ARTICLE IN PRESS

FOODHYDROCOLLOIDS

0268-005X/$ - s

doi:10.1016/j.fo

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(J. Ahmed), Ho

Food Hydrocolloids 22 (2008) 278–287

www.elsevier.com/locate/foodhyd

Thermal and dynamic rheology of insoluble starch from basmati rice

Jasim Ahmed�, Hosahalli S. Ramaswamy, Anwer Ayad, Inteaz Alli

Department of Food Science & Agricultural Chemistry, McGill University, Macdonald Campus, 21111 Lakeshore, Ste Anne de Bellevue, Canada H9X 3V9

Received 27 September 2006; accepted 20 November 2006

Abstract

Insoluble rice starch obtained as a by-product of the sequential solvent extraction of proteins from commercial Basmati rice flour was

evaluated for thermal and rheological properties. Differential scanning calorimetry (DSC) was employed to gather data on gelatinization

temperature and heat of gelatinization of rice starch at a constant heating rate. A small amplitude oscillation shear rheometer was used to

study gel rigidity and other rheological characteristics. Elasticity (G0) of thermally induced starch gel increased as function of starch

concentration and decreased with heating rate. Swelling power (SP), close packing concentration and pasting properties of rice starch

were also investigated. Isothermal heating of starch in the temperature range of 70–85 1C exhibited a systematic decrease in elasticity (G0)

with heating time where as a reverse trend was observed above 90 1C. Non-isothermal heating of starch at a linear rate of temperature

increase with time exhibited peak elastic modulus (G0) values at specific temperatures which corresponded to starch gelatinization

temperatures as observed by DSC. During non-isothermal starch gelatinization kinetics of rice starch, G0�t data up to peak value were

considered and a rate equation was evaluated. A first-order reaction kinetics adequately described the rice starch gelatinization and the

process activation energy obtained ranged between 32.3 and 42.2 kJ/mol.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Basmati rice starch; Gelatinization; Gel rigidity; Thermal characteristics

1. Introduction

Starch is a complex food hydrocolloid (polymer of a-D-glucose and partially crystalline polymer) where one wouldexpect two-phase transitions during heating in presence ofexcess water (Maurice, Slade, Sirett, & Page, 1985). Starchgranules absorb water resulting in swelling up to severaltimes their original size and lose their crystallinity. Thecomplete process is known as gelatinization. Gelatinizationof starch paste involves changes in amylose and amylo-pectin (Kim, Lee, & Yoo, 2006). Kinetics of starchgelatinization can be studied either by heating it in wateror steam in vitro using pure starch or in situ using wholegrain. In case of the in vitro gelatinization, there is nophysical barrier between starch granules and water

ee front matter r 2006 Elsevier Ltd. All rights reserved.

odhyd.2006.11.014

ing author. Tel.: +1514 398 7583; fax: +1 514 398 7977.

esses: jasim.ahmed@mcgill.ca, jahmed2k@yahoo.com

sahalli.ramawamy@mcgill.ca (H.S. Ramaswamy).

molecules, and starch is readily accessible (Turhan &Gunasekaran, 2002).Gelatinization kinetics of starch has been studied

extensively by different techniques. Karapantsios, Sakoni-dou, and Raphaelides (2002) studied starch gelatinizationkinetics by changes in electrical conductance with time.Several investigations on gelatinization kinetics of starcheshave been reported by iodine blue value technique (Birch &Priestley, 1973; Njintang & Mbofung, 2003). However,most of the researchers (Baik, Kim, Cheon, & Ha, 1997;Riva, Schiraldi, & Piazza, 1994; Spigno & De Faveri, 2004)have used differential scanning calorimetry (DSC) for thestudy where the degree of gelatinization is directlymeasured from gelatinization enthalpy. Starch gelatiniza-tion has been reported to follow a first-order reactionkinetics (Njintang & Mbofung, 2003; Ojeda, Tolaba, &Suarez, 2000; Riva et al., 1994; Turhan & Gunasekaran,2002) irrespective of the measuring technique. It is believedthat during starch gelatinization, crystallites melt and bothmolecular and crystalline structures get disrupted (Cooke

ARTICLE IN PRESSJ. Ahmed et al. / Food Hydrocolloids 22 (2008) 278–287 279

& Gidley, 1992). However, gelatinization occurs in a non-equilibrium state and, therefore, knowledge of reactionkinetics is essential to predict reaction mechanism moreprecisely.

Small amplitude oscillatory shear (SAOS) measurementsis a particularly useful method to study the gelation/gelatinization phenomenon, in monitoring the kinetics ofnetwork development, provided that the measurements arewithin viscoelastic limit (Biliaderis, 1992) and the strain isrestricted to less than 5%. The techniques afford themeasurement of dynamic rheological functions, withoutaltering food texture and are far more reliable than steadyshear measurement (Bistani & Kokini, 1983). Comparedwith DSC-based starch gelatinization technique, much lessis known about the rheological approach; nevertheless,some studies on such kinetic approaches have beenpublished (Ahmed, Ramaswamy, & Alli, 2006; Kubota,Hosokawa, Suzuki, & Hosaka, 1979; Yoon, Gunasekaran,& Park, 2004). Recently, Yamamoto, Makita, Oki, andOtani (2006) studied alkali-induced rice starch gelatiniza-tion kinetics using conventional steady shear measure-ments. The results were determined by power-law model toelucidate the normality dependence of a flow behaviorindex and a consistency coefficient.

The reaction kinetics in food systems are commonlystudied under isothermal heating conditions. The processusually is simpler to conduct and evaluate the kineticparameters than under non-isothermal condition (Dolan,2003). However, the isothermal process has some practicallimitations especially when dealing with samples which aredifficult to heat instantaneously to testing temperatures.The rate of heating of food materials depends on the state(liquid vs. solid or semi-solid), size, thermal conductivity,viscosity/density and moisture content. In some instances,especially at higher temperatures, it might take even longerto achieve the target temperature than to hold it at thattemperature to complete the reaction. In most situations,thermal lag corrections are applied to compensate for thenon-isothermal contribution to the total process. Thethermal behavior of starches is complex compared to otherdestruction kinetics because of the several physicochemicalchanges that occur during heating may involve gelatiniza-tion, melting, glass transition, crystallization, change ofcrystal structure, volume expansion, molecular degradationand motion of water (Yu & Christie, 2001). Kinetic datagathering under non-isothermal conditions has beenrecently practiced which allows parameter estimation froma single experiment where temperature is varied over therange of interest, and samples are taken at various intervals(Dolan, 2003; Yoon et al., 2004). Parameters are alsoestimated from a dynamic environment closer to realprocessing conditions, and thermal lag problems areovercome (Cunha & Oliveira, 2000).

Several studies have been carried out on non-isothermalkinetics for food systems (Ahmed et al., 2006; Claeys,Ludikhuyze, van Loey, & Hendrickx, 2001; Dolan, 2003;Rhim, Nunes, Jones, & Swartsel, 1989; Yoon et al., 2004).

It would be interesting to study the influence of non-isothermal heating on starch rheology. Such studies withSAOS measurement techniques in the linear visco-elasticrange would provide a broader insight to the gelatinizationkinetics. The order of reaction and the necessary energyrequirement to achieve critical gel rigidity (activationenergy) can be calculated from such thermorheologicaldata. These studies could provide a better insight intogelling mechanisms as well as useful data for potentialsubstitution of one starch for another in food productdevelopment procedures. Although Basmati rice hasenormous market throughout the globe among Asianpeople for its characteristics popcorn like flavor, there hasbeen no systematic study on gelatinization kinetics anddynamic viscoelastic characteristics which is the objectiveof this study. An implicit objective of the work was toevaluate non-isothermal heating kinetics of starch gelati-nization by rheological approach.

2. Materials and methods

2.1. Materials

Commercial Indian Basmati rice, a non-waxy long grainrice (cultivar Basmati 370) was purchased (4.5 kg in a jutebag) from a Montreal area supplier and one bag of rice wasused for the entire study. Rice was ground to powder usinga laboratory grinder and passed through 1mm screen.

2.2. Starch isolation

Rice starch was obtained as the residue from sequentialprotein extraction of Basmati rice based on the proteinextraction technique of Hu and Essen (1981) and Kwon,Park, and Rhee (1996) with modification. Ground Basmatirice were steeped in distilled water (1:10 ratio) at 25 1C for16 h, centrifuged at 10,000 rpm for 30min. The watersoluble protein part was decanted for further processing.The insoluble part was subjected to further sequentialsolvent extraction using 0.5M NaCl, 0.1M NaOH, 70%ethanol and finally 50% acetic acid (1:10 ratio, meal tosolvent) for an hour. Each solvent extraction step insolubleresidue was separated by centrifugation at 10,000 rpm for30min. The final insoluble starch residue was frozen at�80 1C for 2 h, freeze dried (Model 77550, LabconcoCorporation, Kansas City, USA) at a vacuum pressure of1� 10�2 Pa at �55 1C for 48 h. Freeze dried samples wereused to prepare starch dispersions at various concentrationlevels (10, 20, 25 and 30%w/w) for calorimetric andrheological studies. Ground rice dispersion (25%w/w) wasmade using distilled water to compare gelatinizationbehavior with isolated starch.

2.3. Composition analysis

Moisture, ash, protein and lipid contents of freeze driedstarch sample were determined according to the AOAC

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0.001

0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1 10 100

G' (

Pa)

40C70C90C

10000

Applied stress (Pa)

Fig. 1. Stress sweep for 25% rice starch dispersion at selected temperature

at frequency of 1Hz.

J. Ahmed et al. / Food Hydrocolloids 22 (2008) 278–287280

methods (1984). The analysis indicated that the starchsample contained 3.7% moisture, 0.05% ash, 0.3% proteinand 0.25% lipid. All the chemical analyses were reported asthe means of three replicates. Amylose content wasdetermined according to the procedure of Juliano et al.(1986) and it was found to be 24.6%. The pH of the starchdispersions varied between 3.71 and 3.79. An isolatedIndica rice starch with acidic pH was earlier reported bySingh, Okadome, Toyoshima, Isobe, and Ohtsubo (2000).

2.4. Differential scanning calorimetric (DSC) measurement

A differential scanning calorimeter (DSC) (TA Q100,TA Instruments, New Castle, DE, USA) was employed tomeasure the thermal analysis for rice starches. Starch andrice flour samples (about 5mg) were weighed intoaluminum DSC pans. Test samples were moistened withdeionized water to achieve a moisture content of 78–80%(w/w) in both rice flour and starch and then the pan washermetically sealed. The sample containing pans wereallowed to stabilize at room temperature (21 1C) for 2 hprior to subjecting them to DSC. Thermal scans wereperformed from 30 to 130 1C at a heating rate of 5 1C/min.An empty pan was used as a reference. The DSCmeasurements were done in triplicate. Thermal transitionsof rice starches were measured in terms of onset (To) andpeak (Tm) gelatinization temperatures. The enthalpy (DH)associated with starch gelatinization was evaluated as thetotal peak area of the endotherm using the UniversalAnalysis Software (version 3.6C, TA Instruments, NewCastle, DE, USA).

2.5. Swelling power (SP) and close packing concentration

SP and close packing concentration (C� ¼ concentrationat which starch granules are swollen to equilibrium and fillup the available space in a starch suspension) of 1% starchsuspension at selected temperatures (70, 75, 80, 85 and90 1C) were determined as per method described byVandeputte Derycke, Geeroms, and Delcour (2003) forrice starch.

2.6. Dynamic rheological measurement

SAOS measurement was made with a TA AR 2000controlled-stress rheometer (TA Instruments Inc., NewCastle, DE, USA) interfaced with supplier providedcomputer software (Rheology Advantage Data AnalysisProgram, TA). A parallel plate (6 cm diameter) measuringgeometry was used with a gap width of 1000 mm. Eachstarch sample (approximately 2mL) was transferred to therheometer plate and heating was carried out in situ of therheometer plate. The temperature was increased andcontrolled precisely by a peltier system (attached with theinstrument). A solvent trap was used during rheologicalmeasurement above 60 1C and an additional precaution

was also taken to prevent moisture loss by covering theedge of the plate with a thin layer of light paraffin oil.In order to ensure that all the measurements are carried

out within the linear viscoelastic ranges, initially oscillationstress amplitude sweeps were tested for selected concentra-tions and temperature of rice starch dispersion. Fig. 1illustrates linear viscoelastic range for 25% rice starchdispersion at selected temperature (40, 70 and 90 1C). Basedon these results, oscillation stresses for various concentra-tions were selected. Following an initial equilibration ofsamples for 5min at 30 1C, ramp heating carried was out at2 and 5 1C/min to an endpoint of 100 1C (non-isothermalheating) at a frequency of 1Hz. For isothermal heating,time sweep of 25% starch dispersions were performed at70, 80, 85, 90 and 95 1C for 30min (come up time 60–95 swas excluded) at constant frequency of 1HZ. In anotherset of experiments, starch samples were isothermally heatedto 90 1C and held for 30min with and without time sweepand followed by immediate cooling to 20 1C after which thefrequency sweep tests were carried out at 0.1–10Hz tocompare gel rigidity.All rheological measurements were carried out in

triplicate and the experimental dynamic rheological datawere obtained directly from the TA Rheology AdvantageData Analysis software V 5.1.42 (AR 2000, TA Instru-ments, New Castle, DE, USA). The deviation did notexceed 5% between duplicate runs, as the experiment wasrepeated. The average of the three runs was reported as themeasured value.

2.7. Pasting properties

Pasting properties (gel point temperature, peak viscosityand breakdown viscosity in terms of complex viscosity) of10% rice starch was obtained from a rheogram. Recently,Tsutsui, Katsuta, Matoba, Takemasa, and Nishinari (2005)advocated that a conventional rheometer has more

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advantages over brabender visco-amylograph (BVA) andrapid visco-analyser (RVA) to investigate starch character-istics by not breaking the gel structure during measurementsince the instrument operates at considerably low shearstrain or stress. In this study, the time–temperaturesequences of RVA as described by Vandeputte et al.(2003) were imitated for rice starch. The heating andcooling cycles were programmed as follows: the sample washeld at 50 1C for 2min, heated to 95 1C in 9min, heldat 95 1C for 15min, cooled down to 50 1C in next 9minand finally held at 50 1C for 10min. The starting gel pointtemperature was considered as the temperature wherethere was cross over of G0 and G00 where as themeasurement of peak viscosity was based on the peakvalue of complex viscosity (Z�) which takes into accountboth dynamic modulii G0 and G00 of rice starch duringpasting. The difference between the peak and the minimumof Z� was considered as break down viscosity duringheating ramp.

70 75 80 85

Hea

t flo

w (

W/g

)

Starch-0.8

-0.7

-0.6

Flour

-0.9

-1

-1.160 65

Temperature (°C)

74.2°C

75.5°C

Fig. 2. Thermograms of 25% Basmati rice flour and 25% rice starch

dispersion.

2.8. Kinetic modeling of rheological data

The general form for non-isothermal kinetics combiningreaction rate, time–temperature profile and Arrheniusrelationship can be represented as

Z C

C0

dC

Cn ¼ k0

Z t

0

exp �Ea

RT

� �dt, (1)

where C0 is the concentration at zero time, C theconcentration at time t, k0 the pre-exponential or frequencyfactor, Ea the activation energy (J/mol), T the absolutetemperature (K), and R the universal gas constant (8.314 J/molK).

Heating rate affects the evaluation of this general formof the equation. The non-isothermal kinetic relation basedon the experimental data and regression analysis wascarried out following the steps described by Rhim et al.(1989) for a linearly increasing temperature system.

For nth order decomposition reaction the rate isdescribed by

�dC

dt¼ kCn. (2)

The temperature dependency of reaction rate constant iswell represented by the Arrhenius relationship:

k ¼ k0 exp �Ea

RT

� �. (3)

Combining Eqs. (2) and (3) and integrating yields:

ln �1

Cn

dC

dt

� �¼ ln k0 �

Ea

R

� �1

T

� �. (4)

The kinetic parameters, Ea and k0 are estimated from anArrhenius-type plot of Eq. (4).

2.9. Data analysis

Minitab statistical software package (Minitab Inc. 2000)and Microsoft Excel software (XP Professional 2003) wereused for multiple linear regressions and other data analysis.

3. Results and discussion

3.1. Differential scanning calorimetry

Starch samples were thermally scanned in a DSC atdifferent moisture levels (4–67%) and sample containing67% moisture (1:2) exhibited only an endothermic peak. Itindicated the hydration of starch should be optimum tomelt crystallites (causing the glass transition to be passed)and consequently a DSC thermal transition was obtained.At low moisture contents, the plasticizing effect of water islow that leads to high melting temperature of the crystal-lite. At 4% moisture level, the thermal transition of ricestarch was found at 155 1C. DSC thermograms of Basmatirice flour and isolated starch dispersions are illustrated inFig. 2. One major endothermic transition was observed forboth samples during heating of starch in presence of waterresulting in the irreversible disruption of molecular orderwithin rice/starch granule whereas another peak was alsonoticed at higher temperature range (109–113 1C) for starchdispersion (not shown). This observation was supported byearlier studies reporting a single DSC endotherm for starchgelatinization in presence of excess water (Biliaderis, Page,Maurice, & Juliano, 1986; Calzetta resio & Suarez, 2001).The onset and peak gelatinization temperatures for riceflour dispersion (moisture content, mcE25% w.b.) were69.8 and 75.3 1C where as the corresponding data forisolated starch dispersion (mcE25% w.b.) were 63.0 and74.2 1C, respectively. There was a minor decrease in peak

ARTICLE IN PRESSJ. Ahmed et al. / Food Hydrocolloids 22 (2008) 278–287282

gelatinization temperature by 1 1C for isolated starch. Theobserved onset and peak temperatures values of rice starchgelatinization were higher than reported in the literature(Biliaderis & Juliano, 1993; Li & Yeh, 2001; Tsai & Lii,2000). Rice starch appeared to have a higher heat ofgelatinization (DH) of 16.2 J/g compared to whole flour(4.81 J/g). The observed value of heat of gelatinizationfollowed the same trend as that reported in the literature(10–30 J/g) for starches (Biliaderis & Juliano, 1993; Li &Yeh, 2001; Roos, 1995). The differences in gelatinizationtemperatures (To and Tm) among rice starches may beinfluenced by the following factors: starch composition(amylose to amylopectin ratio), granular architecture(crystalline to amorphous ratio), and molecular structureof amylopectin (extent of branching, unit chain length, andpolydispersity) (Gunaratne & Hoover, 2002). Furthermore,high amylose starches with longer average chain have beenreported to exhibit higher transition temperatures (Janeet al., 1999). However, To and Tm values were found to beindependent of water while the water to starch ratioexceeds 1.5–1 (Calzetta resio & Suarez, 2001). Thegelatinization temperature for rice starch particles repre-sents the amount of crystalline regions within a starchgranule that are composed of small crystallites withdifferent crystal melting temperatures (Biliaderis, Maurice,& Vose, 1980; Vasanthan & Bhatty, 1996).

3.2. Rheological properties of rice starches

3.2.1. Effect of temperature sweep

Effect of heating of rice flour and starch dispersion (both25%) at a constant rate of 2 1C/min on elastic modulus isshown in Fig. 3. Starch dispersion exhibited significantlyhigher G0 value (E100 times) as compared to rice flourdispersion at almost similar moisture content (81% and82% w.b.) in the temperature range of 40–60 1C. However,increase in G0 was relatively slower (amylose enriched solstage) at that temperature range. A sharp increase in G0 was

0.01

0.1

1

10

100

1000

10000

40 50 60 70 80 90 100 110

G' (

Pa)

25% flour dispersion25% starch dispersion

Temperature (°C)

Fig. 3. Thermograms of 25% Basmati rice flour and 25% starch

dispersion at frequency of 1Hz.

noticed between 60 and 82 1C for both samples leading tosol–gel conversion of starch. The peak value of G0 (termedas G0max) for starch at 82 1C was not significantly differentfrom that of flour dispersion (G0max at 80 1C). Tsai, Li, andLii (1997) and later on Li and Yeh (2001) found similarobservations for various starches during dynamic rheolo-gical measurements. A significant increase in G0 of ricestarch on heating is caused by formation of a three-dimensional (3D) gel network developed by leached outamylose and reinforced by strong interaction among theswollen starch particles (Eliasson, 1986; Hsu, Lu, &Huang, 2000; Ring, 1985; Vasanthan & Bhatty, 1996).However, our observations for G0max are not close inagreement with those reported values of for waxy and non-waxy rice starches. Most studies report G0max values for ricestarch at about 62 1C. A major factor for such propertyresponse is presence of significant amount of amylosecontent (24.6%). The gels of all high amylose content ricestarches have been credited with higher G0 indicating awell-cross-linked network structures and increase in G0

followed an exponential relationship with amylose content(Biliaderis & Juliano, 1993).The rice flour dispersion maintained almost constant G0

values in temperature range of 80–85 1C. It is believed thatabove the melting temperature starches exhibit irreversibleswelling and solubilization of leached out amylose (Bilia-deris & Juliano, 1993) and, therefore, a nearly constant G0

was observed in those temperature ranges. However,maintaining constant G0 over wider temperature range offlour dispersion could be contributed by resistance toamylose leaching in presence of lipid and protein. Minorconstituents (lipids, phorphorus) also play important rolein addition to concentration–temperature–time protocol onviscoelasticity of starch dispersions (Biliaderis, 1992).Further heating beyond G0max, the G0 decreased signifi-

cantly, indicating destruction of gel structure duringprolong heating as reported earlier by various researchers(Lii, Lai, & Tsai, 1996; Lii, Shao, & Tseng, 1995). Thedestruction of structure could be due to the ‘melting’ of thecrystalline regions remaining in the swollen starch granuleor resulted from the disentanglement of the amylopectinmolecules in the swollen particles that softens the particles(Keetels & van Vliet, 1994; Tsai et al., 1997). The networkcollapse due to the loss of interaction between particles andnetwork may be another reason for lowering G0 value.

3.2.2. Starch swelling and effect of concentration

SP and close packing concentration (C�) of Basmati ricestarch were studied as a function of temperature(70–90 1C). SP of rice starch increased with temperaturewhere as a decrease in C� was noticed with an increase intemperature. Both parameters were found to follow linearrelationships (R2

X0.87):

SP ¼ 0:23T � 8:76 ðR2 ¼ 0:87Þ, (5)

C� ¼ �0:21T þ 26:3 ðR2 ¼ 0:97Þ. (6)

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2C/min5C/min10C/minG

' (Pa

)

7500

6000

4500

J. Ahmed et al. / Food Hydrocolloids 22 (2008) 278–287 283

Similar observations for various cereal starches includingwaxy and non-waxy rice starches have been reported in theliterature (Li & Yeh, 2001; Vandeputte et al., 2003).

3.2.3. Effect of concentration

Effect of starch concentration on G0 during temperaturesweep at constant frequency (1Hz) is shown in Fig. 4. Itcan be clearly observed from the figure that an increase instarch concentration increases gel rigidity. Starch concen-tration at 10% exhibited a very weak gel while concentra-tion at and above 20% significantly increase the gelrigidity. An increase in starch concentration from 10% to30% showed an enormous increase in gel rigidity even at50 1C (increasing it from fractional decimal levels to severalthousands higher). It can also be clearly seen from thisfigure that a critical concentration (�20%) is required toresult in a moderate gel network. A close look on the figure(Fig. 4) revealed that though gel rigidity differed at thelower temperature range, the bulk of starch gelatinizationoccurred in the temperature range between 70 and 80 1Cexcept for 10% concentration. At higher temperatures(90–100 1C) both 20% and 25% starch concentrationsuperimposed with each other and exhibited almost similargel rigidity. Studies on gel rigidity (G0) as function ofconcentration revealed that a linear relationship exist forwheat and maize starch at concentration range of 6–30%(Ring, 1985) however, a stronger concentration depen-dence as power-law relationship was reported for ricestarch gels (8–40%) (Biliaderis & Juliano, 1993).

Some researchers have advocated that rheology ofstarches depend on starch concentrations and on closepacking concentration (C�) (Steeneken, 1989; Vandeputteet al., 2003). At low concentration (C) levels (CoC�) theviscosity is determined in the first instance by the volumeoccupied by the swollen granules (termed as ‘dilute’regime), and, to a lesser extent, by the soluble fraction.

0.01

2000

4000

6000

8000

10000

50 60 70 80 90 100

10%20%25%30%G

' (Pa

)

Temperature (°C)

Fig. 4. Effect of concentration on gel rigidity of Basmati rice starch

dispersions during temperature ramp at 2 1C/min.

However, in a concentrated regime (C4C�), starchgranules cannot swell to their equilibrium volume dueto limited availability of water. The rheological character-istics of starch suspensions are then primarily determinedby the particle rigidity of the swollen granules. Inexperimental practice though, C� is not very well definedand quite a large transition domain between the concen-trated and dilute regimes is observed (Vandeputte et al.,2003).

3.2.4. Effect of heating rate

A comparison of heating rate on gel rigidity and peakgelatinization temperature of 25% rice starch is illustratedin Fig. 5 and Table 1, respectively. Peak gelatinizationtemperature (Td) increased as function of heating rate asevidenced from Td value of 73.2 1C at 1 1C/min comparedto 80 1C at heating rate of 10 1C/min. This observation wassupported by DSC data (not shown) and also literaturevalues (Calzetta Resio & Suarez, 2001). Marshall (1992)reported that increasing heating rate from 2 to 10 1C/minled to a higher value of gelatinization temperature of riceflour. The use of rapid heating rates may lead totemperature gradients within the sample, which maycontribute to the broadening of the endotherm (Ozawa,1970). Firmer gel rigidity developed at slower heating ratecould be attributed by combining effect of heatingtemperature and duration of heating.

030 40 50 60 70 80 90 100

3000

1500

Temperature (°C)

Fig. 5. Effect of heating rate on gel rigidity of 25% Basmati rice starch.

Table 1

Effect of heating rate on peak gelatinization temperature

Heating rate (1C/min) Peak gelatinization temperature (1C)

1 73.2

2 74.0

3 75.0

5 75.8

7 78.2

10 80

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3.2.5. Effect of heating time (isothermal heating)

Time sweep experiments provide insight whether tem-perature or time predominates in rice gelatinization process(Hsu, Lu, & Huang, 2000). Fig. 6 illustrates the isothermalheating behavior on gel rigidity of rice starch at selectedtemperatures (70–95 1C). The temperature ranges wereselected based on gelatinization temperature of rice starchas observed from DSC thermogram. At the vicinity ofgelatinization temperature ranges (70–75 1C) G0 decreasedwith an increase in temperature; however, the pattern wasnot followed systematically as function of time andtemperature. To make it more clear, the resulting gelrigidity of rice starch as function of time–temperature hasbeen divided into two groups (70–85 and 90–95 1C). At70 1C, a systematic increase in G0 was found as function oftime initially (up to 300 s), then exhibited constant values ofG0 between 300 and 1100 s and finally showed a decreasingtrend above 1100 s. An initial increase in G0 was alsoobserved for first 500 s at 75 1C followed by a decreasingtrend while only a decreasing trend of G0 was observed at80 and 85 1C, respectively, from the beginning. Theseobservations are close in agreement with DSC observationsand clearly indicated that heating time has a pronouncedeffect on gel rigidity of rice starch in addition to

0

G' (

Pa)

70C75C80C85C

0

3000

6000

9000

12000

0 500 1000 1500

Time (s)

Time (s)

2500

2000

1500

1000

500500 1000 1500 2000 2500 3000

90C95C

2000

G' (

Pa)

a

b

Fig. 6. Effect of isothermal heating on gel rigidity of 25% rice starch at

selected temperature.

temperature effect. A contrast temperature–time effect onG0 exhibited by rice starch in the temperature range of90–95 1C. The G0 of starch heated at 95 1C for 15minexhibited lower values than corresponding gel at 90 1C.However, a sharp increase in G0 was noticed at about 850 s.This unusual observation of G0 could be contributed bymelting of amylopectin crystallites above 90 1C whichenhanced gel rigidity significantly.

3.2.6. Pasting properties

The starting gel point temperature for 10% suspensionswas found to be at 65.9 1C for Basmati rice starch whichwas higher than for waxy starches (60–62 1C) but similar tonormal starches (64–77 1C) (Vandeputte et al., 2003). Theobserved starting gel point (10% dm) of the rice starch wasalmost similar to the DSC onset gelatinization temperaturedescribed earlier section (65.6 1C). The viscosity of pastedepends largely on the degree of gelatinization of the starchgranules and the extent of their molecular breakdown. Thepeak complex viscosity for 10% suspensions obtained fromthe rheometer was found to be 13.79 Pa s (at 83.3 1C duringheating ramp). The complex viscosity during pasting at95 1C was recorded as 4.04 Pa s (Fig. 7) which graduallydecreased to 0.88 during holding periods (15min). How-ever, the final Z� value (during cooling) increased againfrom 2.82 to 3.71 during holding period (10min) at 50 1C.The breakdown paste complex viscosity was measured as6.6.8� 10�2 Pa s. Earlier reported values for rice starchvaried between 190 and 380 RVU for normal rice starcheswhere as so far, no information is available on complexviscosity of rice starches during pasting.

3.3. Non-isothermal kinetic studies

The previous discussion on isothermal heating of starchindicated that gelatinization is affected by both tempera-ture and duration of heating. Isothermal starch gelatiniza-tion kinetics can be evaluated from property evaluationunder carefully controlled isothermal time temperature

0.001

0.01

0.1

1

10

100

0 500 1000 1500 2500 300045

60

75

90

105

η* (

Pa.s

)

Pasting η*

Time (s)

2000

Time-viscosity

Time-temperature

Peak η*

Tem

pera

ture

(°C

)

Fig. 7. Representative pasting properties of rice starch in terms of

complex viscosity as function of temperature and time.

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0.1

1

10

100

1000

10000

30 40 50 60 70 800

300

600

900

1200

1500

Tim

e (s

)

-9

-8.5

-8

-7.5

-7

-6.5

-6

0.0028 0.0029 0.0031 0.0032 0.0033

YG

' (Pa

)

y = -3875.7x + 4.42

R2 = 0.87

Temperature (°C)

1/T (K)

0.003

a

b

Fig. 8. Non-isothermal reaction kinetics of rice starch gelatinization: (a)

time–temperature effect; and (b) reaction order study.

J. Ahmed et al. / Food Hydrocolloids 22 (2008) 278–287 285

heating combinations. They can also be evaluated underthe more common non-isothermal heating conditions. Thetechnique has been used for protein gelation (Ahmed et al.,2006; Yoon et al., 2004) and enzymatic sugar hydrolysis(Rhim, Nunes, Jones, & Swartzel, 1989) kinetics. Gel pointgenerally occurred due to early stage of gelation of starchyfoods. At faster rate of heating, the gelation point may bemissed. Lapacin and Pricl (1995) advocated that thegelatinization process can be accurately depicted rheologi-cally only if small deformations are used in the frequencyrange of 0.1–1Hz and also it is advisable to study gelationkinetics at milder condition or slower rate of heating(Labropoulos & Hsu, 1996). In the present study, thetemperature for gelation was selected and evaluated fromDSC thermogram (74.4 1C) and the non-isothermalkinetics of starch gelatinization was considered up to74.4 1C (Fig. 6a) at heating rate of 2 1C/min.

The non-isothermal kinetics is based on combination ofthe Arrhenius equation and time–temperature relationship(Rhim, Nunes, Jones, & Swartzel, 1989). The kineticequation (Eq. (4)) can be changed to Eq. (7) in terms ofrheological parameters (G0 and dG0) instead of reactantconcentration (C) and its change (dC). The negative sign of(Eq. (4)) is replaced by positive sign due to increase in G0

during heating (positive dG0)

ln1

G0n

�dG0

dt

�¼ ln k0 �

Ea

R

� �1

T

� �. (7)

A multiple linear regression technique was employedwith rice starch kinetic data set (at interval of 10 s) todetermine the order of the reaction (n) after changing theabove equations into the following linear forms:

lndG0

dt

� �¼ ln k0 þ n ln G0 �

Ea

R

� �1

T

� �. (8)

The reaction order of the above equation was found tobe 0.95 for 25% rice starch dispersion. An average valuefor the activation energy was found to be 42.2 kJ/mol. Thecoefficient of variation (R2) and standard errors were 0.99and 0.15, respectively, from the data. The other kineticparameter obtained from the equations is: k0 ¼ 3.53� 103.Since the calculated value of n was nearly equal to one, afirst-order reaction kinetics was considered for rice starchgelatinization. The reaction order was further verified byconsidering n ¼ 1 and 2 in Eq. (7) which confirmed betterfit by first-order model. Karapantsios et al. (2002)investigated the non-isothermal gelatinization kinetics ofmaize starch dispersion by analyzing changes in electricalconductance data recorded with time and reported tofollow first-order reaction kinetics.

An Arrhenius-type plot for starch gelatinization withdata generated by linearly increasing temperature ispresented in Fig. 8b. Contrary to isothermal heating, eachdata point in non-isothermal heating kinetics is determinedin a continuous manner to generate kinetic parameters. Aleast-squares linear regression was employed to calculateactivation energy from the slope of Eq. (8). The magnitude

of activation energy (Ea) was 32.2 kJ/mol (R2¼ 0.86;

standard error 0.18). The reported activation energy ofgelatinization for cereal starches ranged between 62 and76 kJ/mol (Okechukwu & Rao, 1995, 1996) which weresignificantly higher (51–234 kJ/mol). A significant lower Ea

for 25% rice starch is because of consideration oftemperature up to gel point where as other works it takesinto account of wider range of temperature and the processof heating (most of the cases isothermal heating).

4. Conclusions

Thermal and dynamic rheological characteristics ofBasmati rice starch isolate were studied. Calorimetrically,starch dispersion exhibited peak gelatinization temperatureat 74.2 1C and heat of gelatinization was found to be16.2 kJ/mol. Starch gelatinization measured by rheometryrevealed that gel elasticity (G0) increased as function ofconcentration and peak gelatinization temperature in-creased with an increase in heating rate. Isothermal heatingof 25% rice starch exhibited temperature dependency gelelasticity and a sharp unusual increase in G0 observedabove 90 1C. Non-isothermally heated starch gelatinization

ARTICLE IN PRESSJ. Ahmed et al. / Food Hydrocolloids 22 (2008) 278–287286

(up to gel point) was adequately described by the first-orderreaction kinetics with process activation energy rangedbetween 32.2 and 42.2 kJ/mol. The gel rigidity was found tobe combined effect of time–temperature. These findingsprovide information on Basmati rice starch gelatinizationthat would help processors to develop and formulate foodproducts incorporating rice starches.

Acknowledgement

This research was partially funded by a grant from theNatural Sciences and Engineering Research Council(NSERC) of Canada.

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