Materials Science and Engineering C 29 (2009) 651–656

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Materials Science and Engineering C

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Chemical and rheological properties of a starch-rich fraction from the pulp of the fruitcupuassu (Theobroma grandiflorum)

Lúcia C. Vriesmann, Joana L.M. Silveira, Carmen L. de O. Petkowicz ⁎Universidade Federal do Paraná, Departamento de Bioquímica e Biologia Molecular, CP 19046, CEP 81531-990, Curitiba-PR, Brazil

⁎ Corresponding author. Tel.: +55 41 3361 1661; fax: +E-mail address: clop@ufpr.br (C.L.O. Petkowicz).

0928-4931/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.msec.2008.12.011

a b s t r a c t

a r t i c l e i n f o

Article history:

The pulp obtained from the Received 8 May 2008Received in revised form 28 November 2008Accepted 9 December 2008Available online 16 December 2008

Keywords:Theobroma grandiflorumCupuassuStarchAmyloseGelRheology

fruit of cupuassu (Theobroma grandiflorum) was extracted with hot aqueous 0.1%citric acid to give fraction 0.1CA-2 in 15% yield. This was the predominant component polysaccharide, 91% ofwhich was composed of starch, by an iodine test and monosaccharide composition, and its 13C NMRspectrumwas consistent with that of a high amylose starch. The content of amylose found in fraction 0.1CA-2was 71%. This value is higher than those of common starches of cereal grains, tubers, roots, and other fruits.The fraction was submitted to rheological examination, gels being prepared on heating with concentrationsof 4 to 7% (w/w). A non-Newtonian behavior was observed, and gel viscosity and strength depended on theconcentration. The presence of starch, as well as the presence of previously investigated pectin, conferred thehigh viscosity and gelling capability of the pulp.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Starch is the main reserve polysaccharide of many plants. It is animportant renewable and a low-cost polymer, occurring as granules,which determine its physical properties. Due its thickening andgelling properties, it is used in the food, pharmaceutical, and chemicalindustry [1–3].

Cereal grains, legume seeds, tubers, and certain fruits contain from30 to 85% starch on a dry-weight basis [3]. Commercial starches areobtained mainly from yellow corn, although potato, wheat, tapioca,rice and sorghum also are significant sources [2,3].

Many reports have described the characterization of starch fromcereals, roots and tubers [4–7]. The fruit starches have also beeninvestigated, mainly those from banana [8,9], mango [8,10], apple [11],squash [12], cherimoya [13] and Kamo Kamo [14].

Starch consists of a mixture of two polymers, amylose andamylopectin. Whereas amylose is an essentially linear molecule,consisting of (1→4)-linkedα-D-glucopyranosyl units, amylopectin is ahighly branched molecule with substitution of these at O-6 by α-D-glucopyranosyl branches. These are composed of (1→4)-linked α-D-glucopyranosyl units with various lengths [1–3].

It has been demonstrated that the pulp of the fruit of Theobromagrandiflorum, Schumann (family Sterculiaceae), growing in theBrazilian Amazon, known locally as cupuassu, contains a considerableamount of starch as well as pectin polysaccharides [15,16]. The pulp is

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greatly appreciated for its pleasant acidic taste, being consumed freshor processed, mainly as juice, ice cream, candy and jellies [17]. It has anintense fragrance [17], whose volatile compounds have been exten-sively analyzed [18], as well as its content of vitamin C [19].

As chemical and rheological characterization of native starch canlead to improvements for predicting desirable functional properties ofstarchy products, we now study some chemical and rheologicalfeatures of the native starch-rich fraction obtained from the pulp ofcupuassu fruit [15].

2. Experimental

2.1. Extraction of fraction 0.1CA-2

After enzyme-inactivation with methanol–H2O (4:1, v/v) underreflux for 20 min, the grounded pulp fruit was defatted with p-toluene-ethanol (2:1, v/v) in a Soxhlet and dried. The residue wassubmitted to sequential extractions with water (25 °C and 60 °C), citricacid (0.1%, 0.5%, 1%, 2.5% and 5%, using temperatures of 50 °C and100 °C at each concentration) and NaOH (2 mol L–1 and 4 mol L–1

NaOH; 25 °C) [15,16]. Fraction 0.1CA-2 was obtained with 0.1% citricacid at 100 °C for 60 min. The extract was concentrated and treatedwith ethanol (2:1 v/v) in order to obtain precipitated polysaccharide,which was then washed three times with ethanol and dried undervacuum.

The fraction was identified according to the conditions applied onextraction: 0.1 displays the concentration of citric acid (CA) and “−2”was used for the second hot extraction (100 °C).

Table 1Monosaccharidea composition of fraction 0.1CA-2 obtained from the pulp of cupuassu

Fraction Rha Ara Xyl Man Gal Glc Uronic acidb

mol%

0.1CA-2 0.6 0.4 0.4 0.2 1.0 90.8 6.6

a Determined by GLC of derived alditol acetates.b Determined by colorimetric method, Filisetti-Cozzi and Carpita [21].

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2.2. Chemical characterization of fraction 0.1CA-2

Total carbohydrate was determined by the phenol–sulfuric acidcolorimetric method [20], employing glucose as standard. Uronic acidwas estimated by the sulfamate/3-phenylphenol colorimetric method[21], using galacturonic acid as standard. Proteinwas measured by theHartree method [22], with BSA as standard.

Amylose content was determined by the Chrastil method [23] witha modification, using 80% amylose and waxy amylopectin (0%amylose) (Sigma) as standards. Intermediate amylose proportionswere obtained by mixing appropriate amounts of each. A straight lineplot was used to determine the content of amylose in the sample.Fraction 0.1CA-2 was solubilized in distilled water at 100 °C during10 min, then alkalinized with 1 mol L–1 NaOH and kept for 5 min at100 °C. After cooling at 25 °C, it was submitted to the procedure foramylose determination.

The monosaccharide composition was determined after total acidhydrolysis with 2 mol L−1 trifluoroacetic acid (5 h, 100 °C). Themonosaccharides, obtained on evaporation to dryness, were reducedwith NaBH4 and then acetylated with pyridine-acetic anhydride(1:1 v/v, 16 h, at 25 °C). The resulting alditol acetates were analyzed bygas–liquid chromatography (GLC) using a model 5890 S II Hewlett-Packard gas chromatograph at 220 °C (flame ionization detector andinjector temperature, 250 °C) with a DB-210 capillary column(0.25 mm internal diameter×30 m, film thickness 0.25 µm), thecarrier gas being nitrogen at 2.0 mL min−1 [16].

For high pressure size exclusion chromatography (HPSEC) ana-lyses, suspension of the sample in 0.1 mol L−1 NaNO2 solutioncontaining NaN3 (0.5 g L−1) was heated to 80 °C during 2 h undermagnetic stirring. HPSECwas carried out onpolysaccharide solution at25 °C, using a multidetection equipment in with a Waters 2410differential refractometer (RI) and a Wyatt Technology Dawn F multi-angle laser light scattering (MALLS) detector were adapted on-line.Four Waters Ultrahydrogel 2000/500/250/120 columns were con-nected in series and coupled to the multidetection equipment. Themobile phase consisted of a 0.1 mol L−1 NaNO2 solution containing0.5 g L−1 NaN3 (vacuum-filtered through a 0.22 µm membrane filter)at a flow rate of 0.6 mL min−1 and pressure 830 psi. The sample,previously filtered (0.22 µm;Millipore), was injected (100 µL loop) at a1.5 mg mL−1. HPSEC data were collected and analyzed by a WyattTechnology ASTRA program.

13C NMR spectra of fraction 0.1CA-2 were obtained using a BrukerDRX 400 Avance spectrometer incorporating Fourier transform (FT) on asolution in D2O at 70 °C. Chemical shifts are expressed as δ (ppm), usingthe resonances of CH3 groups of acetone as internal standard (δ 30.2).

2.3. Rheological analysis of fraction 0.1CA-2

Solutions of fraction0.1CA-2 at concentrationsof 4, 5, 6 and 7% (w/w)were prepared by stirring in water for 16 h at 25 °C. They were thenheated at 92 °C with continuous stirring for 15 min and cooled at roomtemperature. The samples were stored at 5 °C prior to rheologicalexamination.

These were carried out using a model RS 75 Haake Rheometer,coupled with a DC5 heating circulator, with a C60/2° (cone and plategeometry) or PP20 sensor (plate and plate geometry). Mechanicalresponses of the samples were determined by subjecting them to afrequency sweep (0.1–10 Hz) at 25 °C at tension (τ) between 1 and4 Pa. These values refer to the viscoelastic-linear region, where the gelstructure was preserved. The temperature sweeps, heating (5–95 °C)and subsequent cooling (95–5 °C), were performed at a rate of 1 °Cmin−1, at a frequency of 1 Hz. Before starting the experiments, theexposed sample edge was covered with a thin layer of low viscositymineral oil to minimize evaporation losses during measurements.

Continuous flow ramps in the CR mode (controlled rate) wereperformed at 25 °C. The sensor was programmed to increase the shear

rate from 0.1 to 100 s−1 (up curve) in 100 s followed immediately by areduction from 100 to 0.1 s−1 in 100 s (down curve). The shear stress(τ) was then measured as a function of shear rate. As done byTechawipharat, Suphantharika and BeMiller (2008) [24], data from thedown curve of the shear cycle were used to characterize the flowbehavior of the cupuassu starch samples. The experimental data wereevaluated and fitted according to the rheological models of Herschel–Bulkley (τ=τ0+Kγn), Power-law (τ=Kγn), and Bingham (τ=τ0+ηpγ),where τ is the shear stress (Pa), γ is the shear rate (s−1), K is theconsistency coefficient (Pa sn), n is the flow behavior index(dimensionless), τ0 is the yield stress (Pa), and ηp is the Binghamplastic viscosity (Pa s). The software RheoWin 3 Data Manager wasemployed to obtain the rheological and statistical parameters. Thetemperature of all analyses was controlled with a Peltier system.

3. Results and discussion

The pulp of cupuassu, which sums ∼40% of the total weight of thefresh fruit, was used to the isolation and characterization of itspolysaccharides [15,16]. Fraction 0.1CA-2, obtained by extraction withhot 0.1% citric acid, was the main fraction, yielding 15% on an enzyme-inactivated, defatted, dry-weight basis. Due to its high yield, it wasselected for detailed chemical and rheological analysis.

3.1. Characterization of fraction 0.1CA-2

3.1.1. Chemical compositionThe monosaccharide composition of fraction 0.1CA-2 showed it to

contain 91% glucose (Table 1) and this and a blue coloration withiodine test suggested the presence of starch. The glucose content wasclose those reported by Freitas et al. [7] for starches of yam (Dioscoreaalata: 88%) and cassava (Manihot utilisssima: 87.4%). The monosac-charides Rha, Ara, Gal, and uronic acid were detected, typical of pecticpolysaccharides, in small quantities of 0.6, 0.4, 1, and 6.6%, respec-tively. The protein content of fraction 0.1CA-2 was negligible at 1%.

The amylose content of fraction 0.1CA-2 determined by colori-metric method [23] was 71% (R2=0.9993). According to Chrastil [23],the same color intensity is obtained after sample solubilization inNaOH, Me2SO, or urea-Me2SO, and the choice depends only on thesolubility of the sample. Fraction 0.1CA-2 was found to be moresoluble in aqueous NaOH than in urea-Me2SO or Me2SO. When onlyGlc content of fraction 0.1CA-2 (Table 1) is take into account, thecontent of amylose increases to 77%.

Thus, the amylose content of starch present in fraction 0.1CA-2from cupuassu is larger than that reported for starch from roots andtubers (10–38%) [6,7,25] and also of normal corn (21.4–32.5%) [25,26],wheat (18–30%) and rice (5–28.4%) starches [26].

Comparing with other fruits, this fraction from cupuassu pulppresented a native high amylose starch, differing of apple (26.0–29.3%of amylose) [11], cherimoya (15.4–16.3%) [13], squash (12.9–18.2%)[12] and Kamo Kamo fruit starches (17.2%) [14].

Amylose level greatly affects the properties of starch and high-amylose starch has attracted attention because of their beneficialproperties in many food and non-food applications [27]. It has beenreported that long-term intake of dietary amylose may be valuable indecreasing insulin response while maintaining proper control ofglucose tolerance and low levels of blood lipids [28]. Therefore,

Fig. 1. 13C NMR spectra of fraction 0.1CA-2 in D2O at 70 °C (A) and DEPT-135 (B).

653L.C. Vriesmann et al. / Materials Science and Engineering C 29 (2009) 651–656

consumption of cupuassu pulp which contains high-amylose starchcould have some health advantages in relation to the other fruitsknown to contain starch.

3.1.2. 13C-NMR analysisAs expected, the 13C-NMR spectrum of fraction 0.1CA-2 contained

6 main signals (Fig. 1A), typical of starch with a high proportion ofamylose and minor ones of amylopectin [29]. Present were signals of(1→4)-linked units of glucopyranose (C-1; 99.5 ppm), O-substitutedC-4 (77.4 ppm). The resonance at 73.2, 71.5 and 71.2 ppm corre-sponded to C-3, C-2 and C-5 respectively. The signal at 60.6 ppm arosefrom unsubstituted C-6, confirmed by a DEPT-135 spectrum (Fig. 1B),which contained an inverted signal. Minor signals were present nearto the C-1, C-4 and C-5 signals, which should arise from amylopectin.Similar 13C-NMR assignments for a starch fraction of mango pulp andfor a standard corn starch with a high amylose content have beenreported by Iagher et al. and Freitas, respectively [10,30].

3.1.3. HPSEC-MALLS analysisFraction 0.1CA-2 was analyzed by HPSEC using MALLS and RI

detectors (Fig. 2). The RI gives a signal proportional to concentration

Fig. 2. Elution profile of fraction 0.1CA-2 obtained by HPSEC-MALLS/RI.

whereas the MALLS response depends on both concentration andmolar mass.

Although the alkaline media (pH13–14) has been considered goodsolvent for amylose, according to the literature the unperturbeddimensions are adopted at pH 7 and 25 °C, whatever the ionic strength[31]. Milles et al. [32] isolated amylose from starch and determined themolecular weight by light scattering using aqueous solutions ofamylose. In the present work, for HPSEC analysis, fraction 0.1CA-2(71% amylose) was solubilized in 0.1 mol L−1 NaNO2 solutioncontaining NaN3 (0.5 g L−1) at 80 °C.

Fig. 2 showed fraction 0.1CA-2 to be heterogeneous. A broad RIpeak, representing the proportion of the molecules present, appearedbetween 45 and 58 min and is characteristic of molecules with lowermolecular weight, with another being eluted at ∼38 min with highermolecular weight. With MALLS, this appeared overlapping with aminor RI peak of higher molecular weight.

The profile of fraction 0.1CA-2 with both detectors is similar to thatobtained by SEC-MALLS for a low molecular weight polydisperseamylose sample [33]. Its RI response showed a small shoulder elutedbefore a large peak and whose LS profile had a large peak eluted andthen with a shoulder. The authors interpreted these responses asindicating that amylose solutions of low molecular weight areheterogeneouswith respect tomolecular weight distribution, contain-ing a high molecular weight population at a low elution volume andthe majority with lower molecular weight at higher elution volumes.

The RI profile of fraction 0.1CA-2 is similar as those of debranchedamylopectins of corn, sorghum, barley, wheat, and rice, measured by aintermediate SEC pressure [34], or by flow field-flow fractionationanalysis [35], exhibiting in the same way, a multi-modal chaindistribution and also it is close to that of the hot-water soluble starchof Bolivar rice cultivar [36]. The authors suggest that the amylopectinmolecules of Bolivar are bigger, are more capable of inter- and intra-molecular interactions, and may be more difficult to dissolve in hotwater, and thus the hot-water soluble fraction of Bolivar starchpredominantly consisted of amylose.

The results suggest that the peak eluted at ∼38 min could be ahigh-molecular amylopectin, while the peaks eluted later correspondto an intermediate-molecular weight and low-molecular weightamylose fractions.

Fig. 3. Frequency sweeps at 25 °C of fraction 0.1CA-2 at 4, 5, 6 and 7% (w/w) at tensionsof 1.5, 4.0, 1.0 and 3.0 Pa, respectively.

Fig. 4. G′ as a function of concentration for 0.1CA-2 gels at different frequencies (25 °C).

654 L.C. Vriesmann et al. / Materials Science and Engineering C 29 (2009) 651–656

According to Cheetam and Tao [37], the amylose content in cornstarches has a strong correlation with the molecular size of itsamylose, themolecular size of amylose decreasingwith the increase inamylose content. If that was true for other species, amylose fromcupuassu should be a low molecular weight polymer and conse-quently more soluble.

The general results are in agreement with reports that considerstarch to be a polymolecular and polydisperse polysaccharide [2,3].

3.2. Rheological analysis

Starch is a biopolymer largely employed by industry [3]. High-amylose starches have the ability to form strong gels and films. In thefood industry, the high gelling strength of these starches makes themespecially useful for producing sweets. The film-forming ability ofthese starches prevents moisture loss in fried products crispy andreduces their fat uptake upon cooking. High-amylose starches can beprocessed into ‘resistant starch’, which has nutritional benefits, andalso may be used for the creation of biodegradable packing materialsand adhesives [27,38].

As discussed by Prokopowich and Biliaderis [39], gelation ofamylose (concentrationb10%) involves rapid formation of doublehelical structures from an amorphous sol upon cooling, which acts asjunction zones among polymer chains, leading to the establishment ofa three-dimensional hydrated gel network. On the other hand,gelation of amylopectin (concentrationb10%) is a much slowerprocess involving recrystallization of the outer branches.

As fraction 0.1CA-2 was the main polysaccharide fraction obtainedfrom cupuassu pulp and was composed mainly by starch with a highamylose content (71%), it was submitted to rheological analysis.Solutions were prepared at concentrations of 4 to 7% (w/w).

3.2.1. Dynamic rheological properties of fraction 0.1CA-2 at differentconcentrations

Frequency sweeps for gelatinized starch-rich fraction, at differentconcentrations, are shown in Fig. 3. The elastic moduli (G′) at allconcentrations were relatively frequency-independent, while viscousmoduli (G″) were only slightly dependent on frequency. There was agradual increase of G″ with an increase of frequency for all testedconcentrations.

Rosalina and Bhattacharya [40] and Khondkar et al. [41] obtainedsimilar rheological characteristics for some corn and waxy cornstarches, respectively. They proposed that starch samples give rise to aweak-gel behavior or a high viscosity gel respectively.

Dynamic oscillatory results showed a gel-like behavior for allconcentrations of fraction 0.1CA-2 with G′ being significantly largerthan G″ over studied frequencies (0.1–10 Hz). These data, togetherwith G′ being independent of frequency, characterize the presence ofa network structure [42]. Furthermore, G′ of the sample with a higherconcentration (7%) was about ten times higher than G″, characterizinga stronger gel [43].

The G′ of fraction 0.1CA-2 gels as a function of concentrationmeasured at three different frequencies can be seen in Fig. 4. Themagnitude of G′ increased with increasing sample concentration. Thevariation of the elastic modulus with concentration for 0.1CA-2 gelsfollowed the relation G′∝C5.

Iturriaga et al. [44] evaluated starch gels from seven novelargentine rice genotypes and the dependence of storage modulus onconcentration followed the Power-law, G′∝Cn, with n=2.9–3.2 for thenon-waxy genotypes and n=1.1 for waxy starch. Other authors havereported that G′ varies as C2.6–3.2 for non-waxy starches [45,46]. Ring(1983) cited by Miles et al. [47] have reported that G′∝C7 for amylosegels in the concentration range 1.5–7%, which is similar to the resultsfor high-amylose starch from cupuassu (fraction 0.1CA-2). However, ithas been pointed out by the same author that the values of G′ aresensitive to thermal history of the amylose sample. Also, thedifferences in the origin, molecular weight of the samples of amylosecould affect the values of G′ as well as the methods of preparation ofthe gels.

The value of C0 has been used as an estimate of the lowestconcentration where gelation is possible and it is obtained by theextrapolation of the values of G′ to zero [31]. A C0 of 0.9% wasobserved for amylose gels in 0.2 and 0.5 M KCl. Although we have notenough data to determine C0, when the concentration of 0.9% wasused in the relation obtained for 0.1CA-2, the value determined for G′was 0.018 Pa.

Ortega-Ojeda et al. [48,49] reports that at the same concentration,the moduli values for potato starch (20.4% amylose) were not as highas for amylose. A lower minimum concentration (1.7%) was requiredfor amylose to observe solid-like behavior [48]. On the other hand,potato starch showed G′NG″ only when concentration was approxi-mately higher than 4%.

Fig. 5. Temperature dependence of elastic modulus (G′) of fraction 0.1CA-2 fromcupuassu fruit pulp (4–7%, w/w) during heating trace at 1 Hz.

655L.C. Vriesmann et al. / Materials Science and Engineering C 29 (2009) 651–656

Samples of gelatinized fraction 0.1 CA-2 also were submitted totemperature sweeps, by cooling and heating curves from 5 to 95 °Cand 95 to 5 °C, respectively. Fig. 5 shows the temperature dependenceof the elastic modulus (G′) at a heating rate of 1 °C min−1. Two-stepmechanisms were observed at all concentrations of the fraction,namely a slow G′ decreases from 5 to 40 °C, then a sharp decrease ofthe G′modulus between 65 to 70 °C. The profiles revealed losses in thegel strength above this temperature suggesting that structuralchanges occurred in the three dimensional network after the thermalcycle of the sample. This is similar to those described for starches fromrice [50].

A temperature sweep of the 5% fraction 0.1CA-2 during thecomplete thermal cycle (heating followed by cooling) is shown inFig. 6. The G′ underwent a decrease of about ten times during heating,approximately twice that of G″, and during cooling, both moduliunderwent a gradual increase, but more pronounced for G″, so muchso that reaching 5 °C, its value was greater than that of the start of theanalysis. Thus, after thermal variation, the sample had a lower solidcharacter than that of the start of the experiment. According to Yu andChristie [51], the thermal behavior of starches is complex. Severalphysicochemical changes may occur during heating, involvinggelatinization, crystallization, volume expansion, molecular degrada-tion and motion of water, among others.

3.2.2. Flow behavior of fraction 0.1CA-2 at different concentrationsThe viscosity of gelatinized starch is influenced by the extent of

swelling of the granules prior to their rupture, as well as the expansionand dispersion of resistant starch granules after the rupture of the

Fig. 6. Variation of G′ and G″ (1 Hz, 4 Pa) during initial heating from 5 to 95 °C andsubsequent cooling from 95 to 5 °C for a 5% (w/w) 0.1 CA-2 gel.

granular structure. When the viscosity of starch pastes is determined,the volume of swollen granules and their inherent deformability areimportant [52].

Fig. 7 shows viscosity curves for fraction 0.1CA-2 at differentconcentrations (4 to 7 w/w %) at 25 °C. A decrease in the viscosity withshear rate increase was observed, characterizing a non-Newtonianbehavior under the examined conditions. The non-Newtonianbehavior of starch solution was reported by many other researches[24,53–55].

The viscosity value increased with the increase in the fraction0.1CA-2 concentration, as expected for polysaccharides in solution. Forexample, at concentrations of 4, 5, 6 and 7% the absolute viscosity— ηwas 14120; 37230; 63960 and 90410 mPa s respectively, with allmeasurements determined at 1.2 s−1 (low shear rate). These resultsshowed that the viscosity of fraction 0.1CA-2 in aqueous solution isstrongly dependent on starch concentration. A similar behavior wasobserved by Chaudemanche and Budtova [56] for pregelatinized cornstarch (70% amylose) at concentrations higher than 1% at 60 °C. Underhigh shear, the gelatinized starch granule structure was broken down,which caused a drop in paste viscosity [57].

The experimental data for the cupuassu starch (fraction 0.1CA-2)were evaluated according to different rheological models. Theparameters of the Herschel–Buckley, Power-law and Binghammodelsobtained from regression analysis between the shear stress and theshear rate from 4 to 7% (w/w) samples are summarized in Table 2.

The Bingham model is a special case of Herschel–Bulkley modelwhen n is equal to unity [53]. For the values of n in Herschel–Bulkley,which were far from unity, the samples of this study can not beappropriately described by the Bingham model. Considering the R2

values, Herschel–Buckley (R2≥0.98) was found to be the mostadequate model to describe the flow behavior of the sample in thisstudy.

Gelatinized starch dispersions are described as non-Newtonianfluids whichmay also exhibit a yield stress at low shear [53,58], whichseems the case of starch of this study at concentration of 4–7%.According to Lagarrigue and Alvarez [58], the yield stress valuedepends on concentration, mass fraction of swollen granules, granulemean diameter and gelatinization procedure. Although other modelshave been used, gelatinized starch dispersions are usually representedby the Power-law or the Herschel–Bulkley model in the range 1–1500 s−1. The consistency index (K) and the flow behavior index (n)depend on the kind of starch, its concentration and temperature [58].As can be seem from Table 2, K increases with the increasing ofconcentration of 0.1CA-2, showing increasing viscosities. Similar

Fig. 7. Influence of shear rate on the absolute viscosity of gels of fraction 0.1CA-2 at25 °C.

Table 2Comparison of different models for the flow behavior of fraction 0.1CA-2 samplesobtained from the pulp of cupuassu

Concentration(%, w/w)

Model K (Pa sn) τ0 (Pa) n ηp (Pa s) R2

4 Herschel–Bulkley 1.52 10.34 0.70 – 0.980Power-law 5.22 – 0.51 – 0.946Bingham – 15.08 – 0.2630 0.951

5 Herschel–Bulkley 10.22 38.40 0.55 – 0.989Power-law 21.94 – 0.45 – 0.985Bingham – 107.1 – 0.5416 0.960

6 Herschel–Bulkley 11.52 40.43 0.55 – 0.996Power-law 23.66 – 0.45 – 0.993Bingham – 117.8 – 0.5992 0.964

7 Herschel–Bulkley 12.16 44.45 0.55 – 0.997Power-law 24.72 – 0.44 – 0.992Bingham – 120.1 – 0.5998 0.960

656 L.C. Vriesmann et al. / Materials Science and Engineering C 29 (2009) 651–656

observations were reported for cassava starch [53], sago starch pastes[55] and corn starch [59].

4. Conclusions

For the first time, a starch-rich fraction from the pulp of cupuassufruit (T. grandiflorum) is described with a yield of 15% related todefatted-driedmaterial. The fractionwas composed predominantly bya high amylose (71%) starch, characterized by a blue coloration withiodine solution, monosaccharide composition and 13C NMR spectro-scopy. This fraction contained a minor proportion of pectin that maycontribute to its gel-like behavior. Analysis of the rheological proper-ties of the fraction showed a non-Newtonian behavior at concentra-tions from 4 to 7% (w/w). Considering the R2 values, Herschel–Buckleywas found to be the most adequate model to describe the rheologicalcomportment of the samples.

Dynamic oscillatory experiments indicated a highly elastic beha-vior with G′ significantly larger than G″, being independent of therange of examined frequencies (0.1–10 Hz), confirming gel charactersfor all concentrations. A gel-like behavior was demonstrated forfraction 0.1CA-2, a native high amylose starch, suggesting a potential asa gelling additive in pharmaceutical, cosmetic and food applications.

Acknowledgements

The authors thank the Brazilian agencies, CNPq and FundaçãoAraucária-PRONEX for financial support, and Dr. Philip A. J. Gorin forhelp with the English language.

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