 55.45±0.2a
3.75±0.4g
99.63
10
0.142±0.04b
42.27±0.10a
31.57±0.10b
1.90±0.08bc
4.50±0.4b
80.53±0.3a
99.52
14
0.115±0.08cd
30.90±0.10b
25.87±0.05d
1.74±0.07bc
2.98±0.1cd
53.9±0.4c
99.76
18
0.109±0.10d
22.82±0.08d
24.17±0.07e
1.62±0.04c
2.64±0.3d
37.13±0.2d
99.81
10
0.121±0.09c
28.37±0.20c
35.95±0.10a
2.07±0.08b
4.38±0.2b
58.8±0.2b
99.75
14
0.114±0.10cd
18.11±0.20e
28.17±0.20c
1.89±0.07bc
3.21±0.3c
34.32±0.1e
99.42
18
0.098±0.04d
15.57±0.09f 25.08±0.30de
1.66±0.09c
2.46±0.1d
25.97±0.2f
99.58
SC
Extruded sorghum (T: die temperature, M: feed moisture)-wheat dough, Native: native sorghumwheat dough. %SR: % stress relaxation.
TE D
Reported values correspond to the mean±standard deviation. Different letters in the same column indicate significant differences (P<0.05).
EP
160
AC C
110
RI PT
32.510±0.20a
Native
E2
η1 (KPas)
R2
(KPa)
E1 (KPa)
λ2 (s)
η2 (KPas)
λ1 (s)
M (%)
M AN U
T (°C)
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
Figure 1.
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Figure 2.
ACCEPTED MANUSCRIPT
3
M AN U
SC
2
RI PT
1
TE D
4
AC C
EP
6
Figure 3.
5
7
ACCEPTED MANUSCRIPT
- Using sorghum extrudate increased the water absorption and dough development time while it decreased the stability of sorghum-wheat composite dough. - Extrusion cooking decreased sorghum-wheat composite dough elasticity and viscosity.
RI PT
- Maxwell models was well used to describe the viscoelasticity of the sorghum-wheat composite dough. - Sorghum extrudate increased the heating rate of sorghum-wheat composite dough during baking.
AC C
EP
TE D
M AN U
SC
- Sorghum extruded at higher feed moisture and die temperature had negative effect on the uniformity of sorghum-wheat composite dough structure.