Study on the methods for reducing the acrylamide content in potato slices after microwaving and frying processes

Abstract

Acrylamide, a neurotoxic and potential cancer-causing agent, was found in a range of fried and baked starchy foods and has caused worldwide concern. Ever since, it has been an urgent agenda to find out effective ways to limit acrylamide formation during processing. The aim of this work is to examine the effect of immersion in different solutions on the acrylamide content in potato slices after microwaving and frying. Acrylamide levels were analyzed by the HPLC method, which was confirmed by HPLC-MS/MS. The results showed that immersing potato slices in water reduced the amount of acrylamide by 8–40% after microwaving and 19–75% after frying, respectively. For microwave processing, immersion in a NaCl solution at a concentration of 0.5 g L⁻¹ caused a considerable reduction of the acrylamide content by 96%, followed by a treatment with a CaCl₂ solution of 2 g L⁻¹ (80%) and a citric acid solution at a concentration of 1 g L⁻¹ (58%) . For the frying process, the most effective method for acrylamide reduction was the immersion in a citric acid solution at a concentration of 1 g L⁻¹ (77%), followed by a CaCl₂ solution at a concentration of 2 g L⁻¹ (72%) and a NaCl solution at a concentration of 0.5 g L⁻¹ (64%). The optimal soaking treatments could effectively reduce the acrylamide content while reasonably retaining the sensory attributes of the crisps.

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Introduction

On April 2002, Swedish researchers presented their preliminary findings on the contents of acrylamide in some fried and baked foods, most notably potato chips and French fries. As acrylamide had not been detected in unheated or boiled foods, it was considered that it was formed during heating at high temperatures.¹ Acrylamide is a chemical compound, which is widely used in the chemistry industry.² Acrylamide produces DNA adducts, gene mutations and chromosome abnormalities as confirmed by animal tests, and was regarded as probably carcinogenic to humans by the IARC.³ Recently, researchers have focused on the possible reduction of acrylamide formation in foods. Studies have shown that treatments like immersing the samples in citric acid, NaCl, CaCl₂ and other chemical solutions could effectively reduce the acrylamide content in food and the Maillard reaction model system.^4–11

Microwave processing is widely used in our daily life and offers a lot of convenience such as high energy efficiency and fast and precise processing.¹² Recent studies have indicated that microwave processing can likewise facilitate the Maillard reaction, resulting in food browning.^13–16 In our previous study, we found that microwaving could enhance the formation of acrylamide in potato slices,¹⁷ while El-Saied et al.¹⁸ reported that pre-frying treatments with a microwave resulted in a remarked reduction in the formation of acrylamide by 53.77–71.88%. The strategy to reduce the formation of acrylamide in foods – by using citric acid, NaCl and CaCl₂ solutions – has not been investigated by direct microwave processing. Therefore, the main purpose of this investigation was to determine the influence of such chemical treatments on acrylamide formation and to study ways of reducing the acrylamide formation in potato slices under microwaving conditions compared to frying conditions.

Materials and methods

Reagents

Acrylamide (2-propene amide) (purity >99.8%), glucose (purity >99.0%), fructose (purity >99.0%) and (¹³C₃)-acrylamide (99% isotopic purity) were purchased from Sigma-Aldrich (St. Louis, Mo., U.S.A.) and Cambridge Isotope Laboratories (Andover, MA, USA), respectively. Formic acid and n-hexane were obtained from Beijing Chemicals Co. (Beijing, China). HPLC methanol was purchased from Merck Ltd. (Beijing, China). HPLC-grade and 0.20 μm filtered water was prepared. Anhydrous magnesium sulfate, citric acid, calcium chloride and sodium chloride of analytical grade were obtained from Beijing Chemicals Co. (Beijing, China). Solid-phase extraction (SPE) cartridges Oasis MCX (3 ml, 60 mg) were supplied by Waters (Milford, MA, USA).

Stock solutions of 1 mg kg⁻¹ for acrylamide and (¹³C₃)-acrylamide were prepared in methanol. The stock solution was diluted with water to give a series of standard solutions (1.0, 2.0, 5.0, 8.0, 10.0, 15.0, 20.0 μg kg⁻¹). A 90 μg L⁻¹ of (¹³C₃)-acrylamide solution was used as the internal standard for quantification.

Preparation of potato slices and chemical pre-treatments

A batch of potatoes (variety Kexin, with 23% of dry solids) was purchased from a local market. Potato tubers were washed, peeled and the medulla was cut into slices of the same dimension (2.0 cm × 2.0 cm × 2.0 mm). The slices were rinsed immediately for 1 min in water to eliminate some starch adhered to the surface. Sixty raw potato slices were soaked in 1 L of the different solutions to investigate the influence of chemical pre-treatments on the acrylamide formation. The soaking solutions were water, citric acid with a concentration of 0.1, 0.3, 0.5, 0.7, and 1.0 g L⁻¹, calcium chloride with a concentration of 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, and 2.0 g L⁻¹, and sodium chloride with a concentration of 0.1, 0.3, 0.5, 0.7, 1.0, 1.5, 2.0, and 4.0 g L⁻¹, respectively. For the microwave treatments, 30 slices from each pre-treatment were put evenly into a round, flat dish, which was placed in the centre of the bottom of a microwave oven (Galanz WP800TL23-K3, Galanz Group Co., Ltd., Foshan, Guangzhou) and heated at a rotation speed of 30 rpm per min at 850 W for 3 min. For the frying treatment, 30 slices from each pre-treatment batch were placed in a basket, which was large enough to enable the free movement of the chips in the frying oil. The slices were fried at 180 ± 1 °C for 4 min in 5 L of fresh vegetable oil in an electrical fryer (HH-S, Ronghua Instrument Co., Ltd., Jintan, Zhejiang) in order to keep the temperature constant. The chips were then drained over a wire mesh for 5 min. Meanwhile, thirty slices of potato without any soaking treatment were fried at 180 °C for 4 min or heated in the microwave at 850 W for 3 min, respectively, as the control groups to determine the effects of the pre-treatments. Different microwaving and frying times were chosen in order to obtain potato chips with the same conditions. The details for the pre-treatments are shown in Table 1.

Table 1 Microwaving and frying treatments of potato slices

Soaking solutions	Concentration of soaking solutions (g L^−1)	Soaking time (min)	Microwaving treatment	Frying treatment
Water	—	5	850 W for 3 min	180 ± 1 °C for 4 min
		10
		15
		20
		25
		30
Citric acid	0.1	20	850 W for 3 min	180 ± 1 °C for 4 min
	0.3
	0.5
	0.7
	1.0
Calcium chloride	0.1	20	850 W for 3 min	180 ± 1 °C for 4 min
	0.3
	0.5
	0.7
	1.0
	1.5
	2.0
Sodium chloride	0.1	20	850 W for 3 min	180 ± 1 °C for 4 min
	0.3
	0.5
	0.7
	1.0
	1.5
	2.0
	4.0

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Acrylamide analysis by HPLC-MS/MS

The sample preparation was performed according to the method of Liu et al.¹⁹ The analysis of acrylamide in the potato slices was performed by an Alliance 2695 Separation Module (Waters, Milford, MA, USA) coupled to a Micromass Quattro Micro triple-quadrupole mass spectrometer (Micromass, Manchester, UK) with MassLynx software. The final tested solution (20.0 μl) was injected onto a reversed ODS-C₁₈ column (250 × 4.6 mm, 5 μm, Hypersil, Thermo, USA) maintained at 30 °C. The elution mode was isocratic using a mixture of 10% acetonitrile and 90% water containing 0.1% formic acid as mobile phase at a flow rate of 0.4 ml min⁻¹. Acrylamide was detected by MS/MS using electrospray ionization in the positive ion mode. Multiple reaction monitoring (MRM) of the degradation patterns m/z 72 → 55 for acrylamide and m/z 75 → 58 for [¹³C₃]- acrylamide was used for the quantification of acrylamide, respectively. The optimized MS instrument parameters obtained from the calibration were as follows: capillary voltage, 1 kV; cone voltage, 20 V; source temperature, 110 °C; desolvation temperature, 400 °C; desolvation gas flow, 600 per h; cone gas flow, 50 l h⁻¹; the argon collision gas pressure was 2 × 10⁻³ mbar for MS/MS and the collision energy for each transition was 13 eV in MRM mode. In the MRM transitions, the dwell and inter scan times were 0.4 and 0.1 s, respectively. Each determination was performed in triplicate.

Sensory analysis

Ten untrained volunteers were invited to taste and score the control and treated samples in terms of color, crispness and flavor. The sensory analysis was performed in a double blind manner to eliminate the effect of subjective prejudice and the score criteria for the potato slices was performed according to the method by Zhang et al.²⁰

Statistical analysis

Statistical analysis was performed by the Student's t-test with SPSS 15.0 software and Origin 7.0 software.

Results and discussion

In this study, effective ways of decreasing the acrylamide content in potato slices have been found. Crisps with low acrylamide content and good sensory quality can be obtained by immersion of the potato slices in different solutions prior to microwaving and frying. The immersion method does not greatly change the processing technology and it is practical in the food industry for reducing the acrylamide formation during the high-temperature processing. It is important to note that the present results provide guidance for the food industry to reduce the acrylamide content during food processing.

Effect of the immersion in water on the formation of acrylamide

Immersion in water is an effective way for the reduction of the acrylamide content in potato slices both by microwaving and frying (Fig. 1). With immersion times ranging from 5 min to 30 min, the acrylamide content dropped from 8 to 40% (12.42 ± 0.16 mg kg⁻¹ to 8.14 ± 0.14 mg kg⁻¹) after microwaving at 850 W for 3 min, and 19 to 75% (1.74 ± 0.05 mg kg⁻¹ to 0.54 ± 0.05 mg kg⁻¹) after frying at 180 °C for 4 min compared to the control groups, respectively. It is an effective way in reducing the amount of acrylamide for the frying process, while a high concentration of acrylamide could still be found after the microwaving process, which reached 8.14 ± 0.14 mg kg⁻¹ after 30 min of the immersing treatment. The reduction effect of the distilled water on the formation of acrylamide in potato slices may be due to the leaching of important acrylamide precursors, such as reducing sugars and asparagine. These results agree with the studies of Jung et al.⁴ and Pedreschi et al.,^21–23 in which potato strips immersed in distilled water for 1 h gave almost 25% reduction of acrylamide formation in French fries after frying at 190 °C. A much higher amount of acrylamide could be found after microwaving compared to after frying. This is related to the difference in the heating processes. In conventional heating, such as boiling or frying, heat transfer occurs mainly by conduction or irradiation; while in the microwave treatment, it is known that the molecular friction caused by the microwave energy produces heat, which can cause an increase of the sample temperature^11,24,25 and accelerate chemical reactions.²⁶ The present study suggests that the specific microwave effect could play a bigger role than conventional heating in the acceleration of the formation of acrylamide. The mechanism by which the formation of acrylamide was formed upon treatment with the microwave is currently under investigation. In our study, long immersing treatments, such as for 30 min, resulted in a deterioration of the taste and color of the potato slices compared to the control groups, both by microwaving and frying, which would influence the final quality of the potato slices. An effective way to reduce acrylamide formation in potato slices is by immersing the potato slices in water for only 20 min.

Fig. 1 Acrylamide content in potato slices immersed in water for different times.

Effect of the immersion in acid solutions on the formation of acrylamide

The effect of immersing potato slices in citric acid solutions with different concentrations on the formation of acrylamide after microwaving and frying was also studied (Fig. 2). For the microwaving process, the acrylamide content decreased with the increasing citric acid concentration. The acrylamide content in all the citric acid treated potato slices was significantly different from that of the control group (p < 0.05), except for the 0.1 g L⁻¹ and 0.3 g L⁻¹ citric acid solution treatments. The crispness and flavor of the potato slices showed no significant difference (p > 0.05, sensory evaluation, data not shown) with the increase in the citric acid concentration.

Fig. 2 Acrylamide content in potato slices immersed in citric acid solutions with different concentrations. The arrow indicates the limit beyond which the panel of tasters detected a loss of quality.

The acrylamide content in all the citric acid treated potato slices from the frying process was significantly different from that of the control group (p < 0.05). Potato slices immersed in 0.1 g L⁻¹ to 1.0 g L⁻¹ citric acid solutions presented a 47% (0.64 ± 0.07 mg kg⁻¹) to 77% (0.18 ± 0.04 mg kg⁻¹) inhibition of acrylamide formation after frying, respectively. In the sensory evaluation, no significant difference was found with respect to the control for the different citric acid solution concentrations (p > 0.05).

Comparing the two processing methods, immersion in citric acid solutions is an effective way to reduce the acrylamide content in potato slices, having a larger impact in the frying process. The results are in agreement with the study of Jung et al.,⁴ who found that soaking potato cuts in 1% and 2% citric acid solutions for 1 h before frying resulted in a 73.1% and 79.7% inhibition of the acrylamide formation in French fries. Mestdagh et al.²⁷ found that a synergistic acrylamide lowering effect was observed after adding citric acid to the potato powder model system. In the Maillard reaction, the reactivity of the sugars and the amino groups is highly influenced by the pH. The open chain form of the sugars and the unprotonated form of the amino groups, considered to be the reactive forms, are favored at a higher pH.²⁷ The effect of citric acid may be related to the decrease of the surface pH value thereby causing asparagine to be protonated and therefore diminishing its favoured reaction with the carbonyl moiety at the start of the reaction.²⁸ Jung et al.⁴ attributed the reduction of the acrylamide content in French fries by soaking the potato strips in citric acid solutions to both a pH decrease and the leaching out of free asparagine and reducing sugars from the surface layer of the potato cuts into the solution. No significant difference was found from the sensory evaluation of the potato slices both by microwaving and frying with respect to the control group. Immersing the potato slices into a 1 g L⁻¹ citric acid solution is an effective way to reduce the acrylamide content for both the microwave and frying processes.

Effect of the immersion in calcium chloride and sodium chloride solutions on the formation of acrylamide

Fig. 3 shows the results of immersing potato slices in CaCl₂ solutions with different concentrations after microwaving and frying. For the microwave processing, immersed potato slices in CaCl₂ solutions for 20 min showed a reduction in acrylamide formation of 2% (8.29 ± 0.17 mg kg⁻¹) to 80% (1.71 ± 0.02 mg kg⁻¹) at a concentration range from 0.1 g L⁻¹ to 2.0 g L⁻¹. A significant reduction in the acrylamide formation was found after immersing potato slices in calcium chloride solutions compared to the control group (p < 0.05). For the frying process, potato slices immersed in 0.1 g L⁻¹ to 2.0 g L⁻¹ calcium chloride solutions induced a 43% (0.46 ± 0.08 mg kg⁻¹) to 72% (0.22 ± 0.01 mg kg⁻¹) inhibition of the acrylamide formation, respectively. After immersion in a 2.0 g L⁻¹ calcium chloride solution for 20 min, 1.71 ± 0.02 mg kg⁻¹ and 0.22 ± 0.01 mg kg⁻¹ of acrylamide could be detected from the microwaving and frying processes, respectively. Ou et al.²⁹ found that the immersion of potato slices in a 5 g L⁻¹ CaCl₂ solution reduced the acrylamide formation by more than 85% in fried crisps. El-Saied et al.¹⁸ also found that soaking potato strips in a CaCl₂ solution caused a highly considerable reduction of the acrylamide formation by 86% to 92%. The mechanism for the influence of the CaCl₂ solutions on the acrylamide formation may be due to its complexation with amines and some intermediates of the Maillard reaction products, especially the Schiff bases which are the key intermediate leading to acrylamide, and the change in the reaction path towards the dehydration of glucose leading to hydroxymethylfurfural and furfural.^6,8,30–32 Casado et al.⁹ found that CaCl₂ had no effect in reducing acrylamide levels in olive juice. The influence of CaCl₂ on the acrylamide reduction may be dependent on the character and variety of the food products.

Fig. 3 Acrylamide content in potato slices immersed in calcium chloride solutions with different concentrations. The arrow indicates the limit beyond which the panel of tasters detected a loss of quality.

Potato slices were also immersed in NaCl solutions with different concentrations to show the effect of NaCl on the acrylamide formation after microwaving and frying (Fig. 4). The results showed that significant differences were found in the reduction of acrylamide formation with the increase of NaCl concentration compared to the control group, both by microwaving and frying. For microwave processing, the potato slices immersed in NaCl solutions for 20 min showed a reduction in the acrylamide formation of 32% (5.79 ± 0.11 mg kg⁻¹) to 97%(0.26 ± 0.01 mg kg⁻¹) at a concentration range from 0.1 g L⁻¹ to 4 g L⁻¹. In the frying process, the potato slices immersed in NaCl solutions for 20 min showed a reduction in the acrylamide formation of 51% (0.39 ± 0.05 mg kg⁻¹) to 83% (0.14 ± 0.01 mg kg⁻¹) at a concentration range from 0.1 g L⁻¹ to 4 g L⁻¹. Pedreschi et al.²² also found that soaking potato slices in a NaCl solution before frying reduced dramatically the acrylamide formation in potato chips by ∼90% (average value) in comparison to the control chips. In the present study, and especially after immersing the potato slices in the 0.5 g L⁻¹ NaCl solution, the reduction rate of acrylamide formation reached 96% (0.34 ± 0.02 mg kg⁻¹) after microwave processing and 66% (0.27 ± 0.02 mg kg⁻¹) after frying, respectively. No significant differences were found after increasing the NaCl solution concentration further than 0.5 g L⁻¹. In the sensory evaluation, the potato slices soaked in NaCl solutions with concentrations higher than 1 g L⁻¹ after microwaving and frying were much darker, sour and with an acerbic taste. The treatments greatly influenced the appearance as well as the taste and flavor of the potato slices, which were thus not acceptable. These results are consistent with the studies of Kita et al.³³ and Moreau et al.³⁴

Fig. 4 Acrylamide content in potato slices immersed in sodium chloride solutions with different concentrations. The arrow indicates the limit beyond which the panel of tasters detected a loss of quality.

The presence of mono or divalent cations in the potato slices after the soaking treatments, rather than reducing the acrylamide precursors, plays an important role in the reduction of the formation of acrylamide.⁷ Many researchers have reported that, although the monovalent cations prevent the formation of acrylamide to a certain extent, they are less efficient than divalent cations. However, the current results in our study reveal that soaking pre-treatments in NaCl solutions are more effective than those in CaCl₂ solutions for the microwave processing. The mechanism for the different effect of CaCl₂ and NaCl solutions on reducing the acrylamide content may be due to microwave effects such as the effect of the electromagnetic field directly acting on the polar molecules of the sample, causing the rotation or the liberation of chemical bonds. The mechanism for the acrylamide reduction in potato slices and other fried or baked products by microwaving must be investigated further.

Great interest and rapid research efforts on the study of acrylamide in foods have followed after the announcement in April 2002 by the Swedish National Food Authority and the University of Stockholm. It is an urgent research agenda to find out effective ways to limit acrylamide formation in foods. The reduction of acrylamide in high-temperature processed foods, including selection of the raw material and variation of the processing parameters, has been extensively reported. In this report, the effect of immersing potato slices in different solutions on the formation of acrylamide was investigated. The results obtained in these experiments showed that the immersion of potato slices in different solutions is a potential way to reduce the formation of acrylamide in high-temperature processed foods. Thus, this simple, easy approach may be particularly appealing to those people who are unwilling to change their eating habits despite of the known cancer risk factor of acrylamide, and would also probably have a positive impact on public health.

Conclusion

In our present study, the immersing treatments had a great influence on acrylamide formation. Our findings show that immersing the potato slices in water for 20 min, in a citric acid solution of concentration 1 g L⁻¹ for 20 min, in a CaCl₂ solution of concentration 2 g L⁻¹ for 20 min or in a NaCl solution of concentration 0.5 g L⁻¹ for 20 min can effectively reduce the amount of acrylamide in the potato slices. The optimal immersing treatments can effectively achieve the reduction of acrylamide while reasonably retaining the sensory attributes of the crisps.

Acknowledgements

This work was supported by the Fund of the “National Natural Science Foundation (31000750)”, the China Postdoctoral Science Foundation Special Funded Project (201104527), and the Fund for Distinguished Young Scholars of the Heping Campus of Jilin University (4305050102Q9). Accordingly, the authors gratefully acknowledge the funding supports.

References

1. SNFA, Uppsala, Sweden, Swedish Natl. Food Admin., 2002, http://www.slv.se.
2. L. Z. Xu, Z. J. Xu, G. S. Zhang, K. Zhou and Z. W. Zhai, Chem. Res. Chin. Univ., 2008, 24, 575.
3. IARC, International Agency for Research on Cancer (IARC), 1994, Lyon, France.
4. M. Y. Jung, D. S. Choi and J. W. Ju, J. Food Sci., 2003, 68, 1287.
5. E. Kolek, P. Simko and P. Simon, Eur. Food Res. Technol., 2006, 224, 283.
6. V. Gokmen and H. Z. Senyuva, Eur. Food Res. Technol., 2007, 225, 815.
7. V. Gokmen and H. Z. Senyuva, Food Chem., 2007, 103, 196.
8. F. J. Hidalgo, R. M. Delgado and R. Zamora, Food Chem., 2010, 122, 596.
9. F. J. Casado, A. H. Sanchez and A. Montano, Food Chem., 2010, 119, 161.
10. J. Bassama, P. Prat, P. Bohuon, R. Boulanger and Z. Gunata, Food Chem., 2010, 123, 558.
11. X. Zeng, K. W. Cheng, Y. Du, R. Kong and C. Lo, Food Chem., 2010, 121, 424.
12. G. Lewandowicz, J. Fornal and A. Walkowski, Carbohydr. Polym., 1997, 34, 213.
13. M. Maskan, J. Food Eng., 2001, 48, 169.
14. M. E. C. Oliveira and A. S. Franca, J. Food Eng., 2002, 53, 347.
15. S. Tuta, T. K. Palazoğlu and V. Gökmen, J. Food Eng., 2010, 97, 261.
16. B. Ünal, T. K. Palazoğlu, V. Gökmen, H. Z. Şenyuva and H. İ. Ekiz, J. Sci. Food Agric., 2007, 87, 133.
17. Y. Yuan, F. Chen, G. H. Zhao, J. Liu, H. X. Zhang and X. S. Hu, J. Food Sci., 2007, 72, C212.
18. M. H. El-Saied, A. M. Sharaf, M. M. Abul-Fadl and N. El-Badry, World J Dairy Food Sci., 2008, 3, 17.
19. J. Liu, G. H. Zhao, Y. Yuan, F. Chen and X. S. Hu, Food Chem., 2008, 108, 760.
20. Y. Zhang, J. Chen, X. L. Zhang, X. Q. Wu and Y. Zhang, J. Agric. Food Chem., 2007, 55, 523.
21. F. Pedreschi, K. Kaack and K. Granby, LWT–Food Sci. Technol., 2004, 37, 679.
22. F. Pedreschi, K. Granby and J. Risum, Food Bioprocess Technol., 2010, 3, 917.
23. F. Pedreschi, O. Bustos, D. Mery, P. Moyano, K. Kaack and K. Granby, J. Food Eng., 2007, 79, 989.
24. M. T. O. Villanueva, A. D. Marquina, E. F. Vargas and G. B. Abellan, Food Chem., 2000, 71, 417.
25. L. A. Campanone and N. E. Zaritzky, J. Food Eng., 2005, 69, 359.
26. L. Perreux and A. Loupy, Tetrahedron, 2001, 57, 9199.
27. F. Mestdagh, J. Maertens, T. Cucu, K. Delporte, C. Van Peteghem and B. De Meulenaer, Food Chem., 2008, 107, 26.
28. S. I. F. S. Martins, W. M. F. Jongen and M. A. J. S. van Boekel, Trends Food Sci. Technol., 2000, 11, 364.
29. S. Ou, Q. S. Lin, Y. Zhang, C. Huang, X. Sun and L. Fu, Innovative Food Sci. Emerging Technol., 2008, 9, 116.
30. R. H. Stadler, F. Robert, S. Riediker, N. Varga and T. Davidek, J. Agric. Food Chem., 2004, 52, 5550.
31. V. A. Yaylayan, C. P. Locas, A. O. Wnorowski and J. Brien, J. Agric. Food Chem., 2004, 52, 5559.
32. Ö. Ç. Açar, M. Pollio, R. Di Monaco, V. Fogliano and V. Gökmen, Food Bioprocess Technol., 2010, 5, 519.
33. A. Kita, E. Brathen, S. H. Knutsen and T. Wicklund, J. Agric. Food Chem., 2004, 52, 7011.
34. L. Moreau, J. Lagrange, W. Bindzus and S. Hill, Int. J. Food Sci. Technol., 2009, 44, 2407.

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