Food Hydrocolloids Vol.6 no.6 pp.55\l-569, 1993

The effect of gamma irradiation on guar gum, locust bean gum,gum tragacanth and gum karaya

Karen King1.2 and Richard Gray'

JFood and Agricultural Chemistry Research Division, Department of Agriculturefor Northern Ireland, and 2The Queen's University of Belfast, Newforge Lane,Belfast BT9 5PX, UK

Abstract. Changes in rheological properties, as measured by viscosity, of two galactomannans (guargum and locust bean gum) and two acidic polysaccharides (gum tragacanth and gum karaya) werestudied at a range of irradiation doses < 10 kGy. Powdered samples were irradiated, and the viscosityof a I% dispersion prepared at room temperature or by heating to 80°C for 1 h, was determined overa wide shear rate range. All samples showed pseudoplastic behaviour which approached Newtonianwith increasing irradiation dose. Viscosities were calculated at a shear rate of 54 sec" to enablecomparison across the samples. Both galactomannans showed a decrease in viscosity with increasing-y irradiation independent of temperature and a hypothesis is proposed that at low -y irradiation doses«2 kGy) there is a reduction in polymer aggregation in solution, whereas at higher doses polymerhydrolysis occurs. Electron spin resonance spectroscopy data supports this hypothesis, with thedetection of different free radicals at low and high irradiation doses. The viscosity of the acidicpolysaccharides, gum karaya and gum tragacanth, following -y irradiation at low doses « 1 kGy) wasunchanged or slightly higher when compared to the unirradiated control samples. Above 1 kGydispersion viscosity decreased with increasing dose. For these polysaccharides chain hydrolysis seemsto occur during irradiation at all doses resulting in an increase in the amount of soluble polymer andhence increased viscosity at low doses, whilst at high doses viscosity decreases due to extensivepolymer hydrolysis. Similar electron spin resonance (ESR) spectra were obtained at low and highdoses with a stronger signal at the higher dose.

Introduction

Guar gum and locust bean gum are both galactomannans used mainly for theirthickening and stabilizing properties as they bind water when present atrelatively low concentrations, They are structurally similar having a 1-4 linkedI3-D-mannopyranosyl backbone with single I3-D-galactopyranosyl side groups(1). However, the ratio of galactose to mannose is around 1:2 for guar gum and1:4 for locust bean gum (2). The higher galactose content of guar gum leads to agreater solubility, and hence solution is possible at lower temperatures than forlocust bean gum which generally requires heating to 80°C (3). Both themolecular weight and the distribution of the galactose on the mannan chainaffect rheological properties. For neutral polysaccharides such as galacto­mannans further increases in viscosity can occur on aggregation of the polymerin solution to give supra molecular particles (3) or hyperentanglements (4,5).

Gum karaya and gum tragacanth are both acidic polysaccharides. Gum karayais obtained as a dried exudate from the tree Stericula urens; it containsgalacturonic acid (37%) along with neutral sugars such as rhamnose andgalactose (6). Gum tragacanth also contains galacturonic acid which may bemethylated, along with galactose, fucose, xylose and other neutral sugars (7).The methylated form, bassorin, is insoluble in water, swelling to form a gel,

559

K.King and R.Gray

whereas the demethylated form, tragacanthin, is relatively soluble (7). Bothforms are present in commercial tragacanth.

The use of ionizing radiation as a method of increasing shelf-life of foodproducts is becoming increasingly acceptable and "y irradiation has beensuggested as a possible alternative to ethylene oxide for sterilization ofpowdered gums. However, polysaccharides generally show a decrease infunctional properties on irradiation, due to attack by the free radicals generated(8). Considerable work has been carried out on starches and some oncarrageenans (9) but comparatively little is known about the behaviour of otherfood grade hydrocolloids on irradiation. This paper examines the changes inrheological properties, as determined by viscosity, of two galactomannans andtwo acidic polysaccharides when subjected to a range of irradiation doses. Dataon the free radicals generated during irradiation as determined by electron spinresonance (ESR) spectroscopy are also presented.

Materials and methods

The gums were obtained from Sigma Chemical Co. as follows: guar gum (G­4129; Lot No 77F-0639), locust bean gum (G-0753; Lot 87F-0742), gum karaya(G-0503; Lot 37F-0666) and gum tragacanth (G-1128; Lot 58F-0332). All otherchemicals were analar grade.

Moisture content

Moisture content was determined on 1.0 g (to 0.1 mg) samples of gum driedovernight in a vacuum oven at 70°C (±I°C).

Irradiation of gums

Powdered gum samples (-10 g) were sealed in glass vials (5.0 x 2.5 em) andsubjected to given doses ranging from 0.1 to 10 kGy using a cobalt 60 source(Gamma-beam 650, Nordion International Inc., Kanata, Ontario, Canada). Thedose rate was 1.38 kGy h-1 and the environmental temperature was maintainedwithin the range of 16-18°C during irradiation. Gammachrome YR dosi­meters (UK Atomic Energy Authority, Harwell) were attached to vials receiving0.1-3 kGy and amber perspex dosimeters (type 3042 B, UK Atomic EnergyAuthority) were attached to all other samples. The change in absorbance of thegammachrome dosimeters at 530 nm and the amber dosimeters at 603 nm wasmeasured spectrophotometrically. The corresponding doses were obtained fromcalibration graphs provided by the National Physical Laboratory, Teddington,UK. The irradiation doses received by each of the gums is given in Table I.Following irradiation samples were kept in sealed vials at room temperature.

Preparation of aqueous gum suspensions

In order to minimize differences in viscosity due to different methods ofpreparation, all suspensions were prepared using a Silverson mixer (SilversonMachines Ltd, Waterside, Chesham, UK). The following methods were found

560

Effects of -y irradiation on gums

Table I. Irradiation dose received by gum samples

Intended dose 0.1 0.2 0.4 n.n n.s 1.0 z.o 5.0 10.0(kGy)

Sample Actual dose (kGy)

Guar gum 0.11 0.20 0.37 n.50 0.65 o.ss 1.70 5.07 9.07Locust bean gum n.11 n.19 0.35 0.49 0.04 0.81 1.60 5.35 9.84Gum tragacanth 0.11 0.18 0.34 0.46 0.67 0.80 1.76 4.91 8.41Gum karaya 0.11 0.17 0.33 0.48 0.64 0.81 1.63 4.77 8.54

after preliminary experimentation to give the most reproducible results. Asample of 1 g (±0.05 g) of the gum was weighed into a glass vial. Distilled water(100 ml) was put into a 250 ml beaker into which the mixer head of the Silversonmixer was placed. The mixer was set to a low speed (-12-13% of maximum)and the gum added into the vortex. The mixer was immediately turned to therequired speed (expressed as % of maximum) for the gum sample as follows:guar gum, 37-38% for 5 min; locust bean gum, 25% for 2.5 min followed by50% for 2.5 min; gum tragacanth, 25% for 2.5 min, followed by 50% for 2.5min; gum karaya, 50% for 5 min. Two sets of samples were prepared for eachgum: (i) all procedures at room temperature and (ii) heating at 800e in a waterbath for 1 h immediately following mixing.

Viscosity measurements

Viscosity was measured 1 and 24 h after mixing using a Haake rotovisco RV2rotational viscometer at 200 e (Haake, Karlsruhe, Germany). The measuringhead MK50 and reduction gear ZG10 were used with the following sensorsystems: guar gum, MVII, 50-104 mPa/s; locust bean gum and gum tragacanths,MVI, 50-104 mPa/s; gum karaya, NV, 2-10 mPa/s. The systems were calibratedusing standard oils, and the viscosity determined over a range of shear rates<54 s- 1 for the MVII system, 140 s-1 for the MVI system and 325 s- t for theNV system.

Samples were kept at 200 e in a waterbath before measuring and a delay of 2min was allowed to ensure temperature equilibration once the sample, in theappropriate cup, was attached to the viscometer.

Detection of free radicals

Additional samples of each of the gums were irradiated as described previously.The mean dose received by the 3 replicate samples, calculated from dosimeterswere 0.54 kGy (±0.004) and 4.52 kGy (±0.063).

ESR spectroscopy was performed at room temperature using a Bruker ESP300 Spectrometer (Bruker, Karlsruhe, Germany) fitted with a TE I0 4 doublerectangular cavity under the conditions described in Table II. The powderedgum samples were placed in previously weighed ESR tubes (4 mm 1D) and thenreweighed. The tube was placed in the front resonator. Spectra were doubleintegrated over the range 3.44-3.54 mT and calculations on peak height and

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K.King and R.Gray

Table II. Operating conditions for the ESR spectrometer

Centre fieldSweep widthMicrowave frequencyMicrowave powerModulation frequencyModulation amplitudeReceiver gainConversion timeTime constantResolution of field axisNumber of scans per determination

348.7 mT10.0 mT9.79 GHz

15.8 mW100 KHz

0.251 mT3.2 x 10'5.12 ms

40.96 ms4096

4

area corrected for differences in sample weight contained in the 'active length' ofthe ESR tube (the length of the tube over which the spectrometer is sensitive tofree radical content).

Results and discussion

The moisture content of each of the gums before irradiation is presented inTable III. Both guar gum and locust bean gum had very similar moisturecontents indicating that any differences in behaviour of the polymers onirradiation were not due to differences in the amount of water available for freeradical formation. Gum tragacanth had a significantly lower and gum karayasignificantly higher water content compared to the galactomannans, but wereboth <10%.

Viscosity measurements

The viscosity of the gums was determined over various shear rate rangesdepending on the sensor system used on the viscometer. However, in order tocompare the samples, viscosities were calculated at the same shear rates. Allsamples showed pseudoplastic behaviour which approached Newtonian withincreasing 'Y irradiation dose. The trends in viscosity were similar at each shearrate and only the viscosities at a shear rate of 54 sec-I are given for comparisonin Figures 1-4.

Guar gum (Figure 1) showed a decrease in viscosity with increasing irradiationdose when prepared at room temperature and 80°C. At doses < 1.7 kGy agreater viscosity was generally obtained for samples prepared at 80°C, indicatingincomplete solubility at room temperature. Above this dose the viscositiesobtained at the different temperatures were similar.

Locust bean gum (Figure 2) also showed decreasing viscosity with increasingirradiation dose. However, the viscosity of dispersions prepared at 80°C wasabout ten fold higher than those prepared at room temperature, except for thehighest dose (9.8 kGy) examined. This increase in viscosity is due to the reducedsolubility of locust bean gum due to its lower level of galactose side chainsubstitution compared to guar gum.

562

Effects of ,/ irradiation on gums

Table III. Moisture content of sample prior to irradiation

Samp le % moisture (±SD)

Guar gumLocust bean gumGum tragacanthGum karaya

7.36 ± 0.2387.92 ± 0.0966.31 ± 0.3449.43 ± 0.071

10o 2 4 6 8

IRRADIATION DOSE ( kGy )

OL-- - - '------'-- - -"--- --l.- _ _ -.L --'

-2

0.8

,-...'" 0.6

~

>- 0.4E-o....enoU~ 0.2>

Fig. 1. Effect of '/ irradiation dose on guar gum (1%) viscosity. 0, dispersion prepared at roomtemperatu re ; 0 , dispersion prepared at 80°C (1 h). Values obtained 1 h after prepara tion.

<:....0.8

eno0tI)....

0.6 ..,~,-...

0.4 "I:lIII

'"0.2 '-'

1.2

o 2 468

IRRADIATION DOSE ( kGy )

0.1

,-... 0 .08

'"to:~

'-' 0.06

>-E-o....en 0.040Uen....> 0.02

0-2

Fig. 2. Effect of '/ irradiation dose on locust bean gum (1%) viscosity. 0 , dispersion prepared atroom temperature; 0 , dispersion prepared at 80°C (1 h).

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K.King and R.Gray

The viscosity changes observed for locust bean gum at room temperature and80°C with increasing 'Y irradiation dose, although similar. were independent ofone another. This suggests that free radical attack during 'Y irradiation is randomover the whole range of polymer species.

It would be expected that hydrolysis of polymer species soluble at 80°C (butnot at room temperature) during 'Y irradiation would result in more solublepolymer at room temperature and hence increased viscosity, but this did notoccur. At each temperature viscosity changes are due only to changes in thepolymer species soluble at that temperature. Guar gum also showed two distinctpolymer species (based on solubility) but at >1.7 kGy the viscosities obtained atthe two temperatures were the same, indicating that the polymer species solubleat 80°C only is no longer contributing to dispersion viscosity. For locust beangum the decrease in the viscous properties of the material soluble at 80°C onlywas even more significant, though there was still some difference in theviscosities after a dose of 9.8 kGy.

Theoretically a decrease in viscosity of either galactomannan with increasing 'Yirradiation dose could involve: (i) hydrolysis of the mannan backbone; (ii) loss ofgalactose, rendering the polymer less soluble; and (iii) a reduction in polymerchain aggregation to form supramolecular particles.

If chain hydrolysis alone occurred, resulting in the production of lowermolecular weight fragments, an increase or stabilization of the viscosityespecially at room temperature and low irradiation doses would be expected aspreviously insoluble polymer became soluble. This was not observed for eithergalactomannan. However, this could be explained if the polymer solubility wasdictated by the degree of galactose substitution rather than molecular weight.Hydrolysis of high molecular weight galactose poor polymer chains to lowermolecular weight ones, which would still be galactose poor and hence insoluble,would not result in an increase in viscosity at the lower temperature.

McCleary et al. (10) showed that the galactose was irregularly or randomlydistributed on the mannan backbone and once solubilized the galactose contentcould be reduced to <25% with no decrease in viscosity. However, no indicationof solubility from the dry state was given.

Removal of the galactose side chains would result in decreased polymersolubility and hence viscosity, even if the molecular weight was low. However,no free galactose has been found in samples of guar or locust bean gumirradiated at 9.1 and 9.8 kGy respectively (unpublished data).

Alternatively, the supramolecular structures formed by aggregation of thepolymer chains in solution (3) and which make a significant contribution todispersion viscosity, may be affected at low irradiation doses. This would give arapid decrease in viscosity with increasing irradiation dose until no polymeraggregation occurs. At higher doses polymer hydrolysis may occur leading to afurther reduction in viscosity. Little is known about the specific molecularinteractions leading to aggregation and it is possible that either a specificmolecular effect is involved or that chain hydrolysis at low doses influencesviscosity primarily by reducing polymer aggregation, rather than by decreasingpolymer molecular weight. Work on the molecular weight of guar gum following

564

Effects of -y irradiation on gums

irradiation (J .Mitchell and K.King, unpublished results) suggested that at lowdoses aggregation was affected as intrinsic viscosities were similar, whereas lightscattering data showed a decrease in molecular weight. Decreases in molecularweight were found using both methods at 2:2 kGy.

The viscosity of the acidic polysaccharides showed a different response toirradiation dose. Gum karaya (Figure 3) showed relatively stable viscosity at

0.12

0.1,-.

<I.l

= 0.08I:l.o'-"

>< 0.06Eo<....~

0 0.04U~....> 0.02

0-2 o 2 4 6 8 10

IRRADIATION DOSE ( kGy )

Fig. 3. Effect of -y irradiation dose on gum karaya (1%) viscosity. 0, dispersion prepared at roomtemperature; 0, dispersion prepared at 80°C (1 h).

L- -'- ---'--- ~ ___'___ __L ----'

0.25

,-. 0.2

<I.l

=I:l.o 0.15'-"

><Eo<.... 0.1~

0U~....

0.05>

0-2 o 2 4 6 8 10

IRRADIATION DOSE ( kGy )

Fig. 4. Effect of gamma irradiation dose on gum tragacanth (1%) viscosity. 0, dispersion prepared atroom temperature and read after 1 h; ., dispersion prepared at room temperature and read after24 h; 0, dispersion prepared at 80°C and read after 1 h; ., dispersion prepared at 80°C and readafter 24 h.

565

K.King and R.Gray

doses <0.8 kGy when prepared at 80°C, whereas samples prepared at roomtemperature showed an increase in viscosity < 1.6 kGy. Above 1.6 kGy, viscositydecreased with increasing irradiation dose. Gum tragacanth (Figure 4) showed asimilar trend to that of gum karaya. Due to the incorporation of air, data forthese samples are given for 1 and 24 h after preparation, when the air had settledout of the suspension. Both these acidic polysaccharides showed behaviourwhich could be attributed to polymer hydrolysis, where low doses of irradiationhad a net effect of increasing the amount of soluble polymer and hence tended toincrease the resulting dispersion viscosity. This effect was clearly seen to begreater in the gum karaya samples prepared at room temperature where theviscosity was increased by 60% after an irradiation dose of 1.6 kGy (Figure 3). Inthis case the polymer solubility would appear to be dictated by molecular weightrather than structure and composition as was suggested for the galactomannans.

Electron spin resonance

Both guar gum and locust bean gum irradiated at 0.5 kGy gave ESR spectra withasymmetric singlets (Figure Sa and c) suggesting the presence of more than onetype of free radical, but with one dominant (Peak A). Peak area decreased withtime (Table IV). At 4.5 kGy both galactomannans gave an asymmetric doubletwhich decayed with time to an asymmetric singlet (Figure 5b and d) indicatingthat free radical generation at higher doses results in an increase in a radical type(Peak B) which is relatively insignificant at low doses. In addition, this radicaltype is relatively unstable, decaying substantially over 6 days. Interestingly, thedevelopment of the two radical types approximates to the phases of non­aggregation and subsequent polymer hydrolysis, described as a possiblemechanism for the changes observed in viscosity on increasing irradiation dose.Gum karaya and gum tragacanth both gave a symmetric singlet spectra of PeakA at irradiation doses of 0.5 and 4.5 kGy, with a greater peak area at the higherdose indicating an increase in the production of the same free radical type withincreasing "y irradiation dose (Figure 6). All unirradiated samples gave straightline spectra at the same scale.

Samples of each gum stored at 5°C gave similar spectra to those stored at 20°Cbut the rate of decay of the unstable radical was reduced (a mean peak decreaseof 8.5% after 6 days at 5°C compared to a mean peak decrease of 31% after 6days at 20°e).

Conclusions

The viscosity of solutions of guar gum and locust bean gum decrease withincreasing v irradiation dose when irradiated in the dry powder form. Changes inviscosity may be explained by a decrease in polymer aggregation at lowirradiation doses followed by a hydrolysis of the polymer at higher doses.Viscosity changes in dispersions prepared at different temperatures show asimilar trend but are independent of each other at all irradiation doses. ESRspectra indicate two groups of free radicals generated during irradiation whichmay relate to the changes in mechanism proposed for viscosity decreases over

566

Effects of"yirradiation on gums

(a)

A+

-J'y-+

(b)

A+

(c)

(d)

Increasing H --.

Fig. 5. ESR spectra from (a) guar gum at 0.5 kGy, (b) guar gum at 4.5 kGy; (c) locust bean gum at0.5 kGy; (d) locust bean gum at 4.5 kGy. Spectra obtained 0, 1,3 and 6 days after irradiation. PeakA at 348.5 mT. Peak B at 347.9 mT. '----l = 1.0 mT.

the range of irradiation doses studied. Karaya and tragacanth showed viscositychanges with increasing 'Y irradiation dose which can be described by randomhydrolysis of the polymer. At low doses the net effect is to increase the amountof soluble polymer, stabilizing or increasing the viscosity, whereas at high dosespolymer hydrolysis occurs to such an extent that a drop in viscosity is observed.The ESR spectra do not change with increasing dose but the signal strengthincreases indicating an increase in free radical production with dose.

Gamma irradiation of the powdered gums under the conditions used in thisstudy results in a decrease in their viscous properties and would, therefore, beunsuitable for gums intended for use as thickening agents, However, the changesin viscosity with increasing dose are characteristic of each gum and hence the

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K.King and R.Gray

Table IV. Change in ESR signal strength from gums irradiated at 0.5 and 4.5 kGy with time

Sample and irradiation dose ESR signal (arbitrary units)"

Days after irradiation 0 3 6

Guar gum0.5 kGy 677 648 591 5524.5 kGy 1193 1137 1064 964

Locust bean gum0.5 kGy 628 562 469 4224.5 kGy 803 732 624 498

Gum karaya0.5 kGy 278 251 219 2014.5 kGy 438 300 291 200

Gum tragacanth0.5 kGy 334 319 285 2664.5 kGy 893 735 651 565

"Mean of four measurements. Signal corrected for weight of gum in active length of tube.

A+(a)

Increasing H --.

Fig. 6. ESR spectra from (a) gum tragacanth at 0.5 kGy; (b) gum tragacanth at 4.5 kGy; (c) gumkaraya at 0.5 kGy; (d) gum karaya at 4.5 kGy. Spectra obtained 0, 1,3 and 6 days after irradiation.Peak A at 348.5 mT. t--t = 1.0 mT.

568

Effects of'Y irradiation on gums

viscosity of an irradiated gum can be predicted for a given dose, provided theviscosity of the unirradiated gum is known. Gamma irradiation at low dosestherefore may have an application in the production of gums for use asstabilizing agents and will be the subject of further research.

Acknowledgements

The authors wish to thank Mr N.E.Bjergarde, Mrs H.Cahoon and MissE.Stewart for expert technical assistance.

References

I. Dea,I.C.M. and Morrison,A. (1975) Chemistry and interactions of seed galactomannans. Adv.Carbo Chem. Biochem., 31, 241-312.

2. Goldstein,A.M., Atler,E.N. and Seaman,l.K. (1973) In: Whistler,R.L. and BeMiller,l.N.(ed.), Indus/rial Gums. 2nd edn. Academic Press, New York, pp. 303-321.

3. Doublier,l.L. and Launay,B. (1981) Rheology of galactomannan solutions: Comparative studyof guar gum and locust bean gum. J. Text. Stud., 12, 151-172.

4. Kratochvil.P. (1972) In: Huglin,M. (ed.), Light Scattering From Polymer Solutions. AcademicPress, New York, pp. 79-95.

5. Mitchell,l.R. (1978) In: Blanchard,l.M.V. and Mitchell,J.R. (eds), Polysaccharides in Foods,Butterworths, London.

6. Goldstein,A.M. and Atler,E.N. (1973) In: Whistler,R.L. and Bemiller,l.N. (ed.), IndustrialGums, 2nd edn. Academic Press, New York, pp. 273-288.

7. Meer,G., Meer.WA. and Gerard,T. (1973) Gum Tragacanth. In: Whistler,R.L. and BeMiller,1.N. (ed.), Indus/rial Gums, 2nd edn. Academic Press, New York, pp. 289-299.

8. Dauphin,l.F. and Saint-Lobe,L.R. (1977) Radiation chemistry of carbohydrates. In: Elias,P.S.and Cohen,A.l. (eds), Radiation Chemistry of Major Food Components. Elsevier Scientific,Amsterdam, pp. 131-172.

9. Marrs,W.M. (1988) The effect of gamma radiation on the structure of carrageenans In:Phillips,G.O., Wedlock,D.l. and Williams,P.A. (eds), Gums and Stabilisers for the FoodIndustry, 4th edn. IRL Press, Oxford, pp. 399-408.

10. McCleary,B.V., Amado.R.; Waibel,R. and Nenkom,H. (1981) Effect of galactose content onthe solution and interchain properties of guar and carob galactomannan, Carbo Res.; 92, 269­285.

Received on April 24, 1992; accepted on November 9, 1992

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