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International Journal of Biological Macromolecules 49 (2011) 471– 479

Contents lists available at ScienceDirect

International Journal of Biological Macromolecules

jo u r n al hom epa ge: ww w.elsev ier .com/ locate / i jb iomac

ffect of gamma irradiation on rheological properties of polysaccharides exudedy A. fluccosus and A. gossypinus

amira Alijani, Sima Balaghi, Mohammad Amin Mohammadifar ∗

epartment of Food Science and Technology, National Nutrition and Food Technology Research Institute, Faculty of Nutrition Sciences and Food Technology,hahid Beheshti University of Medical Sciences, P.O. Box 19395-4741, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 28 March 2011eceived in revised form 10 May 2011ccepted 25 May 2011vailable online 2 June 2011

eywords:um tragacanth

a b s t r a c t

In this study, Iranian gum tragacanth (GT) exudates from Astragalus fluccosus (AFG) and Astragalus gossyp-inus (AGG) were irradiated at 3, 7, 10 and 15 kGy. Fourier transform infrared spectroscopy (FTIR) datashowed that irradiation did not induce changes in the chemical structure of either type of gum. Althoughparticle size distribution and both steady shear and dynamic rheological properties were considerablyaffected by the irradiation process, the magnitude of the effect of irradiation on each of the rheologicaland size variables was different for the hydrocolloids. For instance, for AGG, increasing the irradiationdose from 3 to 10 kGy, the d(0.5) and D[3,2] values were reduced by one-sixth to one-eighth fold. Colour

amma irradiationiscosityrequency sweeparticle size distributionTIR

measurement revealed that the radiation process led to an increase in the yellow index and b* valuesfor both types of GT in powder form, but it was more pronounced for AGG samples. Irradiation ledto an approximate 13-fold increase in redness in AFG. Surface and shape changes of the gum crystalswere studied by scanning electron microscope (SEM) and a smoother surface for irradiated samples wasdetected. The notable changes in functional properties of each variety of irradiated gum should be taken

usin

into consideration before

. Introduction

Food irradiation has been profoundly investigated, but mainlynitiated as an ultimate minimal processing technology. Food isommonly irradiated with microwaves; however, the term foodrradiation is used to describe a process in which food is exposed toonising energy [1]. Sufficient energy that breaks chemical bondss “ionising irradiation” [2]. Gamma rays involve very high-energyadiation and are able to break chemical bonds when absorbed byaterial [3]. The application potential of specific ionising radiation

s very diverse, from inhibition of sprouting of tuber and bulb cropso production of commercially sterile food products [1]. Irradiationn doses up to 10 kGy has been approved by the IAEA, the WHO, andhe FAO (IAEA, 2003) [4]. Additionally, doses as high as 75 kGy haveeen approved for some products (WHO, 1981) [4]. Specific appli-ations of food irradiation are approved by national legislationsn over 55 countries worldwide [1]. As commercial applications ofood irradiation become more accepted, the technology is increas-ngly applied not only to agricultural products but also to food

ngredients and ready-to-eat meals. Irradiation is often viewed aseing the last process, after packaging, used to control spoilagerom pathogenic organisms. Furthermore, gamma irradiation, as

∗ Corresponding author. Tel.: +98 2122648120; fax: +98 2122376470.E-mail address: mohamdif@ut.ac.ir (M.A. Mohammadifar).

141-8130/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.ijbiomac.2011.05.030

g the radiation technology as a commercial tool for sterilisation.© 2011 Elsevier B.V. All rights reserved.

an ionic and non-thermal process, has received more attention as afunctional modification agent in polymer research and application.It has been applied as a physical modification method for naturalpolysaccharides such as starch [5]. Gamma irradiation treatment,compared to microwave, UV, ultra-high hydrostatic pressure andhydrothermal treatment, is rapid, convenient and more extensive,because ionising energy rapidly penetrates through the polysac-charide granules [6].

Polysaccharide gums provide a wide variety of functionalityin foodstuffs, which includes gelling, thickening, emulsifying, andstabilising [7]. The function of polysaccharides in foods generallyoriginates from chain entanglement, formation of junction zonesbetween the ordered polysaccharide chains and subsequent aggre-gation. These effects are essentially governed by the molecularproperties (e.g., molecular weight, degree of branching, chain rigid-ity, and functional groups) of polysaccharides and factors such asthe concentration of polysaccharide and the quality of solvent,which is usually affected by the presence of sugars or salts [7].Many studies have shown that treatments such as heating, ultra-sonication and irradiation can induce inverse changes in the desiredquality of the end product by influencing the structure and func-tional properties of the polysaccharides. For example, increasing

the irradiation dose in gums such as agar, guar, alginates and car-rageenan induced a decrease in the viscosity of their solutions[8–10], but for other gums, this reduction occurred only after aninitial increase in the viscosity [11]. However, it was reported that

4 Biolog

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he rheological characteristics of pectin and salep did not changeuch with irradiation [10]. The most important change caused by

rradiation in polysaccharide solutions or their solid state [12] is theepolymerisation of basic units through the breakage of glycosidiconds, which yields the radiolytic product’s smaller polysaccha-ide units and results in softer gels [8]. In fact, polysaccharides areegraded by ionising radiation with cross linking as a possible sideeaction via free radical mechanisms, depending on the water con-ent. Radiolysis occurs with all doses of irradiation and is sometimesccompanied by an initial increase in viscosity. Low doses are anffective means of obstructing aggregation [11]. It appears that, to

certain degree, gamma irradiation modification can be useful formproving viscosity stability during storage time [6].

In this study, the effect of gamma irradiation on two varietiesf GT was investigated. GT is defined by the Food Chemical Codexs the ‘dried gummy exudation obtained from Astragalus gummiferabillardiere or other Asiatic species of Astragalus (Fam. Legumi-osae)’ [13]. It is a plant exudates collected from Astragalus specieshrubs grown in the Middle East, which is a complex, heteroge-eous, anionic polysaccharide of high molecular weight [14]. GTonsists of a water-soluble component, tragacanthin, and the insol-ble, albeit swelling component, bassorin [15]. It was shown thatarious species of GT had different chemical compositions, physic-chemical properties and rheological behaviours [16]. GT has beensed as a stabiliser, emulsifier and thickener in food and has beensed in technical applications in the pharmaceutical and cosmetic

ndustries for many years. Its more outstanding characteristics,owever, are its high degree of stability under strong acid condi-ions and its use as a bifunctional stabiliser [17]. Previous studiesave researched the effect of gamma irradiation on the viscos-

ty of GT with unknown botanical sources [18,19]. However, tohe best of our knowledge there is no report on how the struc-ure and viscoelastic properties of GT dispersions are influenced byamma irradiation. Here, an attempt has been made to study theffect of gamma irradiation on steady shear and oscillatory rheolog-cal properties, functional groups, particle size distribution, colourarameters and the surface and shape of crystals of two varietiesf GT.

. Materials and methods

.1. Gum tragacanth

GT exuded by two species of Astragalus (gossypinus and flucco-us) was collected from plants growing in different provinces ofran. The plants were tapped with a knife by making careful longi-udinal incisions in the tap root and the bark of the branches [17].he gum readily exuded from these cuts in the form of ‘ribbons’hat became brittle on drying. Taxonomic identification of the spec-mens was done by Dr. Ali Masoumi, an academic member of theorest, Range, and Watershed Management Organisation of Iran.

.2. Preparation of samples

The raw GT was powdered in a high-speed mechanical blendernd later sieved to obtain uniform samples. Powdered gum with aesh size between 200 and 500 �m was used in this study.

.3. Irradiation

The powder form of two varieties of GT were irradiated at 3,, 10 and 15 kGy at ambient temperature and at a fixed dose rate

f 5.4 Gy/s from a Co-60 gamma irradiator (Gammacell 220, AECL)t Nuclear Science and Technology Research Centre (Tehran, Iran).he gamma irradiator was calibrated using the Fricke dosimeter. Aobalt-60, manufactured in Nordion International Co. Ltd., Ottawa,

ical Macromolecules 49 (2011) 471– 479

ON, Canada, was used for �-ray source. The samples were kept insealed polyethylene bags at room temperature.

2.4. Rheological measurements

GT powder (1 g), both irradiated and non-irradiated, was accu-rately weighed and dispensed into 100 g of pure deionised water.Deionised (Milli-Q) water was used for all experimental work. Thewhole gum dispersions were kept on a magnetic stirrer at roomtemperature and gently stirred overnight, which led to completehydration of the biopolymer. Steady state, as well as oscillatoryshear measurements, were performed with a Physica MCR 301rheometer (Anton-Paar, GmbH, Graz, Austria) using a serratedplate-and-plate system (40 mm in diameter, 0.6 mm gap). Temper-ature control was carried out with a Peltier system equipped with afluid circulator. In the experiments, the samples were covered witha solvent trap to prevent evaporation. Flow curves were obtained atshear rates of 0.1–1000 (1/s). Before the steady and dynamic shearrheological measurements, all samples were left standing for 4 minat 25 ◦C to allow for structure recovery and temperature equilib-rium. The rheological measurements were performed in triplicate.

The power-law model was used to describe the rheologicalproperties of AGG dispersions over mid-range shear rates. Logarith-mic plots of shear stress versus shear rate were used to calculatethe consistency coefficient (m) and the flow behaviour index:

� = m �̇n (1)

Eq. (2) was used to evaluate the Hershel–Bulkley model consis-tency coefficient, m′ and flow behaviour index and n′ values for AFGdispersions:

� = �o + m′�̇n′(2)

A satisfactory fitting of data are provided with the Cross equa-tion, using shear rate as the independent variable:where �o and �∞are the asymptotic values of the viscosity at zero and infinite shearrates, respectively, � is the characteristic time and k rules the sheardependence in the power-law region [20].

� = �∞ + �o − �∞1 + (� �̇)k

(3)

Strain sweep tests were performed at (0.01–1000%, 1 Hz) todetermine: (1) the limiting value of the linear viscoelastic range(LVE or �L); (2) the structural strength (G′ at LVE); (3) the resistanceto mechanical force or yield stress (�y), which is also a measure ofstructural strength and can be calculated from the limiting value ofLVE range in terms of shear stress; (4) the flow point (�f), the stressat which the internal structure breaks to the extent that it causesthe material to flow (G′ = G′′); and (5) the damping factor (tan ı)or the ratio of loss modulus to elastic modulus to provide a directview of whether the samples behaved as liquids or solids [21]. Fre-quency sweep tests were performed using a frequency ramp from0.01–100 Hz. All experiments were performed at 25 ◦C.

2.5. Colour measurement

The L*a*b* values and yellowness index were determinedfor gum powders with a Colour-Eye 7000A reflectance spec-trophotometer. The instrument has d/8◦ geometry with a 6-inchintegrating sphere and a pulsed xenon lamp. Reflectance datawere collected over the full wavelength range of the instruments(360–750 nm, 10-nm interval) with a specular component includedthrough a SAV aperture size. The results were expressed in accor-

dance with the CIELAB system with reference to illuminate D65and a visual angle of 10◦. CIE L*a*b* space uses three terms L*,a* and b* to represent colour. L* represents lightness; 100 rep-resents a perfect white sample, and 0 represents a perfect black.

S. Alijani et al. / International Journal of Biological Macromolecules 49 (2011) 471– 479 473

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* represents the redness–greenness quality of the colours; posi-ive values denote redness, and negative values denote greenness.* represents the yellowness and blueness quality of the colours.ositive values denote yellowness and negative values denote blue-ess, which are recommended by the International Commission on

llumination, 1978. The yellowness index was also measured usinghis spectrophotometer [22]. All measurements were performed inriplicate and the results were averaged.

.6. Fourier transform IR spectra measurements

FTIR was recorded on a (MODEL: Spectrum one) Perkin-Elmerpectrometer (USA) in a range from 4000 to 400 cm−1 using aBr-pellet method. The GT samples were pressed directly on anttenuated reflectance KBr tablet (KBr was supplied with the FTIRnit) into the sampling unit. Infrared measurement was carriedut in the transmission mode, in which the infrared beam passedirectly through the sample. Spectral data were then convertedrom transmittance into absorbance units. Deconvolution of thenfrared spectra was performed using Perkin-Elmer Software.

.7. Particle size distribution

The particle size distribution of gum dispersions 1% (w/w), bothrradiated and non-irradiated, was determined at room tempera-ure with a laser diffraction particle size analyser equipped withn accessory Hydro 2000S(A) (Malvern Mastersizer 2000 particlenalyser, Malvern Instruments Limited, UK). Size measurementsre reported as the volume weighted mean diameter:

[4, 3] =∑

nidi

4

∑nidi

3

nd surface weighted mean diameter:

[3, 2] =∑

nidi

3

∑nidi

2

here ni is the number of particles and di is the diameter.

-1

line) and irradiated (interrupted line) of AGG.

Span is the distribution width and has no relation with middleparticle diameter. Absolute deviation from the median, which is anindicator of uniformity, was also reported.

2.8. Scanning electron microscope (SEM)

SEM LEO 1455 VP (Cambridge, UK) was used to provide infor-mation about morphology of the samples. GT samples were fixedon copper stubs bearing sputtered gold. The electron energy was10 kV. The images at different magnification powers were viewedon a TV screen and photographed.

2.9. Statistical analysis

Analytical values are based on the mean and standard devi-ation of three replicates. For all rheological measurements, thereported values are based on the mean of three replicates. Analy-sis of variance (ANOVA) was used for the data analysis (SPSS 16.0).When the F-values were significant (p < 0.05) in ANOVA, Duncan’smultiple-range test was used to compare treatment means. Sta-tistical differences between samples were also calculated usingStudent’s t-test for selecting data. Values were considered to besignificantly different if p < 0.05 (SPSS 16).

3. Results and discussion

3.1. Colour

The L*, a*, b* and yellowness index of non-irradiated and irradi-ated AGG and AFG were summarized in Table 1. Student’s t-testresults revealed that for AGG and AFG, the differences betweenthe non-irradiated and irradiated samples were statistically sig-nificant (p < 0.05). The lightness of both species increased slightlyafter irradiation. On the other hand, after irradiation, increasing a*reflected a darkening of the GT colour. Irradiation led to an approx-

imate 13-fold increase in redness in AFG. The greenness of AGG didnot change significantly. The b* and yellowness index of both sam-ples increased after irradiation. Gamma rays darkened the AFG andincreased the yellowness of the AGG. Radiation-induced darkening

474 S. Alijani et al. / International Journal of Biological Macromolecules 49 (2011) 471– 479

Table 1Colour parameters (L*a*b*) measured for non-irradiated and irradiated samples oftwo varieties of gum tragacanth.

Species/colour parameters AGG AFG

0 (kGy) 10 (kGy) 0 (kGy) 10 (kGy)

L* 85.20 86.60 79.94 81.30a* −0.65 −0.74 0.086 1.14b* 6.17 12.62 12.92 18.24Yellow index 18.72 29.64 36.66 41.81

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AGG 0 kGy

3kGy

7 kGy

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15 kGy

tudent’s t-test results revealed that for AGG and AFG, the differences between theon-irradiated and irradiated samples were statistically significant (p < 0.05).

n all cases the standard deviations were lower than 1%.

f polysaccharides, such as gum arabic, was previously observed4].

.2. FTIR spectra

FTIR spectra of non-irradiated and irradiated AGG are shown inig. 1. Due to the fact that the spectra of the AGG and AFG wereimilar, only the spectra of the AGG have been presented in Fig. 1.n the case of irradiated GT, for both AGG and AFG, no significantew chemical groups were introduced into the structure. Thereppeared to be no obvious effect of gamma irradiation on the mainransmittance peaks for GT. The AGG spectrum was compared toFG spectrum. AGG spectra exhibited big similarities in its absorp-

ion pattern to that of AFG over a wide range of wave number,ncluding the fingerprint region. Further analysis of the FTIR spec-ra for both AGG and AGF indicated that the absorption bands at434, 2925, 1751 and 1636 cm−1 were assigned to O–H stretch-

ng, C–H stretching, ester carbonyl (C O) groups and carboxyl iontretching band (COO−), respectively. At 1000–1500 cm−1 whicheferred to as finger print region, the absorption bands were due to–O, C–H, C O, C–O and C–N for AGG, as well as N–O and C–O forFG.

.3. Particle size distribution

Table 2 shows the particle size distribution for AFG and AGG

ispersions. Accordingly, comparison of the non-irradiated AGGnd AFG samples shows that the particle size of AGG dispersionn terms of D[3,2], D[4,3] and d(0.5) was larger than those of theFG dispersion. According to the ratio of DeBrouckere to the Sauter

able 2article size distribution for the dispersions (0.05%, w/w) of two varieties of gumragacanth at 3 doses of �-irradiation.

Species/PSD Irradiation doses (kGy)

0 3 10

AGGD[3,2] 143.63b 237.79a 30.97c

D[4,3] 357.63a 319.26b 59.63c

d(0.5) 262.11b 279.35a 48.73c

d(0.9) 773.00a 562.08b 116.21c

d(0.1) 61.32b 134.59a 17.50c

Uniformity 0.83a 0.47c 0.63b

D[4,3]/D[3,2] 2.50a 1.34c 1.92b

AFGD[3,2] 98.63b 241.87a 74.86c

D[4,3] 136.34c 325.57a 239.59b

d(0.5) 125.47b 286.72a 126.13b

d(0.9) 231.38c 571.26b 593.36a

d(0.1) 57.57b 137.00a 36.35c

Uniformity 0.43c 0.47b 1.38a

D[4,3]/D[3,2] 1.38b 1.35b 3.20a

alues with different letters in each raw are significantly different (p < 0.05).n all cases the standard deviations were lower than 3%.

Fig. 2. Effect of shear rate on the apparent viscosity of two varieties of gum traga-canth dispersions (1%, w/w) over different doses of �-irradiation at 25 ◦C.

diameters, D[4,3]/D[3,2], the polydispersity of AGG was higher thanthat of AFG, which aligns with the study performed by Balaghiet al. [16].After dosage amounts of gamma irradiation up to 3 kGy,the D[3,2], d(0.1) and d(0.5) increased significantly in both sam-ples. However, with increasing radiation dosages from 3 to 10 kGy,all particle size parameters for both samples exhibited a sharpdecrease. It is interesting to note that irradiation up to 3 kGy causedpolydispersity in AGG dispersion to decrease and at 10 kGy it causedpolydispersity to increase, but it was still lower than that in thenon-irradiated sample.

In contrast to AGG, irradiation of the AFG sample caused poly-dispersity to increase. The ratio of the DeBrouckere to the Sauterdiameters which reflects the broadening of the size distribution asit increases from 1.38 at 0 kGy to 3.2 at 10 kGy. For AFG dispersions,however, it decreased from 2.49 at 0 kGy to 1.34 at 3 kGy and 1.92at 10 kGy.

3.4. Flow properties

Dependence of the apparent viscosity on the shear rate for AGGand AFG dispersions (1%) in different doses of the irradiation at25 ◦C is illustrated in Fig. 2. All samples showed a non-Newtonianflow, or pseudoplasticity. As the shear rate increased, the apparentviscosity of all samples decreased. For the non-irradiated disper-sion of AGG, according to the Herschel–Bulkley model and asseen in Fig. 2, the yield value was observed; that is, in the low

shear rate range when the concentration of the dispersing phaseor of the macromolecules in the continuance phase was high,the strong interactions generate a yield value [23]. AGG consistsof a large amount of high molecular weight bassorin (67%). For

S. Alijani et al. / International Journal of Biologi

Table 3Parameters of Power-law, Herschel–Bulkley and Cross models for two varieties ofgum tragacanth over different doses of irradiation.

Sample/dose (kGy) Power law parameters

AGG m (Pa sn) n R2

0 3.79b 0.40e 0.993 4.35a 0.46d 0.997 0.63c 0.66b 0.99

10 0.52d 0.58c 0.9515 0.17e 0.68a 0.98

Sample/dose (kGy) Herschel–Bulkley parameters

AFG �0 (Pa) m′ (Pa sn) n′ R2

0 – – – –3 0.28a 0.56a 0.65c 0.967 0.18b 0.11b 0.80b 0.99

10 0.17b 0.06c 0.85a 0.9915 0.08c 0.03d 0.85a 0.99

Sample/dose (kGy) Cross parameters

AFG �0(Pa s) �∞(Pa s) �(s) k R2

−9

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that were exposed to higher doses of irradiation. D[4,3] and D[3,2]

0 198.71 7 × 10 69.85 0.54 0.99

alues with different letters in each column are significantly different (p < 0.05).

on-irradiated AFG, the zero shear viscosity was observed accord-ng to the Cross model (Table 3) and as shown in Fig. 2. In fact,or a low concentration of structuring components, the Brown-an motion dominates and maintains an isotopic structure withegligible interactions between the particles or macromolecules.herefore, the apparent viscosity is independent of shear rate, and

zero shear viscosity is observed [23]. In the case of AFG, there is low amount of the high-molecular weight, water-swellable andtructuring compound of bassorin (25%), instead there is a largemount of water-soluble fraction of the GT, tragacanthin (75%).

Increasing irradiation up to 3 kGy increased the apparent vis-osity of AGG dispersion over all ranges of shear rates, but for AFGispersion, an increase in apparent viscosity was observed over theange of 40–900 1/s and 0.9–1.89 1/s. For AFG dispersion, increasinghe irradiation dose from 3 to 10 kGy caused the apparent viscos-ty to decrease. For AGG dispersions, there was a rapid decrease inpparent viscosity of gum samples up to a dose of 15 kGy (Fig. 2).

According to other studies, irradiation induced a decrease in thepparent viscosity of Konjac glucomannan, agar, alginates, gumrabic and carrageenan in an aqueous solution as a function ofosage [4,6,8,10]. Side branches of carbohydrates affect a varietyf functional properties such as solubility, gelling, film formationnd rheological properties and function to prevent intermolecularssociation of carbohydrate polymers. Thus, water molecules canore readily penetrate the intermolecular spaces caused by steric

indrance, resulting in enhancement of solubility. The viscosity ofinear polymers is higher than that of the branched polymers. Onhe contrary, solubility in branched polymers is higher than that ininear polymers. In addition, the shear rate dependency of viscos-ty among linear polymers is lower than that of branched polymers.

hen polymer molecules are branched, their rheological behaviouraries with the number, location and length of the branches [24]. Itas reported that the content of galacturonic acid in AGG and AFGas 37% and 34%, respectively; and the backbone of GT consists

f 1,4-linked �-d-galacturonic acid along which the side branchesay be distributed regularly or in a random fashion. In this study,

robably the irradiation dose of 3 kGy reduced the side branches ofhe gums to the extent that, when the gum dispersed in water, they

re able to develop intermolecular associations or new entangle-ents between the poorly side-branched main chains. The increase

n the intermolecular associations or entanglements between the

cal Macromolecules 49 (2011) 471– 479 475

main chains of the gum may probably have contributed to theenhancement or stabilisation of the viscosity of 3-kGy-irradiatedAGG dispersion overall the shear rate range. The viscosity of the 3-kGy-irradiated AFG dispersion was in the range of 40–900 1/s and0.9–1.89 1/s. However, increasing the irradiation doses over 3 kGyappears to cause the glycoside bonds or covalent bonds betweenthe units of the backbone to be broken down, which results in theproduction of lower molecular weight fragments. Therefore, theviscosity of the fragmented gum dispersions would be drasticallylow compared with non-irradiated and 3-kGy-irradiated gums. Ithas been reported that radiation effects on the natural polymershave resulted in the drastic decrease of molecular weight withincreasing irradiation dose [25]. The particle size distribution ofthe samples showed that the dispersion of the 10-kGy-irradiatedgums were of high polydispersity and had considerably low valuesof D[3,2], D[4,3] and d(0.5). It should also be noted that the vis-cosity of the 15-kGy-irradiated AFG was higher than that of 7- and10-kGy-irradiated gums up to a shear rate of around 90 (1/s) witha very shear thinning behaviour. According to [4], the viscosity ofgum arabic at 10 kGy was higher than that at 2.5 kGy.

The Power-law model was closely fitted to the flow curves ofAGG dispersion. The Herschel–Bulkley model, as well as the Crossmodel were fitted to the flow curves of AFG dispersion; the param-eters of these models are summarized in Table 3. For AGG, theconsistency coefficient increased up to 3 kGy; however, havingapplied a 7–15-kGy irradiation dose to the gum samples, the con-sistency coefficient of the dispersions decreased to 0.17 (Pa sn). Theconsistency coefficient for the dispersion of non-irradiated AGGsample was 22-fold greater than that of 15-kGy-irradiated gum.For the dispersions of irradiated and non-irradiated samples ofAGG, the Spearman correlation coefficient indicated that there wasa statistically significant relation between the consistency coef-ficient and D[3,2] (Spearman correlation coefficient +1, p = 0), aswell as between the consistency coefficient and d(0.5) (Spearmancorrelation coefficient +1, p = 0). It appears that exposing the AGGpowder samples to different doses of gamma irradiation changedthe gum structure. These changes were reflected on the rheolog-ical properties of the gum dispersions. The d(0.5) and D[3,2] forthe dispersion of 3-kGy-irradiated AGG have higher values com-pared with those for non-irradiated samples. It may be that theincrease in the particle size of the dispersion caused the consis-tency coefficient to increase. On the other hand, by increasing theirradiation dose from 3 to 10 kGy, the d(0.5) and D[3,2] values werereduced by approximately one-sixth to one-eighth fold. This sharpreduction in the d(0.5) and D[3,2] values may have contributedto the one-eighth-fold decrease in the consistency coefficient. Itis interesting to note that up to 3-kGy irradiation, D[4,3]/D[3,2],which is indicative of polydispersity [26], decreases, but the con-sistency coefficient increases. Raising the irradiation doses from3 to 10 kGy increased the polydispersity and severely decreasedthe consistency coefficient. Therefore, it appears that increasingthe consistency coefficient at an irradiation dosage of 3 kGy maybe due to decrease in polydispersity and an increase in the par-ticle size in the dispersion. There was an increasing trend in theflow behaviour index with the increasing irradiation dosage, indi-cating that the gum samples, which were exposed to higher dosesof irradiation when dissolved in water, were tending to show lesspseudoplasticity and were nearing Newtonian behaviour (Table 3).The power-law and Herschel–Bulkley models were not fitted on theflow curve for the dispersion of non-irradiated AFG gum. However,for irradiated samples, the Herschel–Bulkley model was well-fitted.In this study, lower-yield stress values were observed for samples

for the dispersion of 3-kGy-irradiated AFG were increased approx-imately 2.5-fold compared with values for non-irradiated samples.Enhanced diameter of particle size for the dispersion of 3-kGy-

476 S. Alijani et al. / International Journal of Biological Macromolecules 49 (2011) 471– 479

0.01

0.1

1

10

10001001010.1

G' a

nd G

"(Pa

)

strain(%)

A. gossypinusG'-

A. gossypinusG''-

A. fluccosusG'-

A. fluccosusG''-

F3

itidio0Spssi

3

3

lqo

ktAa7acfisseadFstacivid+

ivs

Table 4Structural strength G′

LVE and G′′LVE, limiting value of strain �L, and loss-tangent value

tan ıLVE in the linear viscoelastic range, yield stress at the limit of the LVE range, �y,and flow-point stress with corresponding modulus Gf: G′ = G′′ for 1% dispersions ofthe two varieties of gum tragacanth over different doses of �-irradiation(0, 3, 7, 10,and 15 kGy), as determined by strain sweep tests at 25 ◦C and a frequency of 1 Hz.

G′LVE (Pa) G′′

LVE (Pa) �L (%) Tan ıLVE �y (Pa) �f (Pa) Gf (Pa)

AGG0 46.07a 13.62b 4.37e 0.39d 1.69b 2.32 15.73 24.47b 16.13a 32.4a 0.75c 8.49a 24.47 14.157 0.78c 1.88c 28.1b 2.60a 0.53c – –10 0.45d 1.05d 19.8c 2.58a 0.21d – –15 0.32e 0.54e 13.7d 1.97b 0.08e – –

AFG0 4.69a 5.94a 15d 1.45d 1.03a – –3 0.83b 1.85b 31.1a 2.43c 0.58b – –7 0.21d 0.58c 21.8b 3.12a 0.12c – –10 0.15e 0.39d 19.8c 2.91b 0.07c – –15 0.47c 0.36d 9.97e 0.86e 0.05d 0.14 0.27

3.5.2. Frequency sweepAs shown in Fig. 4, for the dispersion of non-irradiated AGG,

increasing the frequency increased both the G′ and G′′ with a domi-

0.01

0.1

1

10

1010.10.01

G',

G''(

Pa)

ω ( rad/s)

AGG

G': 0 KGy

G'': 0 KGy

G': 3 KGY

G'': 3 KGy

G': 7 KGy

G'': 7 KGy

G': 10 KGy

G'': 10 KGy

G': 15 KGy

G'': 15 KGy

0.001

0.01

0.1

1

10

1010.10.01

G',

G''(

Pa)

AFG

G': 0 KGy

G'': 0 KGy

G':3 KGy

G'': 3 KGy

G': 7 KGy

G'': 7 KGy

G': 10 KGy

G'': 10 KGy

G': 15 KGy

G'': 15 KGy

ig. 3. Strain sweep of moduli G′and G′′ for AGG and AFG dispersions (1%, w/w) at kGy �-irradiation at 25 ◦C.

rradiated AFG may have contributed to the increased viscosity ofhe dispersion in low and high shear rate ranges. By increasing therradiation doses from 0 to 3 kGy, the degree of polydispersity in theispersion did not change significantly. It was also observed that by

ncreasing the irradiation doses from 3 to 10 kGy, for AFG, the valuef D[3,2] decreased by 69%, but the value of D[4,3] decreased only by.26%. Thus, the degree of the polydispersity increased about 3-fold.ince the median diameter of the non-irradiated sample in com-arison with that of the 10-kGy-irradiated sample did not changeignificantly, it appears that increasing the polydispersity in theystem rather than increasing the D[4,3] and D[3,2] was effectiven the drastic decrease in viscosity of the 10-kGy-irradiated sample.

.5. Dynamic viscoelastic properties

.5.1. Strain sweepThe main purpose of strain sweep tests is to determine the strain

imits of linear viscoelastic behaviour in samples for future fre-uency sweep experiments. The mechanical spectra for dispersionsf 3-kGy-irradiated AGG and AFG are presented in Fig. 3.

The mechanical spectra for dispersions of non-irradiated and 3-Gy-irradiated AGG showed that the elastic behaviour dominateshe viscous one, consequently a weak gel structure is observed.lthough this is a weak gel structure it shows form stability at thepplied frequency of 1 Hz. Increasing the irradiation dosage from

to 15 kGy, viscous behaviour is dominate over elastic behaviour,nd the sample exhibits a more liquid character in the LVE range. Inontrast to the AGG sample, G′′ values prevail G′ over the LVE rangeor both irradiated and non-irradiated AFG dispersions. The limit-ng value of strain (�L), damping factor (tan ı), yield stress (�y) andtructural strength (G′

LVE) obtained within the LVE range are pre-ented in Table 4. Results indicated that AGG dispersion irradiationxpanded the length of LVE from 4.4% at 0 kGy to 32% at 3 kGy, andfter this initial increase, the �L started to decrease with increasingose of irradiation. A similar trend was observed in the AFG sample.or AGG samples, the Spearman correlation showed a statisticallyignificant relation between G′

LVE and D[4,3] (Spearman correla-ion coefficient +1, p = 0). Dispersions with greater particle size had

higher value of structural strength. The �L displayed a signifi-ant inverse relationship with polydispersity (D[4,3]/D[3,2]); thats, dispersions with a higher degree of polydispersity had a loweralue of �L (Spearman correlation coefficient −1, p = 0). The resultsndicated that the �L of the AFG samples varied in relation to the(0.5) and in relation to D[4,3] (Spearman correlation coefficient1, p = 0).

Dispersions and gels have a yield point, which is due tontermolecular forces. Here, the test material shows reversible-iscoelastic behaviour, and no significant change in the internaltructure occurs as long as applied stresses are below the yield

For both AGG and AFG, values with different letters in each column are significantlydifferent (p < 0.05).In all cases the standard deviations were lower than 2%.

point. It should be noted that a yield point is not a material con-stant because it is a time-dependent and also technique-dependentproperty. Determination of the yield point can be done using sev-eral steady and unsteady shear techniques. In this study, the yieldpoint was determined by performing an amplitude sweep test andmeasuring the limiting value of LVE range in terms of shear stress[21]. According to this method, only non-irradiated and 3-kGy-irradiated samples of AGG exhibited yield points. Although thestructural strength (G′

LVE) of non-irradiated samples was higherthan that of 3-kGy-irradiated samples, the yield point of the non-irradiated samples was approximately 8-fold lower than that of theirradiated samples.

ω ( rad/s)

Fig. 4. Frequency sweep of moduli G′and G′′ for A. gossypinus and A. fluccosus dis-persions (1%, w/w) at LVE range over different doses of �-irradiation at 25 ◦C.

S. Alijani et al. / International Journal of Biological Macromolecules 49 (2011) 471– 479 477

0.01

0.1

1

10

100

1010.10.01

G' a

nd G

"(Pa

)

ω( rad/s)

G' Bassorin

G" Bassorin

G' Tragacanthin

G" Tragacanthin

FB

nartqaflqttaews

1Maofiqstet

ionqta

Fr

1

10

1010.10.01

tanδ

AFG

0 KGy

3 KGy

7 KGy

10 KGy

15 KGy

0.1

1

1010.10.01

tanδ

AGG

0 KGy3 KGy7 KGy10 KGy15 KGy

ω( rad/s)

ω( rad/s)

ig. 5. Frequency sweep of moduli G′and G′′ for AGG tragacanthin solution andassorin dispersion (1.3%, w/w) at LVE range at 25 ◦C.

ance of elastic behaviour up to the crossover point (ωc = 4.75 rad/snd Gc = 10.98 Pa). At higher frequencies, the value of G′′ risesapidly; therefore, viscous behaviour is dominant. In comparisono most of the other gums, this indicates an unusual dynamic fre-uency dependency of the modulus for the samples and represents

transition from a weak-gel-like structure at low frequencies to auid-like one at high frequencies. This unusual pattern for the fre-uency sweep data may be related to the fact that AGG consists ofwo fractions (each one with its own particular molar mass distribu-ion). It has been shown that AGG is composed of 33% tragacanthinnd 67% bassorin [16]. The mean value for the hydrodynamic diam-ter for tragacanthin was determined at approximately 0.12 �m,hile the Sauter diameter for bassorin aggregates in 0.2 M NaCl

olution was approximately 302 �m.We found that dynamic oscillatory viscoelastic behaviour for

.3% solution of tragacanthin fraction of AGG is similar to aaxwellian liquid, with a relatively short average relaxation time

nd dominant elastic behaviour at high frequencies (Fig. 5). On thether hand, the water-swellable fraction (bassorin) is responsibleor the formation of aggregates in the system, which may be due tots hydrophobicity and higher amount of methoxyl groups. The fre-uency sweep data for 1.3% dispersion of bassorin fraction of AGGuggests that it has a gel-like structure (Fig. 5). Hence, bassorin andragacanthin, the two constituents of the whole gum tragacanth,ach with their own specific behaviour (shown in Fig. 5), contributeo the behaviour of whole gum tragacanth (Fig. 6)

For non-irradiated AGG, it appears that at low frequencies, its no longer possible for bassorin long molecules to glide alongne another, so their entanglements begin to form a temporaryetwork. Therefore, at frequencies lower than the crossover fre-

uency, the bassorin fraction of the non-irradiated AGG governshe behaviour of the whole GT and causes the G′ to dominate G′′. Bypplying higher-frequency oscillatory motions to the sample, the

1

10

100

1000

1010.10.01

G' a

nd G

" (P

a)

G' Gum Tragacanth

G" Gum Tragacanth

ω( rad/s)

ig. 6. Frequency sweep of moduli G′and G′′ for AGG dispersion (1.3%, w/w) at LVEange at 25 ◦C.

Fig. 7. Tan ı curves of A. gossypinus and A. fluccosus dispersions (1%, w/w) as afunction of angular frequency at 25 ◦C over different doses of �-irradiation.

bassorin temporary network, due to its interactive forces betweenlong molecules, remained relatively unchanged, therefore, the G′

did not increase as fast as G′′ did. In fact, the tragacanthin molecules,due to their lower molecular weight, are able to show more relativemotion at higher frequencies; hence, internal friction, and thereforeenergy dissipation, rises. Thus, beyond the crossover frequency anddue to the effect of tragacanthin molecules on increasing the fre-quency dependency of G′′, the viscoelastic behaviour of the systemis governed by tragacanthin molecules (G′′ > G′).

As shown in Fig. 7, for non-irradiated AGG, at low frequencies,the trend of tan ı curve and its absolute value (tan ı < 1) imply thatthe bonds between structural flow units stretch and relax withvery little breakage. This can be observed in viscoelastic-solid orsoft-solids and may be related to the presence of a large amountof the gel-forming bassorin fraction. As the frequency increases,some of the bonds or entanglements between the flow units, andeven within the flow units, break and do not have enough time toform new linkages before a second oscillation occurs. The materialthen becomes increasingly more liquid-like, as reflected by highervalues for tan ı (tan ı > 1) [27].

Exposure of the AGG to 3-kGy gamma irradiation caused thecrossover point to shift to the lower frequency (Table 5). At ω > ωc,

the viscous behaviour prevailed, and the second crossover pointoccurred at ωc = 15.2 rad/s and Gc = 10.37 Pa. It appears that, byexposing the long molecular size of polysaccharides to 3-kGy irra-

Table 5Crossover points as determined by frequency sweep test for A. gossypinus dispersion(1%, w/w) over different doses of �-irradiation at 25 ◦C.

Irradiation doses (kGy) Gf (Pa) ωf (rad/s)

0 10.98 4.743 0.88 0.08

10.37 15.27 0.19 0.17

10 0.25 0.8615 0.15 1.25

4 Biolog

ddtorqtqtrWwqemasmlbnmaap

dd(qdos

78 S. Alijani et al. / International Journal of

iation (which may be related to the bassorin fraction), they areegraded into lower molecular-size fragments along with a reduc-ion in polydispersity value. Size reduction and higher motionf molecules due to the irradiation effect on the polysaccharidesesulted in lower values for G′ and G′′ and also an increase in fre-uency dependency. It appears that 3-kGy irradiation decreaseshe degree of cross linking within the bassorin fraction; conse-uently, the gel-like system formed by bassorin no longer hashe increased structural strength of the non-irradiated one atest, so the crossover point (tan ı = 1) transferred to lower values.

ith little increase of frequency, the gel-like system immediatelyeakened, and the viscous behaviour dominated (tan ı > 1). At a fre-

uency of 15.2 rad/s, the second crossover point occurred, and thelastic behaviour again prevailed. The high-frequency behaviouray be explained by the Deborah number [28], which states that

ll materials tend to show greater solid-like characteristics overhorter experimental time scales. In the high frequency range, theotion between molecule chains is very limited; all parts of the

arge molecules and even the smaller molecules are nearly immo-ilised. The molecules’ chains may vibrate somewhat, but they areo longer able to follow rapid oscillatory motion. It appears that theost tenacious bonds did not have time to break and reform during

rapid period of oscillation. This is shorter than the average char-cteristic bond–relaxation times; therefore, the elastic behaviourrevails, and tan ı decreased again [27].

By increasing irradiation doses up to 10 kGy, for AGG the poly-ispersity in the system, and also the size of the particles wasramatically reduced, compared with the non-irradiated samplesTable 2). Thus, the G′ and G′′ values decreased over the entire fre-

uency range (Fig. 4 AGG). Although the G* values of the systemecreased with increasing irradiation dose, the general behaviourf the system remained nearly similar to that of non-irradiatedamples. The structural strength and the ωc for irradiated sam-

Fig. 8. Photomicrograph as recorded by scanning electron microscopy of pow

ical Macromolecules 49 (2011) 471– 479

ples were significantly lower than those for non-irradiated samples.For AGG dispersions, there was a significant relation between thedegree of polydispersity and crossover frequency values; that is,dispersions with a higher degree of polydispersity had a highervalue of crossover frequency (Spearman correlation coefficient,+1, p = 0). It may be that, at irradiation doses higher than 7 kGy,tragacanthin molecules are being destroyed and bassorin chainsincreasingly change into lower molecular weight fragments; how-ever, it appears that there are small amounts of long moleculesof bassorin within the system. Fig. 4 illustrates that, by increasingirradiation doses from 7 to 15 kGy, the structural strength of the sys-tem is drastically reduced. This may be attributed to the increaseddetrimental effect of irradiation on both tragacanthin and bassorinfraction of GT obtained from AGG. The dominant elastic behaviourat lower frequencies may be related to the presence of small bas-sorin fragments, which have still the ability to swell and induceelastic behaviour through inter-particle interactions.

As illustrated in Fig. 4, for the dispersion of the non-irradiatedsample of AFG, increasing the frequency enhanced both G′ andG′′ with viscous behaviour dominating over the entire frequencyrange. This frequency sweep behaviour pattern is similar to thatof unlinked polymers and indicates the behaviour of a viscoelasticliquid [21]. It has been shown that AFG is composed of 75% tragacan-thin and 25% bassorin [16]. It appears that, in an AFG dispersion, thepresence of a high amount of small molecular-sized tragacanthin(soluble fraction) is responsible for the liquid viscoelastic (G′′ > G′)behaviour over an observable frequency range of oscillation.

Reduced values for tan ı, and therefore more solid-likebehaviour, were found for non-irradiated AFG samples at lower

frequencies (Fig. 7). This may have been due to the fact that, atvery low frequencies, the small amount of gel-forming bassorin(25%) governed the rheological behaviour of the system. With aslight increase in frequency, the entanglements between flow units

dered gum tragacanth for non-irradiated and irradiated AGG and AFG.

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3

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4

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S. Alijani et al. / International Journal of B

reak and do not have enough time to form new entanglements.hus, the major component of the system, tragacanthin (75%), asn energy dissipating unit, governs the dynamic mechanical spec-ra and speeding up molecular motion with more dissipated energyesulting in a higher value for tan ı. As shown in Fig. 7, the behaviourf the non-irradiated system becomes increasingly inflexible withn increase in frequency (0.66 < ω < 50 rad/s). The spectra for theispersions of the irradiated samples showed that, by increasinghe irradiation dosage, both moduli were decreased. In all samples,′′ predominates over G′. The results indicated that, with irradia-

ion up to 3 kGy, AFG samples exhibited a remarkable decrease inlastic modulus. In comparison to G′′, elastic modulus becomes lessensitive to irradiation at higher doses.

.6. SEM

Scanning electron microphotographs of two varieties of GT foroth non-irradiated and irradiated samples are shown in Fig. 8t 40× magnification and a 200-�m scale. The microphotographsepict the surface morphology of the gums in both non-irradiatednd 10-kGy-irradiated forms. It is clear from Fig. 8 that for bothums, irradiation caused a shape deformation in which the surfacef the crystals became smoother compared with non-irradiatedamples. It seems that the increase in the yellow index (Table 1)ay be related to the change of smoothness in the surface of the

amples. This may be due to the destructive effects of irradiationn the integrity of gum structure.

. Conclusion

Radiation processing of AFG and AGG resulted in a significantodification in their colour parameters, particle size distribu-

ion, steady flow and dynamic viscoelastic properties. Irradiationesulted in significant and observable differences in the sensitivityf the rheological properties of different types of GT such as increas-ng the viscosity of the dispersions up to 3 kGy and then decreasinghe viscosity from 7 kGy to 15 kGy as well as decreasing the G′ and′′ of the dispersions by increasing the irradiation dosage from 0

o 15 kGy. It is suggested that, prior to using the irradiation pro-ess for GT for decontamination purposes, notable changes in the

unctional properties of each variety of irradiated gum tragacanthhould be taken into consideration. FTIR results showed that thereere no major functional group transformations during irradiation.

EM showed that non-irradiated and irradiated AFG and AGG have

[

[[[

cal Macromolecules 49 (2011) 471– 479 479

irregular particle size; however, irradiation made the crystal sur-face of the gums softer. The results revealed that the techniquesmentioned in this paper could be successfully used to differenti-ate between irradiated and non-irradiated GT. Further study onthe effects of irradiation on the functionality of water-soluble andwater-swellable fractions is suggested to gain more insight into themechanism of irradiation on whole gum.

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