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Radiation induced degradation of xanthan gum in the solid state
Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Radiation induced degradation of xanthan gum in the solid state Murat Şen a,n, Hande Hayrabolulu a, Pınar Taşkın a, Murat Torun a, Maria Demeter b, Mihalis Cutrubinis c, Olgun Güven a a
Hacettepe University, Department of Chemistry, 06800 Ankara, Turkey National Institute for Lasers, Plasma and Radiation Physics, (INFLPR), Atomistilor 409, Magurele, Romania c Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), Reactorului 30, Magurele, Romania b
H I G H L I G H T S
Radiation-induced degradation of xanthan gum (XG) described. The influences of dose rate on the radiation degradation of XG were examined. G(S) and degradation rate of XG were calculated by using molecular weights data.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 September 2015 Received in revised form 1 October 2015 Accepted 3 October 2015
In this study, the effect of ionizing radiation on xanthan gum was investigated. Xanthan samples were irradiated with gamma rays in air at ambient temperature in the solid state at different dose rates and doses. Change in their molecular weights was followed by size exclusion chromatography (SEC). Chain scission yield (G(S)), and degradation rate constants (k) were calculated. The calculated G(S) values are 0.0151 7 0.0015, 0.0144 7 0.0020, 0.0098 70.0010 mmol/J and k values are 1.4 10 8 71.4 10 9, 1.3 10 8 7 2.0 10 9, 8.7 10 9 71.0 10 9 Gy 1 for 0.1, 3.3 and 7.0 kGy/h dose rates, respectively. It was observed that the dose rate was an important factor controlling the G(S) and degradation rate of xanthan gum. Considering its use in food industry, the effect of irradiation on rheological properties of xanthan gum solutions was also investigated and flow model parameters were determined for all dose rates and doses. Rheological analysis showed that xanthan solution showed non-Newtonian shear thinning behaviour and ionizing radiation does not change the non-Newtonian and shear thinning flow behaviour of xanthan gum solutions in concentration ranges of this work. It was determined that, Power Law model well described the flow behaviour of unirradiated and irradiated xanthan solutions. & 2015 Published by Elsevier Ltd.
Keywords: Xanthan gum degradation chain scission yield shear thinning
1. Introduction Xanthan gum, a heteropolysaccharide produced from the fermentation of corn sugar by bacterium Xanthomonas campestris, is widely used as food additive and rheology modifier; thickener, stabilizer, emulsifier, gelling agent and water-binding agents in the food, textile, cosmetic, pharmaceutical and oil industry (Barrére et al., 1986; Cottrell et al., 1980). Its very high molecular weight (2 106–2 107 g/mol) and its chemical structure makes it a highly stable polysaccharide against enzymolysis and acidolysis. The main chain consists of trisaccharide side chains containing a Dglucuronic acid unit between two D-mannose units linked at the n
Corresponding author. Fax: þ 90 312 2977989. E-mail address: [email protected] (M. Şen).
O-3 position of every other glucose residue in the main chain, (Cottrell et al., 1980; Katzbauer, 1998; Garcia-Ochoa et al., 2000). Approximately one-half of the terminal D-mannose contains a pyruvic acid residue linked via keto group to the 4 and 6 positions, with an unknown distribution. The presence of acetic and pyruvic acids produces an anionic polysaccharide type (Sandford and Baird, 1983). In recent years, there has been growing interest in understanding the effect of ionizing radiation on the chemical structure of polysaccharides such as chitin, chitosan, sodium alginate, and kappa carrageenan due to their versatile food and non-food applications (Choi et al., 2002; Nagasawa et al., 2000). The research in this field is mostly concentrated on the degradative effect of radiation to produce polysaccharides with desired low molecular weight ranges. We have carried out numerous studies on radiation induced degradation of polysaccharides in our research group. In
http://dx.doi.org/10.1016/j.radphyschem.2015.10.005 0969-806X/& 2015 Published by Elsevier Ltd.
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
2007 radiation-induced degradation of galactomannan polysaccharides such as locust bean gum (LBG), guar gum (GG) and tara gum (TG) have been investigated by Şen et al. (2007). It was concluded that G/M (galactose/mannose) ratio did not change the chain scission yield of galactomannans, whereas, initial molecular weight was more important in affecting degradation rate of galactomannans under gamma rays. On the other hand, it was found that G/M (guluronic acid/mannuronic acid) ratio was an important factor affecting chain scission yield of sodium alginate (Şen et al., 2010). In 2014 Şen et al. found that one of the important parameters of radiation-induced degradation of chitosan is its degree of deacetylation value (Taşkın et al., 2014). It is well known that the main effect of ionizing radiation on polysaccharides is chain scission of C–O bond connecting the glycoside groups on the main chain (Kume and Takehisa, 1982; Qin et al., 2003; Fei et al., 2000). However, there is no sufficient information about the effect of ionizing radiation on xanthan gum in the literature. In this research, we have investigated the chain scissioning effect of ionizing radiation on xanthan gum in solid state at different dose rates and doses and determined the flow behaviour of xanthan solutions before and after the irradiation.
2. Experimental A commercial xanthan sample (food grade, Batch number: M1207A G48 1207007, produced by Jungbunzlauer, Austria) in the powder form was used. Waters 2000-1000-500 ultrahydrogel columns were used for molecular weight analyses and universal calibration was constructed by using narrow molecular weight pullulan standards obtained from Shodex Company. NaCl (0.1 M) was used as the eluting solvent. The Mark–Houwink parameters used for pullulan standards and xanthan gum were K ¼2.31 10 4 dL/g, a¼ 0.65 and K ¼1.7 10 6 dL/g, a ¼1.14, respectively (Mendichi and Scieroni, 1998; http://www.ampolymer. com/Mark-HouwinkParameters.html). Xanthan samples irradiated in powder and in dilute solution form to obtain various molecular weight fractions by using Co-60 gamma source in air at ambient temperature, at different dose rates (0.1, 3.3 and 7.0 kGy/h) and doses (2,5, 5.0, 10, 20, 30 and 50 kGy). Irradiations were carried out in the Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFINHH), Romania by using Gamma Chamber 5000 for the 3.3 and 7.0 kGy/h dose rates and Gamma Irradiator SVSTCo-60/B for the 0.1 kGy/h dose rate. Spectroscopic characterization of unirradiated and irradiated xanthan gum samples were performed by recording their Nuclear Magnetic Resonance (NMR) and Fourier Transform Infra-red (FTIR) spectra. The changes in the ratio of hydrogen atoms in methyl groups of pyruvate and acetate were followed with Bruker 400 MHz ultrashield model digital FT-NMR model spectrometer operated at 90 °C in which trimethylsilane (TMS) was used as an internal standard. The possible functional groups formed after irradiation of xanthan gum were followed with Spectrum 100 model, Perkin Elmer FT-IR spectrometer in Attenuated Total Relectance (ATR) mode with a resolution of 4 cm 1 in a range of 4000–600 cm 1. Rheological properties of xanthan gum solutions were determined by using a Thermo-Haake Modular Advanced Rheometer System (MARS) equipped with a cone-plate fixture with a radius of 35 mm, cone angle of 4° and a gap size of 0.139 mm. All measurements were performed at a fixed temperature (30 °C) and concentration (1% w/v) over a wide range of shear rates from 0.01 to 500 s 1.
3. Results and discussion 3.1. Effect of gamma rays on the chemical structure of xanthan gum Spectroscopic characterization of xanthan samples was performed by recording their FTIR and 1H-NMR spectra. The minor increases in the area of –OH group region (3000–3500 cm 1) was observed while no change in finger print region indicates the lack of formation of carbonyl and carboxylic acid groups. It is suggested that degradation of xanthan proceeds by main chain scission on glycosidic bonds and ended with –OH termination. 1H-NMR spectra confirms the scission on main chain of xanthan. On the other hand, there is no effect on side groups after irradiation, while the ratio of number of protons in pyruvate group to acetate group is 0.77 in pure xanthan, it is 0.65–0.85 for xanthans irradiated to 2.5–50 kGy. 3.2. Effect of gamma rays on the molecular structure of xanthan gum It is very well known that polysaccharides in dry form or in solution degrade when exposed to ionizing radiation (Kume and Takehisa, 1982; Qin et al., 2003; Fei et al., 2000). For the investigation of the effect of gamma rays on the molecular weight of ¯ w ) and number average molecular weight ( M ¯ n) xanthan, weight ( M values of the polymers were determined by size exclusion chromatography (SEC) analysis. SEC chromatograms obtained for 7.0 and 0.1 kGy/h dose rates are given in Fig. 1. As seen in Fig. 1, there are two main peaks observed for xanthan in the high and low molecular weight regions. As the absorbed dose increased, the main peak the xanthan sample at low retention volumes shifted to higher retention volumes indicating a decrease in the molecular weight of the samples as a function of irradiation. As can be seen in Fig. 1 this shift is much higher at 0.1 kGy/h dose rate than 7.0 kGy/h. A slight increase on the intensity of low molecular ¯ w and M ¯ n with abweight peak was also observed. Changes in M sorbed dose are given in Figs. 2 and 3, respectively. As can be seen from these figures, both average molecular weights decreased rapidly up to 50 kGy and this decrease is more effective for low dose rate irradiations. Fast decrease in molecular weight was attributed to longer irradiation period of samples in the presence of oxygen i.e enhancing of oxidative degradation at low dose rates. 3.3. Radiation stability of xanthan gum The efficiency of radiation-induced events is expressed by Gvalue. The G-value is defined as the number of events as mol per J, and has been customarily used to measure radiation – chemical yield. The molecular weight values of the xanthan were used for the determination of G(S) and degradation rate. If scission is the only mode of action of radiation then the radiation – chemical yield of degradation (scission) G(S) is determined from the Alexander–Charlesby–Ross equation given below for polymers irradiated in dry and solid form (Charlesby, 1960):
1 1 − = G (S ) D ¯n ¯ n0 M M
(1)
¯ n and M ¯ n0 (kg/ where, the absorbed dose D is in J/kg (Gy) and M mol) are the number average molecular weights of the polymer before and after irradiation. ¯ n 1/ M ¯ n0 For the determination of the G(S) (mol/J) values, 1/ M was plotted against dose for all irradiations and given in Fig. 4. Then, the G(S) values were calculated by using the relevant slopes (Table 1). The equation given by Jellinek (1955) modified by Şen et al. (2010) is used for the determination of degradation rate constants (Eq. (2)):
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
1,7
7.0 kGy/h
0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
1,6
0.0 kGy Mw x10
-6
1,5
2.5 kGy 5.0 kGy
1,4 1,3 1,2 1,1
10.0 kGy
1,0 0,9
20.0 kGy 30.0 kGy 50.0 kGy 25
30
35
10
20 30 Dose (kGy)
40
50
¯ w of xanthan gum with Fig. 2. The change in weight average molecular weight M dose. and dose rates.
⎛ k ⎞ 1 1 − =⎜ ⎟D ¯ ¯ MnD Mn0 ⎝ mo ⎠
40
45
50
0.1 kGy/h
0.0 kGy
0
2.5 kGy 5.0 kGy
(2)
¯ n and M ¯ n0 (kg/mol) are where, the absorbed dose D is in J/kg and M the number average molecular weights of the polymer before and after irradiation. mo is the molecular weight (kg/mol) of monomer unit and k (Gy 1) is the degradation rate constant. Degradation rate constants for xanthan were determined by using the (1/ Mn ) (1/ Mn0 ) vs. dose plots. The determined degradation rate constants for xanthans are given in Table 1. It can be seen from Table 1 that, there is a slight decrease in G (S) and k values as the dose rate increased. This behaviour was attributed to the enhancing of oxidative degradations at lower dose rates since longer times are required to achieve the desired absorbed dose.
10.0 kGy
1,6 0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
1,4
Mn x 10
-6
20.0 kGy 30.0 kGy
1,2 1,0
50.0 kGy 0,8 25
30
35
40
45
50
ElutionVolume (ml) Fig. 1. Size exclusion chromatograms of irradiated xanthan gum (a) dose rate 7.0 kGy/h (b) 0.1 kGy/h.
0,6
0
10
20 30 Dose (kGy)
40
50
60
¯ n of xanthan gum with Fig. 3. The change in number average molecular weight M dose. and dose rates.
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
3
10
8 0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
(1/Mn-1/Mn0) x 10
7
6
2.5 kGy 5.0 kGy 10 kGy 20 kGy 30 kGy 50 kGy
2
10
1
η (Pa.s)
10
4
0
10
-1
10
2
-2
10
0
-3
0
10
20
30
40
50
10
Dose (kGy)
-1
10
0
10
1
2
10
3
10
10
-1
Shear rate (s )
¯ n0 vs. dose plot to determine degradation rate constants of ¯ n 1/ M Fig. 4. 1/ M xanthan gum. Dose rates are 0.1, 3.3 and 7.0 kGy/h. Table 1 G(S) and k values of xanthan gum irradiated at different dose rates. Dose rate
G(S) (lmol/J)
k (Gy 1)
0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
0.01517 0.0015 0.01447 0.0020 0.0098 7 0.0010
1.4 10 8 7 1.4 10 9 1.3 10 8 7 2.0 10 9 8.7 10 9 7 1.0 10 9
Fig. 6. Effect of irradiation on the viscosity of xanthan gum at a dose rate of 3.3 kGy/h.
″ became larger than G′, due to the significant decrease in molecular weight with dose. Among various rheological models, Power Law Model (or the Ostwald–de Waele relationship) describes the data of shear-thinning and shear thickening fluids with the equation given below (Eq. (3)):
τ = Kγ n 3.4. Effect of gamma rays on the rheological properties of xanthan gum For the investigation of viscoelastic properties of irradiated xanthan gum, first, linear viscoelastic region was determined by strain-sweep tests for all dose rates and doses, at a fixed frequency of 1 rad/s and at a deformation range of γ ¼ 1 10 4%–10%, for 1% (w/v) xanthan solutions at 30 °C. Then the change in elastic (G′) and viscous (G″) modulus values depending on frequency were determined by frequency-sweep analysis at a frequency range of 0.01–100 Hz, for all dose rates and doses. The representative frequency vs. modulus curves are given 3.3 kGy/h dose rate in Fig. 5. It was observed that, G′ was larger than G″ for unirradiated xanthan solution, and as the absorbed dose increased, G′ and G″ values became closer up to 20 kGy and when the absorbed dose was 50 kGy, G″ became larger than G′. It was concluded that, up to 20 kGy, or approximately 1.0 106 g/mol molecular weight the xanthan retains the elastic behaviour After 20 kGy, especially at high frequencies, viscous behaviour became more dominant and G 3
10
2
G', G" (Pa)
10
1
10
0.0 kGy- G' 0.0 kGy- G'' 2.5 kGy- G' 2.5 kGy- G'' 20 kGy- G' 20 kGy- G'' 50 kGy- G' 50 kGy- G''
0
10
-1
10
-2
10
-1
10
0
10
1
10
Frequency (Hz)
(3)
where K is the flow consistency index (Pa s) (shear stress at a shear rate of 1.0 s 1), dimensionless exponent n is the flow behaviour index, reflects the closeness to Newtonian flow. If the fluid is a Newtonian fluid, n ¼1 and the consistency index (K) is identically equal to the viscosity of the fluid, η. When the magnitude of n o1, the fluid shows shear-thinning and when n 41, shear thickening behaviours (Rao, 2014). For the investigation of the shear thinning and/or shear thickening behaviour of xanthan gum and the effect of dose rate and dose on the rheological properties of its solutions, flow curves of 1% (w/v) xanthan gum solutions were constructed at 30 °C (Fig. 6) over a wide range of shear rates from 0.01 to 500 s 1 and rheological properties and flow behaviours were reported from the experimentally obtained data. A linear regression analysis was used to determine the material parameters of Power Law Model. Calculated model parameters for xanthan gum irradiated at a dose rate of 3.3 kGy/h were given in Table 2. Same behaviour was observed for the other dose rates. Fig. 6 shows that the viscosity of unirradiated and irradiated xanthan solutions decreases exponentially with shear rate which indicates that xanthan solution shows a non-Newtonian and shear-thinning behaviour. Irradiation did not change the nonNewtonian, shear-thinning character of xanthan solution. This behaviour confirmed by evaluating the material parameters of Power Law Model. As seen in Table 2, Power Law Model well describes the shear response of xanthan gum solution. Since n o1, Table 2 Variation of Power Law Model parameters of xanthan gum with irradiation dose (Dose rate of 3.3 kGy/h). Irradiation dose (kGy)
K (Pa.s)
n
r2
0.0 2.5 20 50
6.268 4.269 2.298 0.497
0.195 0.155 0.295 0.487
0.9973 0.9793 0.9979 0.9996
Fig. 5. Change in G′ and G″ with dose and frequency for xanthan gum.
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
unirradiated and irradiated xanthan solutions shows a non-Newtonian and shear thinning behaviour. The consistency index (K) of xanthan solution decreases with dose due to the gradual decrease in molecular weight, whereas, the flow behaviour index (n) increases showing that xanthan solution gaining Newtonian flow behaviour as the molecular weight of xanthan gum decreases.
4. Conclusions The effect of ionizing radiation on xanthan gum in solid state, at different dose rates and doses have been investigated and the flow behaviour of xanthan solutions determined before and after the irradiation. It was determined that the main effect of γ radiation in air for xanthan gum is chain scission. Chain scission reactions become more effective when the dose rate decreased due to the enhanced oxidative degradations during irradiation. Dynamic rheological analysis of irradiated xanthan solutions showed that up to 20 kGy, elastic behaviour was more dominant than viscous behaviour. After 20 kGy, especially at high frequencies, viscous behaviour became more pronounced and G″ became larger than G ′, due to the significant decrease in molecular weight with dose. Rheological analysis also showed that xanthan solution (1.0%) showed non-Newtonian and shear-thinning behaviour and irradiation did not change the non-Newtonian, shear-thinning character of xanthan solution. Calculated model parameters of power law showed that flow consistency index (K) of xanthan solution decreases with dose due to the gradual decrease in molecular weight, whereas, n increases showing that xanthan solution gaining Newtonian flow behaviour as the molecular weight of xanthan gum decreases. At the end of this study, the effect of gamma irradiation on xanthan gum in solid form is clarified. For the understanding of radiation stability of xanthan gum irradiated in aqueous solutions as a function of dose and dose rates our work is still in progress.
Acknowledgements
5
the Scientific and Technological Research Council of Turkey (TUBITAK) research project 112T628, UEFISCDI research project 598/ 2013 and the support provided by the International Atomic Energy Agency through Research Contract no 14475/R2.
References Barrére, G.C., Barber, C.E., Daniels, M.J., 1986. Molecular cloning of genes involved in the production of the extracellular polysaccharide xanthan by Xanthomonas campestris pv. campestris. Int. J. Bio. Macromol. 8 (6), 372–374. Charlesby, A., 1960. Atomic Radiation and Polymers. Pergamon Press, New York, NY. Choi, W.S., Ahn, K.J., Lee, D.W., Byun, M.W., Park, H.J., 2002. Preparation of chitosan oligomers by irradiation. Polym. Degrad. Stab. 78, 533–538. Cottrell I. W., Kang K. S., and Kovacs P., Xanthan Gum, In Handbook of Water Soluble Gums and Resins (Ed. Davidson R. L.) 1980, McGraw-Hill; NewYork, Chap. 24. Fei, B., Wach, R.A., Mitomo, H., Yoshii, F., Kome, T., 2000. Hydrogel of biodegradable cellulose derivatives. I. Radiation-induced crosslinking of CMC. J. Appl. Polym. Sci. 78, 278–283. Garcia-Ochoa, F., Santos, V.E., Casas, J.A., Gomez, E., 2000. Xanthan gum: production, recovery and properties. Biotechnol. Adv. 18, 549–579 http://www.ampo lymer.com/Mark-HouwinkParameters.html. Jellinek, H.H.G., 1955. Degradation of Vinyl Polymers. Wiley and Sons, New York. Katzbauer, B., 1998. Properties and applications of xanthan gum. Polym. Degrad. Stab. 59, 81–84. Kume, T., Takehisa, M., 1982. In: Proceedings of the 2nd International Conference on Chitin and Chitosan, Sapparo, Japan, pp. 66–70. Mendichi, R., Scieroni, A.G., 1998. Evaluation of a single-capillary viscometer detector on line to a SEC system used with a new pulse-free pump. J. Appl. Polym. Sci. 68, 1651–1659. Nagasawa, N., Mitomo, H., Yoshii, F., Kume, T., 2000. Radiation-induced degradation of sodium alginate. Polym. Degrad. Stab. 69, 279–285. Qin, C., Du, Y., Zong, L., Zeng, F., Liu, Y., Zhou, B., 2003. Effect hemicellulase on the molecular weight and structure of chitosan. Polym. Degrad. Stab. 80, 435–441. Rao, M.A., 2014. Rheology of Fluid, Semisolid, and Solid Foods, Food Engineering Series. Springer Science Business Media, New York. Sandford, P.A., Baird, J., 1983. Aspinall, G.O. (Ed.), The Polysaccharides. Academia Press, Prague, pp. 470–473. Şen, M., Rendevski, S., Akkaş Kavaklı, P., Sepehrianazar, A., 2010. Effect of G/M ratio on the radiation induced degradation of sodium alginate. Radiat. Phys. Chem. 79, 279–282. Şen, M., Yolaçan, B., Güven, O., 2007. Radiation-induced degradation of galactomannan polysaccharides. Nucl. Instrum. Meth. Phys. Res. B 265, 429–433. Taşkın, P., Canısağ, H., Şen, M., 2014. The effect of degree of deacetylation on the radiation induced degradation of chitosan. Radiat. Phys. Chem. 94, 236–239.
The authors gratefully acknowledge the support provided by
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Radiation induced degradation of xanthan gum in the solid state Murat Şen a,n, Hande Hayrabolulu a, Pınar Taşkın a, Murat Torun a, Maria Demeter b, Mihalis Cutrubinis c, Olgun Güven a a
Hacettepe University, Department of Chemistry, 06800 Ankara, Turkey National Institute for Lasers, Plasma and Radiation Physics, (INFLPR), Atomistilor 409, Magurele, Romania c Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH), Reactorului 30, Magurele, Romania b
H I G H L I G H T S
Radiation-induced degradation of xanthan gum (XG) described. The influences of dose rate on the radiation degradation of XG were examined. G(S) and degradation rate of XG were calculated by using molecular weights data.
art ic l e i nf o
a b s t r a c t
Article history: Received 26 September 2015 Received in revised form 1 October 2015 Accepted 3 October 2015
In this study, the effect of ionizing radiation on xanthan gum was investigated. Xanthan samples were irradiated with gamma rays in air at ambient temperature in the solid state at different dose rates and doses. Change in their molecular weights was followed by size exclusion chromatography (SEC). Chain scission yield (G(S)), and degradation rate constants (k) were calculated. The calculated G(S) values are 0.0151 7 0.0015, 0.0144 7 0.0020, 0.0098 70.0010 mmol/J and k values are 1.4 10 8 71.4 10 9, 1.3 10 8 7 2.0 10 9, 8.7 10 9 71.0 10 9 Gy 1 for 0.1, 3.3 and 7.0 kGy/h dose rates, respectively. It was observed that the dose rate was an important factor controlling the G(S) and degradation rate of xanthan gum. Considering its use in food industry, the effect of irradiation on rheological properties of xanthan gum solutions was also investigated and flow model parameters were determined for all dose rates and doses. Rheological analysis showed that xanthan solution showed non-Newtonian shear thinning behaviour and ionizing radiation does not change the non-Newtonian and shear thinning flow behaviour of xanthan gum solutions in concentration ranges of this work. It was determined that, Power Law model well described the flow behaviour of unirradiated and irradiated xanthan solutions. & 2015 Published by Elsevier Ltd.
Keywords: Xanthan gum degradation chain scission yield shear thinning
1. Introduction Xanthan gum, a heteropolysaccharide produced from the fermentation of corn sugar by bacterium Xanthomonas campestris, is widely used as food additive and rheology modifier; thickener, stabilizer, emulsifier, gelling agent and water-binding agents in the food, textile, cosmetic, pharmaceutical and oil industry (Barrére et al., 1986; Cottrell et al., 1980). Its very high molecular weight (2 106–2 107 g/mol) and its chemical structure makes it a highly stable polysaccharide against enzymolysis and acidolysis. The main chain consists of trisaccharide side chains containing a Dglucuronic acid unit between two D-mannose units linked at the n
Corresponding author. Fax: þ 90 312 2977989. E-mail address: [email protected] (M. Şen).
O-3 position of every other glucose residue in the main chain, (Cottrell et al., 1980; Katzbauer, 1998; Garcia-Ochoa et al., 2000). Approximately one-half of the terminal D-mannose contains a pyruvic acid residue linked via keto group to the 4 and 6 positions, with an unknown distribution. The presence of acetic and pyruvic acids produces an anionic polysaccharide type (Sandford and Baird, 1983). In recent years, there has been growing interest in understanding the effect of ionizing radiation on the chemical structure of polysaccharides such as chitin, chitosan, sodium alginate, and kappa carrageenan due to their versatile food and non-food applications (Choi et al., 2002; Nagasawa et al., 2000). The research in this field is mostly concentrated on the degradative effect of radiation to produce polysaccharides with desired low molecular weight ranges. We have carried out numerous studies on radiation induced degradation of polysaccharides in our research group. In
http://dx.doi.org/10.1016/j.radphyschem.2015.10.005 0969-806X/& 2015 Published by Elsevier Ltd.
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
2007 radiation-induced degradation of galactomannan polysaccharides such as locust bean gum (LBG), guar gum (GG) and tara gum (TG) have been investigated by Şen et al. (2007). It was concluded that G/M (galactose/mannose) ratio did not change the chain scission yield of galactomannans, whereas, initial molecular weight was more important in affecting degradation rate of galactomannans under gamma rays. On the other hand, it was found that G/M (guluronic acid/mannuronic acid) ratio was an important factor affecting chain scission yield of sodium alginate (Şen et al., 2010). In 2014 Şen et al. found that one of the important parameters of radiation-induced degradation of chitosan is its degree of deacetylation value (Taşkın et al., 2014). It is well known that the main effect of ionizing radiation on polysaccharides is chain scission of C–O bond connecting the glycoside groups on the main chain (Kume and Takehisa, 1982; Qin et al., 2003; Fei et al., 2000). However, there is no sufficient information about the effect of ionizing radiation on xanthan gum in the literature. In this research, we have investigated the chain scissioning effect of ionizing radiation on xanthan gum in solid state at different dose rates and doses and determined the flow behaviour of xanthan solutions before and after the irradiation.
2. Experimental A commercial xanthan sample (food grade, Batch number: M1207A G48 1207007, produced by Jungbunzlauer, Austria) in the powder form was used. Waters 2000-1000-500 ultrahydrogel columns were used for molecular weight analyses and universal calibration was constructed by using narrow molecular weight pullulan standards obtained from Shodex Company. NaCl (0.1 M) was used as the eluting solvent. The Mark–Houwink parameters used for pullulan standards and xanthan gum were K ¼2.31 10 4 dL/g, a¼ 0.65 and K ¼1.7 10 6 dL/g, a ¼1.14, respectively (Mendichi and Scieroni, 1998; http://www.ampolymer. com/Mark-HouwinkParameters.html). Xanthan samples irradiated in powder and in dilute solution form to obtain various molecular weight fractions by using Co-60 gamma source in air at ambient temperature, at different dose rates (0.1, 3.3 and 7.0 kGy/h) and doses (2,5, 5.0, 10, 20, 30 and 50 kGy). Irradiations were carried out in the Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFINHH), Romania by using Gamma Chamber 5000 for the 3.3 and 7.0 kGy/h dose rates and Gamma Irradiator SVSTCo-60/B for the 0.1 kGy/h dose rate. Spectroscopic characterization of unirradiated and irradiated xanthan gum samples were performed by recording their Nuclear Magnetic Resonance (NMR) and Fourier Transform Infra-red (FTIR) spectra. The changes in the ratio of hydrogen atoms in methyl groups of pyruvate and acetate were followed with Bruker 400 MHz ultrashield model digital FT-NMR model spectrometer operated at 90 °C in which trimethylsilane (TMS) was used as an internal standard. The possible functional groups formed after irradiation of xanthan gum were followed with Spectrum 100 model, Perkin Elmer FT-IR spectrometer in Attenuated Total Relectance (ATR) mode with a resolution of 4 cm 1 in a range of 4000–600 cm 1. Rheological properties of xanthan gum solutions were determined by using a Thermo-Haake Modular Advanced Rheometer System (MARS) equipped with a cone-plate fixture with a radius of 35 mm, cone angle of 4° and a gap size of 0.139 mm. All measurements were performed at a fixed temperature (30 °C) and concentration (1% w/v) over a wide range of shear rates from 0.01 to 500 s 1.
3. Results and discussion 3.1. Effect of gamma rays on the chemical structure of xanthan gum Spectroscopic characterization of xanthan samples was performed by recording their FTIR and 1H-NMR spectra. The minor increases in the area of –OH group region (3000–3500 cm 1) was observed while no change in finger print region indicates the lack of formation of carbonyl and carboxylic acid groups. It is suggested that degradation of xanthan proceeds by main chain scission on glycosidic bonds and ended with –OH termination. 1H-NMR spectra confirms the scission on main chain of xanthan. On the other hand, there is no effect on side groups after irradiation, while the ratio of number of protons in pyruvate group to acetate group is 0.77 in pure xanthan, it is 0.65–0.85 for xanthans irradiated to 2.5–50 kGy. 3.2. Effect of gamma rays on the molecular structure of xanthan gum It is very well known that polysaccharides in dry form or in solution degrade when exposed to ionizing radiation (Kume and Takehisa, 1982; Qin et al., 2003; Fei et al., 2000). For the investigation of the effect of gamma rays on the molecular weight of ¯ w ) and number average molecular weight ( M ¯ n) xanthan, weight ( M values of the polymers were determined by size exclusion chromatography (SEC) analysis. SEC chromatograms obtained for 7.0 and 0.1 kGy/h dose rates are given in Fig. 1. As seen in Fig. 1, there are two main peaks observed for xanthan in the high and low molecular weight regions. As the absorbed dose increased, the main peak the xanthan sample at low retention volumes shifted to higher retention volumes indicating a decrease in the molecular weight of the samples as a function of irradiation. As can be seen in Fig. 1 this shift is much higher at 0.1 kGy/h dose rate than 7.0 kGy/h. A slight increase on the intensity of low molecular ¯ w and M ¯ n with abweight peak was also observed. Changes in M sorbed dose are given in Figs. 2 and 3, respectively. As can be seen from these figures, both average molecular weights decreased rapidly up to 50 kGy and this decrease is more effective for low dose rate irradiations. Fast decrease in molecular weight was attributed to longer irradiation period of samples in the presence of oxygen i.e enhancing of oxidative degradation at low dose rates. 3.3. Radiation stability of xanthan gum The efficiency of radiation-induced events is expressed by Gvalue. The G-value is defined as the number of events as mol per J, and has been customarily used to measure radiation – chemical yield. The molecular weight values of the xanthan were used for the determination of G(S) and degradation rate. If scission is the only mode of action of radiation then the radiation – chemical yield of degradation (scission) G(S) is determined from the Alexander–Charlesby–Ross equation given below for polymers irradiated in dry and solid form (Charlesby, 1960):
1 1 − = G (S ) D ¯n ¯ n0 M M
(1)
¯ n and M ¯ n0 (kg/ where, the absorbed dose D is in J/kg (Gy) and M mol) are the number average molecular weights of the polymer before and after irradiation. ¯ n 1/ M ¯ n0 For the determination of the G(S) (mol/J) values, 1/ M was plotted against dose for all irradiations and given in Fig. 4. Then, the G(S) values were calculated by using the relevant slopes (Table 1). The equation given by Jellinek (1955) modified by Şen et al. (2010) is used for the determination of degradation rate constants (Eq. (2)):
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
3
1,7
7.0 kGy/h
0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
1,6
0.0 kGy Mw x10
-6
1,5
2.5 kGy 5.0 kGy
1,4 1,3 1,2 1,1
10.0 kGy
1,0 0,9
20.0 kGy 30.0 kGy 50.0 kGy 25
30
35
10
20 30 Dose (kGy)
40
50
¯ w of xanthan gum with Fig. 2. The change in weight average molecular weight M dose. and dose rates.
⎛ k ⎞ 1 1 − =⎜ ⎟D ¯ ¯ MnD Mn0 ⎝ mo ⎠
40
45
50
0.1 kGy/h
0.0 kGy
0
2.5 kGy 5.0 kGy
(2)
¯ n and M ¯ n0 (kg/mol) are where, the absorbed dose D is in J/kg and M the number average molecular weights of the polymer before and after irradiation. mo is the molecular weight (kg/mol) of monomer unit and k (Gy 1) is the degradation rate constant. Degradation rate constants for xanthan were determined by using the (1/ Mn ) (1/ Mn0 ) vs. dose plots. The determined degradation rate constants for xanthans are given in Table 1. It can be seen from Table 1 that, there is a slight decrease in G (S) and k values as the dose rate increased. This behaviour was attributed to the enhancing of oxidative degradations at lower dose rates since longer times are required to achieve the desired absorbed dose.
10.0 kGy
1,6 0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
1,4
Mn x 10
-6
20.0 kGy 30.0 kGy
1,2 1,0
50.0 kGy 0,8 25
30
35
40
45
50
ElutionVolume (ml) Fig. 1. Size exclusion chromatograms of irradiated xanthan gum (a) dose rate 7.0 kGy/h (b) 0.1 kGy/h.
0,6
0
10
20 30 Dose (kGy)
40
50
60
¯ n of xanthan gum with Fig. 3. The change in number average molecular weight M dose. and dose rates.
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
4
3
10
8 0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
(1/Mn-1/Mn0) x 10
7
6
2.5 kGy 5.0 kGy 10 kGy 20 kGy 30 kGy 50 kGy
2
10
1
η (Pa.s)
10
4
0
10
-1
10
2
-2
10
0
-3
0
10
20
30
40
50
10
Dose (kGy)
-1
10
0
10
1
2
10
3
10
10
-1
Shear rate (s )
¯ n0 vs. dose plot to determine degradation rate constants of ¯ n 1/ M Fig. 4. 1/ M xanthan gum. Dose rates are 0.1, 3.3 and 7.0 kGy/h. Table 1 G(S) and k values of xanthan gum irradiated at different dose rates. Dose rate
G(S) (lmol/J)
k (Gy 1)
0.1 kGy/h 3.3 kGy/h 7.0 kGy/h
0.01517 0.0015 0.01447 0.0020 0.0098 7 0.0010
1.4 10 8 7 1.4 10 9 1.3 10 8 7 2.0 10 9 8.7 10 9 7 1.0 10 9
Fig. 6. Effect of irradiation on the viscosity of xanthan gum at a dose rate of 3.3 kGy/h.
″ became larger than G′, due to the significant decrease in molecular weight with dose. Among various rheological models, Power Law Model (or the Ostwald–de Waele relationship) describes the data of shear-thinning and shear thickening fluids with the equation given below (Eq. (3)):
τ = Kγ n 3.4. Effect of gamma rays on the rheological properties of xanthan gum For the investigation of viscoelastic properties of irradiated xanthan gum, first, linear viscoelastic region was determined by strain-sweep tests for all dose rates and doses, at a fixed frequency of 1 rad/s and at a deformation range of γ ¼ 1 10 4%–10%, for 1% (w/v) xanthan solutions at 30 °C. Then the change in elastic (G′) and viscous (G″) modulus values depending on frequency were determined by frequency-sweep analysis at a frequency range of 0.01–100 Hz, for all dose rates and doses. The representative frequency vs. modulus curves are given 3.3 kGy/h dose rate in Fig. 5. It was observed that, G′ was larger than G″ for unirradiated xanthan solution, and as the absorbed dose increased, G′ and G″ values became closer up to 20 kGy and when the absorbed dose was 50 kGy, G″ became larger than G′. It was concluded that, up to 20 kGy, or approximately 1.0 106 g/mol molecular weight the xanthan retains the elastic behaviour After 20 kGy, especially at high frequencies, viscous behaviour became more dominant and G 3
10
2
G', G" (Pa)
10
1
10
0.0 kGy- G' 0.0 kGy- G'' 2.5 kGy- G' 2.5 kGy- G'' 20 kGy- G' 20 kGy- G'' 50 kGy- G' 50 kGy- G''
0
10
-1
10
-2
10
-1
10
0
10
1
10
Frequency (Hz)
(3)
where K is the flow consistency index (Pa s) (shear stress at a shear rate of 1.0 s 1), dimensionless exponent n is the flow behaviour index, reflects the closeness to Newtonian flow. If the fluid is a Newtonian fluid, n ¼1 and the consistency index (K) is identically equal to the viscosity of the fluid, η. When the magnitude of n o1, the fluid shows shear-thinning and when n 41, shear thickening behaviours (Rao, 2014). For the investigation of the shear thinning and/or shear thickening behaviour of xanthan gum and the effect of dose rate and dose on the rheological properties of its solutions, flow curves of 1% (w/v) xanthan gum solutions were constructed at 30 °C (Fig. 6) over a wide range of shear rates from 0.01 to 500 s 1 and rheological properties and flow behaviours were reported from the experimentally obtained data. A linear regression analysis was used to determine the material parameters of Power Law Model. Calculated model parameters for xanthan gum irradiated at a dose rate of 3.3 kGy/h were given in Table 2. Same behaviour was observed for the other dose rates. Fig. 6 shows that the viscosity of unirradiated and irradiated xanthan solutions decreases exponentially with shear rate which indicates that xanthan solution shows a non-Newtonian and shear-thinning behaviour. Irradiation did not change the nonNewtonian, shear-thinning character of xanthan solution. This behaviour confirmed by evaluating the material parameters of Power Law Model. As seen in Table 2, Power Law Model well describes the shear response of xanthan gum solution. Since n o1, Table 2 Variation of Power Law Model parameters of xanthan gum with irradiation dose (Dose rate of 3.3 kGy/h). Irradiation dose (kGy)
K (Pa.s)
n
r2
0.0 2.5 20 50
6.268 4.269 2.298 0.497
0.195 0.155 0.295 0.487
0.9973 0.9793 0.9979 0.9996
Fig. 5. Change in G′ and G″ with dose and frequency for xanthan gum.
Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i
M. Şen et al. / Radiation Physics and Chemistry ∎ (∎∎∎∎) ∎∎∎–∎∎∎
unirradiated and irradiated xanthan solutions shows a non-Newtonian and shear thinning behaviour. The consistency index (K) of xanthan solution decreases with dose due to the gradual decrease in molecular weight, whereas, the flow behaviour index (n) increases showing that xanthan solution gaining Newtonian flow behaviour as the molecular weight of xanthan gum decreases.
4. Conclusions The effect of ionizing radiation on xanthan gum in solid state, at different dose rates and doses have been investigated and the flow behaviour of xanthan solutions determined before and after the irradiation. It was determined that the main effect of γ radiation in air for xanthan gum is chain scission. Chain scission reactions become more effective when the dose rate decreased due to the enhanced oxidative degradations during irradiation. Dynamic rheological analysis of irradiated xanthan solutions showed that up to 20 kGy, elastic behaviour was more dominant than viscous behaviour. After 20 kGy, especially at high frequencies, viscous behaviour became more pronounced and G″ became larger than G ′, due to the significant decrease in molecular weight with dose. Rheological analysis also showed that xanthan solution (1.0%) showed non-Newtonian and shear-thinning behaviour and irradiation did not change the non-Newtonian, shear-thinning character of xanthan solution. Calculated model parameters of power law showed that flow consistency index (K) of xanthan solution decreases with dose due to the gradual decrease in molecular weight, whereas, n increases showing that xanthan solution gaining Newtonian flow behaviour as the molecular weight of xanthan gum decreases. At the end of this study, the effect of gamma irradiation on xanthan gum in solid form is clarified. For the understanding of radiation stability of xanthan gum irradiated in aqueous solutions as a function of dose and dose rates our work is still in progress.
Acknowledgements
5
the Scientific and Technological Research Council of Turkey (TUBITAK) research project 112T628, UEFISCDI research project 598/ 2013 and the support provided by the International Atomic Energy Agency through Research Contract no 14475/R2.
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Please cite this article as: Şen, M., et al., Radiation induced degradation of xanthan gum in the solid state. Radiat. Phys. Chem. (2015), http://dx.doi.org/10.1016/j.radphyschem.2015.10.005i