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Food Hydrocolloids 25 (2011) 1785e1792

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Food Hydrocolloids

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Rheology and functional properties of starches isolated from fiveimproved rice varieties from West Africa

Olayide S. Lawal a,d,*, Romano Lapasin b, Barbara Bellich c, Tajudeen O. Olayiwola a,Attilio Cesàro c, Miki Yoshimura d, Katsuyoshi Nishinari e

aDepartment of Chemical Sciences, Olabisi Onabanjo University, P.M.B 2002, Ago-Iwoye, NigeriabDepartment of Industrial and Information Engineering, University of Trieste, piazzale Europa 1, 34127 Trieste, Italyc Physical and Macromolecular Laboratory, Department of Life Sciences, University of Trieste, Via Giorgieri 1, 34127 Trieste, Italyd School of Human Science and Environment, University of Hyogo, 1-1-12, Shinzaike-honcho, Himeji, Hyogo, JapaneGraduate School of Human Life Science, Osaka City University, 3-3-138, Sumiyoshi, Osaka 558-8585, Japan

a r t i c l e i n f o

Article history:Received 12 August 2010Accepted 11 April 2011

Keywords:West African Rice Development AgencyRice starchPhysico-chemical propertiesRheology

* Corresponding author. Tel.: þ234 7061898525, 81E-mail address: laidelawal2@yahoo.com (O.S. Law

0268-005X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.foodhyd.2011.04.010

a b s t r a c t

Starches were isolated from five improved rice varieties developed by West African Rice DevelopmentAgency (WARDA) namely FARO 32, FARO 51, FARO 52, FARO 54 and NERICA. Starch yield and amylosecontent varied between 73.77e70.02% and 22.88e24.48% respectively. Starches were polyhedral inappearances and within the size range 1.5e6.1 mm. The X-ray diffraction patterns of the starches showa peak centered on 2q¼ 15.1�, a doublet on 17.1� and 18.1�, and another single peak at 23.12�. However,NERICA shows no doublet but a single peak at 2q¼ 17.1� with a small shoulder. The peak viscosity of thestarches ranged between 147.48 and 209.17 RVA corresponding to FARO 52 and NERICA respectively. Aneat distinction is observed between the marked shear thinning of FARO 52 and the apparently plasticbehavior of the other samples. Important differences appear in the low shear region where the viscosityincreases in the following order: FARO 52< FARO 54< FARO 51< FARO 32<NERICA. The mechanicalspectra exhibit similar profiles as the storage modulus (G0) prevails over the viscous component (G00) andis weakly dependent on the frequency. However, the storage modulus increases in the same orderobserved for low shear viscosity. The percentage retrogradation was between 61.9 and 86.6% and NERICAstarch showed the least retrogradation indication. NERICA starch exhibited highest swelling and solu-bility, while the least was observed in FARO 52. Rheology and functional properties are dependent ofamylose composition. This study provides knowledge for the utilization of starches isolated fromimproved rice varieties that would be relevant for both domestic and industrial applications.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Rice is one of the major cereal crops worldwide and its starch isone of the key ingredients of various food products. Commercially,more than two thousand varieties of rice are grown throughout theworld (Deepa, Singh, & Naidu, 2008). It has been revealed that ricestarch has bland taste; it is smooth, creamy and spreadable. Thesequalities make it valuable in the formulation of food products.Because of the value-added application of rice starch in differentfood applications, its price is often above the average price of otherstarches. In addition, rice starch is non-allergenic because of thehypoallergenicity of the associated protein (Champagne, 1996).

8038357393.al).

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Physico-chemical properties of rice starch depend largely on thevariety of the rice itself and there are several varieties of rice basedon their genetic background (Bao, Corke, & Sun, 2004; Singh, Kaur,Sodhi, & Sekhon, 2005). Also, properties of rice starch depend uponthe isolation procedure, climatic and soil conditions during ricegrain development (Asaoka, Okuno, & Sugimoto 1985). As it occursin other starches, rice starch contains amylose (linear) and amylo-pectin (branched) components with a 1e4 and, a 1e4 and a 1e6respectively. Rice starches could be categorized into waxy, very low,low, intermediate and high amylose, based on amylose contents inthe range 0e2, 5e12, 12e20, 20e25 and 25e33% respectively.

In the present investigation, we report on starches isolated fromfive improved rice varieties from West African Rice DevelopmentAgency (WARDA). The efforts of WARDA in collaboration withConsultative Group on International Research on Agriculture(CGIAR) in Africa Rice Initiative (ARI) project are targeted atdeveloping a rice variety that could revolutionalize rice production

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e17921786

in Africa. In particular, an intensive research is focused on theproduct NERICA. NERICA (New Rice for Africa) is the new high-yielding upland rice which has been widely recognized as a prom-ising technology for addressing the food shortage in sub-SaharanAfrica. It is the product of interspecific hybridization between thecultivated rice species of Africa (Oryza glaberrima) and Asia (Oryzasativa). It combines the local-stress adaptation of African rice withthe high-yielding potential of Asian rice. In addition, it also has theadvantage of early maturity, drought tolerance, resistance todiseases and very good aroma.

Functional characterizations such as solubility, swelling, waterabsorption capacity provide useful insights into the diversepotential applications of the starches for domestic and industrialpurposes (Hoover, 2001). Gelatinization and pasting properties areindications of the ease or the difficulty of cooking the rice starch.The rheological characterizations provide information about theviscoelastic properties of the starches (Hsu, Lu, & Huang, 2000;Lapasin, Pricl, & Tracanelli, 1992; Lii, Tsai, & Tseng, 1996).

Many papers in literature report on rice starch characterizationfrom rice cultivars grown in India (Sodhi & Singh, 2003) to rice fromThailand (Noosuk, Hill, Farhat, Mitchell, & Pradipasena, 2005), aswell as properties of starches separated from indica rice cultivars(Singh, Nakaura, Inouchi, & Nishinari, 2007). The structure andphysico-chemical properties of six wild rice starches were reported(Wang, Wang, & Porter, 2002). The studies on the in vitro starchdigestibility and the glycemic index of six different indigenous ricecultivars from Philippines was also reported (Frei, Siddhuraju, &Becker, 2003).

In Sub-Saharan Africa, the development of rice starch based foodproducts is expanding with the introduction of improved ricevarieties that encourage local farmers’ production. It is reasonablethat increase in both domestic and industrial utilization would bea strategic factor that could further enhance the production of theimproved rice varieties. However, the information on functionalproperties of the starches under investigation is lacking andencourages characterization studies in order to enhance knowledgeabout starch gel, thus improving the potentials of manipulatingtheir texture and developing new products.

The varieties chosen for this study are those with the genericnames: NERICA, FARO 54 (upland varieties), FARO 51, 52 (lowlandvarieties) and FARO 32 (irrigated variety). No previous investigationabout the starches isolated from these varieties has beenmade and,therefore, the characterization of these starches would stimulateboth industrial and domestic applications and hopefully boostingtheir acceptance globally.

2. Materials and method

2.1. Samples and dehulling

NERICA, FARO 32, FARO 51, FARO 52 and FARO 54 rice sampleswere generously provided by the WARDA, Ibadan station, Nigeria.All other reagents used in this study were of analytical grade. Thepaddy rice samples were dehulled to remove the branwith a Satakedehuller (Satake co, Stockport, UK).

2.2. Starch extraction

The dehulled starch was extracted by alkali extraction of theprotein. Rice (1 kg) was steeped in sodiumhydroxide solution (0.3%,10 L) at 25 �C for 24 h to soften the endosperms. The steep liquorwas drained off, then the endosperms were washed and groundwith an Oster blender (6646 Oster 12 speed blender, SunbeamProducts, Inc., Boca Raton, FL). The slurry was again dispersed insodium hydroxide solution (0.3%, 10 L) stirred manually for 20 min

and allowed to settle for 6 h and the supernatant was drained off.This process was repeated until the supernatant gave negativeresponse to the protein test (Biuret). The slurry was suspended indeionized water, the pH was adjusted to 7.0 with HCl (0.5 M) andpassed through nylon screen (53 mm). Afterward, it was allowed tosettle for another 6 h and the clear supernatant was discarded. Thestarch obtained as sediment was dried in a convection oven at 40 �Cfor 72 h.

2.3. Chemical composition

The moisture content was determined with a moisture analyzer(Sartorius MA, 40; NJ, USA). Ash, crude fiber, crude protein and fatwere determined according to AACC (1984) procedures. Amylosecontent was determined using the improved colorimetric methoddescribed by Chrastil (1987).

2.4. Mineral analysis

Rice starch sample (0.52 g) in ceramic crucible was placed ina muffle furnace and the temperature was ramped to 500 �C overa period of 2 h, and was kept at 500 �C for 2 h before cooling. Theashed samplewas poured into a 50 mL centrifuge tube. The cruciblewas rinsed with 5 mL of distilled water into the centrifuge tube,after that, it was also rinsed with 5 mL of aqua regia into thecentrifuge tube and the process was repeated twice to makea volume of 20 mL. The samples were centrifuged for 10 min at3000 rpm and the mineral composition was determined usingatomic absorption spectrophotometer.

2.5. Starch granule morphology

Starch granule morphology was examined with a Leo 1550 ultrascanning electron microscope. The samples were mounted onstuds, sputter-coated with gold (Balzers, SCD-040; Norderstedt,Germany) and examined under the scanning electron microscope.

2.6. X-ray diffraction (XRD)

The X-ray diffraction pattern of rice starches (moisture content,8.11e8.3%) was recorded with X’Pert pro X-ray diffractometerequipped with X’celerator as detector. The diffractograms wereregistered at Bragg angle (2q)¼ 5e50�. The operating conditionswere: target voltage 40 kV, target current 100 mA, aging time5 min; step time 4.55 min, divergence slit width 1.00; scatter slitwidth 1.00 mm and receiving slit width 0.6 mm. Multi-peak fittingwas performed to get the integrated area of crystalline peaks (Ac)and amorphous peak (Aa), and the crystallinity index [Xc (%)] wasdetermined as follows:

Xc (%)¼ 100�Ac/(AcþAa)

2.7. Swelling power and solubility

Swelling power and solubility determinations of the ricestarches (5.0% w/v) were carried out in the temperature range30e90 �C, using the method previously described with slightmodifications (Lawal, 2004).

2.8. Rapid visco-analyzer (RVA)

The pasting properties were determined with a Rapid Visco-Analyzer (Newport Scientific Pty Ltd, Warriewood, Australia).Starch (3.0 g) was suspended in water (25 mL) and the suspension

Table 1The yield and the proximate composition of rice starches.a

Sample Yield (%) Moisture (%) Protein (%) (N� 65) Fat (%) Ash (%) Fiber (%) Amylose (%)

FARO 51 71.23� 1.5 8.21� 0.4 0.81� 0.0 0.71� 0.4 1.77� 0.1 0.88� 0.4 23.75� 0.7FARO 52 71.22� 1.0 8.30� 0.2 0.61� 0.0 0.66� 0.3 1.69� 0.0 0.11� 0.0 22.88� 0.9FARO 54 70.02� 1.0 8.11� 0.2 0.45� 0.1 0.45� 0.1 1.81� 0.0 0.21� 0.1 23.44� 1.1FARO 32 72.64� 2.1 8.12� 0.4 0.45� 0.2 0.65� 0.1 1.55� 0.1 0.35� 0.1 24.05� 1.1NERICA 73.77� 1.7 8.12� 0.6 0.71� 0.5 0.78� 0.4 1.47� 0.0 0.65� 0.3 24.48� 0.9

a Results are means of triplicate determinations� standard deviation.

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e1792 1787

was stirred in aluminum RVA sample canister (950 rpm,10 s) and itwas held at 50 �C for 1 min. A programmed heating and coolingcycle was used: heated to 95 �C at 6 �C/min, held at 95 �C for1.5 min and cooled to 50 �C where it was held for 2 min. Pastingtemperature, peak viscosity, hot paste or trough viscosity, final orcool paste viscosity were determined.

2.9. Rheological properties

Starch dispersions (5.0% solid content) were left to rehydratebefore boiling at 99 �C for 45 min. During heating, the sampleswere constantly agitated until pasting occurred. Starch paste wasleft to cool to 25 �C and the water lost during heating, determinedby re-weighing was added back to paste which was thoroughlymixed before measurements. The water was added back at 25 �C.The rheological measurements were carried out at 25� 0.1 �C witha controlled stress rheometer Haake RS150 using a parallel plategeometry (PP35 Ti) with serrated surfaces (diameter: 35 mm, gap:1 mm). Two different procedures were followed to characterizeboth the shear- and time-dependent properties. The procedure a)starts with a constant shear rate segment (2 min at 10 s�1) followedby a shear rate ramp (logarithmic increase from 0.4 to 1000 s�1 in10 min): The stepwise procedure b) is composed of consecutiveconstant stress segments (with a logarithmic stress increase from 1to 1000 Pa); the duration of each segment was controlled by twocut-off criteria (approximation to the stationary state, maximumtime: 1 min).

The viscoelastic properties were analyzed under oscillatoryshear conditions. The transition from linear to nonlinear regimewas detected through stress sweep tests at constant frequency(1 Hz), with a logarithmic increase from 0.4 to 1000 Pa in 40 steps.The mechanical spectra were obtained from frequency sweep testsat constant stress (within the linear viscoelastic regime), withlogarithmic decrease from 100 to 0.01 Hz in 25 steps.

Recovery tests were carried out to characterize the effectsproduced by shearing conditions on the viscoelastic properties ofthe samples. They were composed of three consecutive segments:1) time sweep under small oscillatory shear conditions at constantfrequency (1 Hz) and stress amplitude (1 Pa or 0.4 Pa) min) for3 min 2) constant shear rate segment (4 min at 10 s�1) 3) timesweep under small oscillatory shear conditions at constantfrequency (1 Hz) and stress amplitude (1 Pa or 0.4 Pa) min) for10 min.

Table 2The mineral composition of rice starches.a

Sample Ca2þ (mg/kg) Kþ (mg/kg) Naþ (mg/kg) Mg2þ (mg/kg)

FARO 51 99.5� 2.5 45.5� 3.2 16.2� 0.5 100.2� 2.7FARO 52 100.5� 2.1 43.3� 2.2 13.8� 0.4 92.2� 1.5FARO 54 92.5� 4.4 38.8� 3.2 14.7� 0.4 101.5� 2.1FARO 32 88.5� 3.2 40.5� 2.1 13.3� 0.4 96.6� 1.2NERICA 89.6� 1.2 37.8� 2.1 11.8� 0.2 104.4� 1.4

a Results are means of triplicate determinations� standard deviation.

2.10. Thermal and retrogradation properties

Distilledwater (6.0 mL)was added to starch (2.0 mg) in DSC pans.Pans were sealed, reweighed and kept at room temperature for 2 hto ensure equilibration of the starch sample and water. The sampleswere scanned from 30 to 130 �C at 10 �C/min using empty pans asreference. The heated pans were then cooled immediately and keptat 4 �C inside a refrigerator for 24 h, followingwhich theywere keptfor 1 or 6 days at room temperature giving a complete storageperiod of 2 and 7 days, respectively. Following these periods ofstorage, the sampleswere scanned under the same conditions as thefirst scanning. Indium and zinc were used for calibration, while anempty pan was used as the reference scale. Onset temperature (To),peak temperature (Tp), conclusion temperature (Tc) and enthalpy(DH, J/g), for gelatinization and retrogradation, were determined.

3. Results and discussion

3.1. Proximate and chemical composition

The starch yield, proximate and chemical compositions arepresented in Table 1 .The yield was based on the whole weight ofthe dehulled rice samples. The result shows that the highest starchyield was obtained in NERICA (73.77%) while the lowest wasobtained in FARO 54 (70.02%). Starch yield is function of the geneticcomposition of the source rice itself, the planting environment aswell as the method of the starch extraction. In previous works, ricestarch yields from 59% (Mohan, Gopal, Malleshi, & Tharanathan,2005) to 71.6% (Wang & Wang, 2004) have been reported. Thehigh starchyield recorded in the present investigation suggests thatthe sourcematerials, particularly NERICA, could be viable sources ofstarch for both domestic and industrial applications. Protein, fatand fiber compositions are generally below 1% attesting the purity

Fig. 1. Scanning Electron Micrograph of representative rice starch (NERICA).Magnification �2000.

5 10 15 20 25 30 35 40 45 50

Diffraction angle 2

Inte

nsity

NERICA

FARO 32

FARO 51

FARO 54

FARO 52

Fig. 2. Wide angle X-ray diffraction pattern of rice starches. Moisture contents of thestarches were within the range 8.11e8.30%.

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e17921788

of the starch samples. These results are also comparable with thevalues previously reported for indica rice starches (He, Song, Ruan,& Chen, 2006).

The ratio of amylose to amylopectin is crucial in affecting manyphysico-chemical parameters of the starches, since the amorphouscomponent of starches is made up mainly of amylose. In thesamples studied, the amylose contents vary from 22.88 to 24.48%,the highest value belonging to NERICA. The amylose content iswithin the range of values reported for rice starches from othersources (Noosuk, Hill, Farhat, Mitchell, & Pradipasena, 2005).Generally, the ratio of amylose to amylopectin is crucial in affectingmany physico-chemical parameters of the starches, since theamorphous component of starches is made up mainly of amylose.

3.2. Mineral analysis

For potential dietary relevance, mineral composition has alsobeen analyzed. The results (Table 2) show that the magnesiumcontent was the highest in all the rice starches except FARO 52,which on the other hand has calcium content of 100.5 mg/kg.Sodium content was the lowest in all the starch samples, withvalues in the range 11.8e16.2 mg/kg. It is reasonable; however, thatmetal element composition and concentration among differenttypes of starches would also depend on other factors such as thelocation of the plant source and the starch extraction method.

3.3. Starch granule morphology

Scanning electron microscopy was used to determine themorphology of the starch granules. Generally, the starches were

Table 3The numerical results of pasting properties of the rice starches.a

Sample Pasting temperature(�C)

Peak viscosity(RVA)

Peak time(min)

H(

FARO 51 85.75� 3.1 190.08� 9.2 6.07� 0.5 1FARO 52 85.65� 5.7 147.58� 12.4 6.27� 1.0 1FARO 54 86.35� 5.6 162.58� 13.6 6.13� 1.0 1FARO 32 86.45� 7.1 196.17� 15.1 6.00� 1.2 1NERICA 87.22� 5.2 209.17� 8.1 5.67� 0.2 1

Starch paste concentration, 12.0% w/v.a Results are means of triplicate determinations� standard deviation.

polyhedral in appearance with irregular shapes. An example isshown in Fig. 1 for NERICA starch. The sizes determined by takingaverages of 20 measurements ranged between 1.5 and 6.1 mm. Nopronounced differences were observed among the rice starchsamples investigated. These values are similar to those reportedfrom some Indian rice cultivars (Singh, Kaur, Sandhu, & Kaur, 2006;Sodhi & Singh, 2003), only slightly smaller than that reported by Liand Yeh (2001) for Taiwan rice starches. Previous studies have alsoshown that rice starch granules are very small in size and this givesa texture perception similar to that of fat (Champagne, 1996).

3.4. X-ray diffraction (XRD)

The X-ray diffraction patterns of rice starches are depicted inFig. 2. The diffractograms show a region of prominent peaks, onecentered on 2Q¼ 15.1�, a doublet on 17.1� and 18.1�, and anothersingle peak at 23.12�. This diffraction pattern is consistent withcereal starches and they are generally classified as “A” pattern ofdiffraction (Imberty & Perez, 1988; Imberty, Chanzy, Perez, Buleon,& Tran, 1988). All the samples investigated here have similardiffraction pattern except the NERICA starch which shows nodoublet but a single peak at 2Q¼ 17.1� with a small shoulder. InNERICA, it was also observed that the peak centered on 2Q¼ 23.12�

was less intense and broader. Generally, the results obtained hereare consistent with those reported in the literature (Grenat,Radosta, Anger, & Damaschum, 1993; He et al., 2006; Zobel,1988). Analysis of the crystallinity showed that the starches have32.71, 32.62, 31.05, 30.88 and 29.64% crystallinity for FARO 52, FARO54, FARO 51, FARO 32 and NERICA, respectively. Starch crystallinityaccounts for its gelatinization and other parameters. In this report,crystallinity decreases with amylose composition of the starchesand, technically, this also relates with gelatinization parameters ofstarches. In addition, the order of crystallinity would account forthe swelling pattern of the starches because it would entailbreaking of intermolecular bonds within the crystalline compo-nents of the starch granules. The results are in general agreementwith the crystallinity of Indica rice starches with various amylosecontents (He et al., 2006).

3.5. Rapid visco-analyses (RVA)

The numerical results of pasting properties are presented inTable 3. The peak viscosity of the starches ranged between 147.48and 209.17 RVA, corresponding to FARO 52 and NERICA respec-tively. Hot paste viscosity was highest in FARO 32 (157.08 RVA)while the lowest value of 123.83 RVA was obtained in FARO 52.Pasting occurs when the starch granules absorb sufficient waterand swell after gelatinization. The initial increase in viscosity withtemperature during heating could be attributed to the increase inthe leachates from the starch granules and the formation ofa homogeneous mass resulting from the remaining fragile starchgranules (Atwell, Hood, Lineback, Marston, & Zobel, 1988). Also, thebreakdownwas highest in NERICA (65.75 RVA) and lowest in FARO

ot paste viscosityRVA)

Final viscosity(RVA)

Breakdown(RVA)

Setback(RVA)

53.67� 7.6 273.50� 14.3 36.42� 2.8 119.83� 10.423.83� 8.6 182.67� 15.3 23.75� 3.4 58.83� 12.437.75� 9.3 229.75� 17.1 24.83� 3.6 92.00� 13.357.08� 12.1 270.08� 17.6 39.08� 3.9 113.00� 14.643.42� 6.6 265.83� 12.6 65.75� 2.4 122.42� 9.8

22.8 23.0 23.2 23.4 23.6 23.8 24.0 24.2 24.4 24.6 24.80

50

100

150

200

250

300

350

Visc

osity

/RVU

% amylose

Fig. 3. RVA parameters as a function of amylose content. (;) final viscosity, (:) peakviscosity, (-) setback, (C) breakdown.

Table 4Parameters of the power law used to describe the shear-dependence of viscosity athigher shear rates (10e1000 s�1).

Parameters Rice starch sample

FARO 51 NERICA FARO 32 FARO 52 FARO 54

K (Pa sn) 62.6 137.5 93.4 18.5 34.6n (e) 0.21 0.13 0.12 0.26 0.20RMSRD 0.025 0.036 0.020 0.034 0.015

Parameters of the CarreaueYasuda equation and their derived quantities

h0 (Pa sn) 33305 140031 88612 489 12102hN (Pa sn) 0.042 0.2050 0.001 0.081 0.027m (e) 0.852 0.955 0.866 0.748 0.829_g c (s�1) 0.0015 0.0012 0.0006 0.0110 0.0029sc (Pa) 25.7 84.8 25.4 2.7 17.7RMSRD 0.042 0.055 0.047 0.130 0.062

Starch paste concentration (5.0% w/v); measurements were carried out at25� 0.1 �C.The root mean square relative deviation is defined as: RMSRD ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP�hexpi � hcali

hexpi

�2s

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e1792 1789

52 (23.75 RVA), as a measure of the structural disintegration of thestarch during cooking. The setback was also highest in NERICA(122.42 RVA) and lowest in FARO 52 (58.83 RVA). The setback iscommonly used to describe the increase in viscosity that occurs oncooling a pasted starch (Fisher & Thompson, 1997). Thus, the highamylose content of NERICA starch reasonably accounts for the highvalues in the setback as well as the breakdown. All the abovereported properties, indeed, clearly depend on the content ofamylose, as shown in Fig. 3. Only sample FARO 51 presents someslight deviations, which can be due to either some higher value ofother active components or some preparation differences. Whilethe dependence of these functional properties on the amylosecontent is technologically important, a detailed study of the rheo-logical behavior could better assist in the interpretation of themolecular origin of the textural properties of the different starches.

1,E+06

3.6. Rheological properties

3.6.1. Continuous shearFor all the samples similar exponential stress decays were

observed in the constant shear rate segment of procedure a) and

1,E-02

1,E-01

1,E+00

1,E+01

1,E+02

1,E+03

1,E-01 1,E+00 1,E+01 1,E+02 1,E+03

shear rate [s-1]

visc

osity

[Pa

.s]

Faro 51

Nerica

Faro 32

Faro 52

Faro 54

Fig. 4. Flow curves for the viscosity as a function of shear rate for all starches. Starchpaste concentration (5.0% w/v); measurements were carried out at 25� 0.1 �C.

a stationary state was almost attained within 2 min. Such a tran-sient behavior is the consequence of the structural breakdowninduced by the applied shearing condition (10 s�1).

Also the flow curves hð _gÞ obtained from shear rate ramps werequalitatively similar, as illustrated in Fig. 4. Their profiles can bedescribed by the power law equation h ¼ K _gn�1 with satisfactoryapproximation in the higher shear rate range (above 10 s�1) wherethe viscosity data are closer to the steady responses. The parametervalues are reported in Table 4. More appreciable differences can benoticed for the consistency index K whose values cover a relativelylarge interval while the power law exponent n lies in a narrowerrange, so underlying the similar shear thinning behavior exhibitedby the samples examined in the experimental shear conditionsexplored.

The stepwise procedure b allowed expanding the characteriza-tion below the power law region toward very low shear conditionsand then to individuate the upper Newtonian plateau and thecritical stress interval whichmarks the onset of shear thinning, as itcan be seen in Fig. 5. Here, a neat distinction can be made betweenthe marked shear thinning of FARO 52 and the apparently plastic

1,E-02

1,E-01

1,E+00

1,E+01

1,E+02

1,E+03

1,E+04

1,E+05

1,E+00 1,E+01 1,E+02 1,E+03shear stress /Pa

visc

osity

/Pa.

s

Faro 51NericaFaro 32Faro 52Faro 54

Fig. 5. Flow curves for the shear viscosity as a function of shear stress for all starches.Starch paste concentration (5.0% w/v); measurements were carried out at 25� 0.1 �C.

1,E-01

1,E+00

1,E+01

1,E+02

1,E+03

1,E-02 1,E-01 1,E+00 1,E+01 1,E+02

[-]

G',

G'' /

Pa

Nerica - G' Nerica - G''

Faro 52 - G' Faro 52 - G''

Faro 54 - G' Faro 54 - G''

Fig. 6. Oscillatory stress sweep tests of rice starches. Starch paste concentration (5.0%w/v); measurements were carried out at 25� 0.1 �C.

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e17921790

behavior of the other samples. Important differences appear in thelow shear region whereas all the flow curves converge to similarviscosity values at high stresses.

The experimental data can be described in the plane loghelogswith satisfactory approximation only resorting to models contain-ing more than 5 adjustable parameters. An example is given inFig. 5 where the curves are calculated with a modified version ofthe Ellis model (Roberts, Barnes, & Carew 2001). If the same dataare treated in terms of viscosity vs shear rate, a good fitting isprovided by the CarreaueYasuda equation (Carreau, 1972; Yasuda,Armstrong, & Cohen, 1981)

h ¼ h0 þhN � h0h

1þ ða* _gÞbip (1)

where h0 and hN are the zero-shear viscosity and the infinite-shearviscosity corresponding to the two limiting Newtonian plateaux, for_g/0 and _g/N, respectively. The other adjustable parameters (a, band p) serve to characterize the shear thinning behavior throughtwo derived quantities: the critical shear rate _g cð¼ 1=aÞ (or the

Fig. 7. Mechanical spectra of rice starches. Starch paste concentrat

corresponding critical stress sc) marking the onset of the power lawregion and the shear thinning indexm (¼bp), related to the slope ofthe flow curve in the same region (m corresponds to 1� n). Table 4also reports the two viscosity parameters, h0 and hN, and thederived quantities.

A good correlation can be established between the twoparameters which define the viscous response at low shear and theonset of shear thinning, i.e. the zero-shear viscosity h0 and thecritical stress sc. Both these parameters and the shear thinningindex m increase in the following order: FARO 52< FARO54< FARO 51< FARO 32<NERICA, so underlining the stronginfluence exerted by the amylose content on the rheologicalresponses of the systems. In particular, the increase of zero-shearviscosity h0 with amylose content follows a sigmoidal trend.

3.6.2. Oscillatory shearSome examples of the experimental responses obtained from

stress sweep tests are reported in Fig. 6. The upper limit of thelinear viscoelastic regime can be located at low strains, below 0.05,for all the samples examined; such a limit corresponds to stressvalues comprised between 0.3 (FARO 52) and 15 Pa (NERICA).

At low strains the storage modulus generally prevails over theloss modulus, as the G0 values and the distance between moduliincrease in a parallel form in the following order FARO 52< FARO54< FARO 51< FARO 32<NERICA. This observation is due to theincreasing elastic character of the sample response. For all thesamples, the storage modulus monotonically decreases withincreasing strain, while the profile of the loss modulus shows anincrease at the exit of the linear viscoelastic regimewhich is almostnegligible for FARO 52 but quite evident for other samples. Athigher strains, G00 reaches a maximum and then decreases ina lesser extent than G0. Such a behavior is classified as Type III inaccordance with the reports of Hyun, Kim, Ahn, and Lee (2002) andSim, Ahn, and Lee (2003) and is often exhibited by disperse systemsand polymeric weak gels where particle concentration and/orinterparticle attractive interactions or interchain associations posesevere constraints to incipient flow of the relevant structural unitssuch as particles, flocs or gel microdomains.

The experimental data obtained from frequency sweep testsperformed under linear viscoelastic conditions are reported inFig. 7. All the mechanical spectra exhibit similar profiles since thestorage modulus prevails over the viscous component along thewhole experimental window and is weakly dependent on w. Asanticipated through the results of the stress sweep, the different

ion (5.0% w/v); measurements were carried out at 25� 0.1 �C.

1

10

100

1000

0 200 400 600 800 1000

time/s

G',

G" /

Pa

1

10

100

visc

osity

(Pa.

s)

G'

G''

G''

G'

G''

G'

G''

G'

Fig. 8. Recovery tests of rice starches (black symbols: NERICA; red symbols FARO 52).Starch paste concentration (5.0% w/v); measurements were carried out at25� 0.1 �C.(For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

70

80

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e1792 1791

distances between the two traces reveal the different elastic char-acter of the samples.

The curves reported in the Fig. 7 are calculated from the Songand Jiang (1998) model, which represents the most parsimonioussolution to describe the mechanical spectra since it contains onlyfive adjustable parameters. Alternatively, the frequency depen-dence of G0 can be approximated with a power law correlationG0 ¼ k0un0, being the power law exponent n0 comprised between0.04 (NERICA and FARO 32) and 0.13 (FARO 52).

Fig. 8 reports two examples of the results obtained fromrecovery tests. The two data sets are referred to the samples whichexhibit the largest differences in their rheological properties, as itwas illustrated in the previous sections. For the most structuredsystem (NERICA) the application of a constant shear rate givesorigin to significant effects on both viscoelastic moduli, whichundergo a substantial decrease, whereas an appreciable decreasecan be detected only for G0 in the case of FARO 52.

3.7. Thermal and retrogradation studies

The summary of thermal properties of the starches and thestored gelatinized rice starches are presented in Table 5. Thethermal event for the gelatinization of starches occurs in a wide

Table 5Thermal properties associated with gelatinization and retrogradation properties ofrice starches.

1st day scan 5th day scan RT (%)

To (�C) Tp (�C) Tc (�C) DH(J/g)

To (�C) Tp (�C) Tc (�C) DHR5

(J/g)

FARO 52 39.7 85.2 156.2 24.1 33.5 84.8 141.0 20.9 86.6FARO 54 37.2 81.5 150.4 28.2 36.4 84.0 150.8 22.7 80.7FARO 51 37.7 88.5 154.1 28.4 36.8 87.7 148.8 23.3 82.1FARO 32 36.8 83.6 150.4 28.5 35.6 82.7 139.8 19.5 68.5NERICA 38.6 84.4 147.1 29.1 40.5 90.5 158.6 18.0 61.9

To: Onset of gelatinization temperature; Tp: Peak of gelatinization temperature; Tc:conclusion of gelatinization temperature; DH: Enthalpy of first scanning; DHR5

:Enthalpy of retrogradation after 5 days of storage; R5: Percentage retrogradationafter 5 days of storage. Starch paste preparation: 6.0 mL distilled water was added tostarch 2.0 mg in DSC pans.

range (ca 30e160 �C). A direct relationship was observed betweenthe amylose content of the starches and their gelatinizationtemperature. This seems reasonable because the amylose compo-nent of a starch constitutes the larger percentage of its amorphouscomponent, while amylopectin constitutes the crystalline compo-nent. Gelatinization as a process involves the breakdown of inter-molecular bonds of starches in the presence of heat and water,leading to the loss of birefringence; since water penetration wouldbe hampered in the crystalline region of the starches, hence it isreasonable that higher crystalline composition would increase thegelatinization temperature. When the gelatinized starches werestored for 5 days to monitor retrogradation, the onset temperaturefor gelatinization is always reduced, but for NERICA. It is alsoinstructive that the enthalpy of gelatinization reduced in the storedstarch gels DHR5

compared with the native starches DH. Thepercentage retrogradation was between 61.9 and 86.6% and theleast retrogradation was observed in NERICA starch. Possibly this isbecause NERICA has the lowest index of crystallinity among thestarches. It is to note that the process of starch recrystallization onstorage after gelatinization is a temperature and time-dependentprocess. This may be due to the weaker starch crystallinity of ret-rograded starch (Sasaki, Yasui, & Matsuki, 2000).

3.8. Swelling and solubility

The temperature dependent swelling profiles and the solubilityprofiles are presented in Figs. 9 and 10 respectively. Within thetemperature range studied (30e90 �C), NERICA starch exhibitedhighest swelling and solubility, while the least was observed inFARO 52 in both swelling and solubility. It is instructive that thisobservation is in good agreement with the amylose content of thestarches as presented in Table 1. The swelling capacity is a measureof the ability of the starch to hydrate under specific conditions suchas temperature and water availability. Previous studies have indi-cated that greater swelling capacity is an indication of weakerbinding forces in the starch granules (Hoover, Sailaja, & Sosulski,1996). Swelling capacity and solubility increased with increase intemperature. Upon increasing temperature in the presence ofwater, starch molecules mobility is increased thereby weakeningthe binding forces; thus, a parallel effect is obtained in bothswelling capacity and water diffusion into starch granules. Thisprocess also enhances the leaching of the soluble components of

30 40 50 60 70 80 900

10

20

30

40

50

60

Swel

ling

capa

city

(g/g

)

Temperature (OC)

NERICA FARO32 FARO51 FARO52 FARO54

Fig. 9. Effect of Temperature on the swelling properties of rice starches. Each valuerepresents mean of triplicate determinations. Error bars are standard deviations.

30 40 50 60 70 80 900

1

2

3

4

5

6

7

8

Solu

bilit

y (%

)

Temperature (OC)

NERICA FARO32 FARO54 FARO52 FARO51

Fig. 10. Effect of temperature on the solubility of rice starches. Each value representsmean of triplicate determinations. Error bars are standard deviations.

O.S. Lawal et al. / Food Hydrocolloids 25 (2011) 1785e17921792

the starch granules thereby leading to improved starch solubility. Ithas been reported that amylose constitutes the larger percentage ofthe amorphous component of the starch granules where waterpenetration into the granule is more pronounced. Hence, highestswelling and solubility of NERICA starch could be attributed to itshigh amylose component.

4. Conclusions

This work concerns detail report on the rheology and otherfunctional properties of starches isolated from some rice varietiesdeveloped byWest AfricanRiceDevelopment Agency. A good yield ofstarch could be obtained from the rice samples (70.02e73.77%). Thestarch isolated from NERICA rice elicited the lowest tendency forretrogradation. The data obtained from the rheological tests per-formed under both continuous and oscillatory shear are correlatedwith simple models (the power law and the CarreaueYasuda equa-tions, the SongeJiangmodel, respectively) and then summarized intofewparameterswhich serve as a useful guide indescribing the effectsrelated to the amylose content. These parameters as well as otherfunctional properties would provide relevant information for usefulapplications of the starches in food industries. The data in this workwould increase both domestic and industrial utilization of thestarches and thereby enhance the production of the rice varieties.

Acknowledgment

The authors would like to express their thanks to Dr. O. Osinameof West African Rice Development Agency, Ibadan, Nigeria for theprovision of rice grains. The corresponding author is grateful to theAlexander von Humboldt Foundation in Germany, the Japan Societyfor the Promotion of Science in Japan and the Abdus Salam Inter-national Centre for Theoretical Physics, Italy for the awards offellowships which facilitated the completion of this work.

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