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Rheological behavior of borate complex and polysaccharides
Materials Science and Engineering C 29 (2009) 607–612
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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e c
Rheological behavior of borate complex and polysaccharides M.R. Gouvêa, C. Ribeiro, C.F. de Souza, I. Marvila-Oliveira, N. Lucyszyn, M.-R. Sierakowski ⁎ Laboratório de Biopolímeros, Departamento de Química, Universidade Federal do Paraná, P.O. Box 19081, 81531-990 Curitiba, Paraná, Brazil
a r t i c l e
i n f o
Article history: Received 31 May 2008 Accepted 8 October 2008 Available online 1 November 2008 Keywords: Starch Xyloglucan Rheology Borate ions Viscoelasticity Gel
a b s t r a c t In this work the rheological behavior of manioc starch (S) industrially modified and its blends with a xyloglucan (XG) in the presence of tetraborate (T) ions at pH 12 was described. At rotational measurements the viscosity values showed a good interaction between polysaccharides (20/5 g/l, respectively, to S and XG), which were highly modified by the presence of tetraborate (7 g/l) resulting in better pseudo or plasticity. To system S/XG/T at 20/5/7 or 40/10/14 g/l the rheological properties were dependent of polysaccharides/T concentration. Mixtures at 25/7 g/l performed a viscoelastic solution, and at 50/14 g/l a weak gel. After the temperature sweep analyses (heating and cooling), a more solid character was obtained. This performance could be explained as a result of total gelatinization process that benefits the structural reorganization and better interaction between polysaccharides and the tetraborate complex formed with the hydrocolloid. Also, it was observed that the S/XG/T system, after heating/cooling together with a shear stress, adopted a helical conformation similar to that obtained with amylose standard, since it was colored in blue with lugol. So, the interactions are related with the conformational change of S and XG and also with shear processes, which aid the reptation phenomena and improvement of the solid character. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The polysaccharides are utilized in several industries as functional ingredients for controlling stability, structure, and texture of products. Xyloglucan is a water-soluble neutral polysaccharide, found in the primary cell walls of non-graminaceous (monocotyledons) and in the cotyledon of some dicotyledonous seeds, where it has functioned as a storage [1]. It is the commercial form extracted from Tamarindus indica (tamarindo) seeds which is used in cosmetic, biomedical and food applications [2–4]. One of the properties of xyloglucan is its high viscosity in aqueous solutions. This biopolymer shows a main chain with (1→4)-linked β-D-glucan, and in the side chains xylose units are linked to the glucose units in the C-6 position. Some of the xylose units are also substituted at C-2 by β-D-galactose units [5]. In the Federal University of Paraná (UFPR) considerable attention has been paid to the xyloglucan extracted from H. courbaril (jatobá) seeds obtained at different Brazilian locations, whose partial fine chemical structure and properties have been determined [6–11]. Other polysaccharide widely used is the starch, that is composed of anhydrous glucose units linked primarily through α-D-(1→4) glycosidic bonds. While the detailed fine structure has not been fully elucidated, it has been firmly established that starch is a heterogeneous material consisting of varying proportions of amylose and amylopectin ⁎ Corresponding author. Present address: Laboratório de Biopolímeros (BIOPOL), Departamento de Química, Universidade Federal do Paraná, C.P. 19081, 81531-990 Curitiba, PR, Brazil. Tel.: +55 41 33613260; fax: +55 41 33613186. E-mail address: [email protected] (M.-R. Sierakowski). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.10.019
[12,13]. Some properties of starch can be modified by the presence of hydrocolloids, to improve their rheological characters, since that mixture of polysaccharides in solution is different from pure systems [13,14]. The synergic interactions between starch and hydrocolloids are of great interest commercially, since they resulted in systems with different functionalities, new rheological characteristics or better textures using minor quantity of polysaccharides, properties which can possibly be used for other applications also [11,15]. Regarding the industrial potential in the processing of polysaccharides, the knowledge of the rheological behavior of starches and other biopolymers is very useful in quality/process control and equipment selection. It is known that at alkaline pH, polysaccharides complex with borate ions forming some heterocyclic boron compounds [16–20]. In the literature it was related that the presence of borate ions in xyloglucan (tamarindo seeds) made up viscous and elastic solutions [21]. This effect was demonstrated previously by Ghose and Krisna [22]. The studies involving guar gum and hydroxypropyl guar (HPG) in high concentrations and pH variation related that the behavior of relaxation of a gel depends on the nature of links due to borate ions [23]. Other studies rheologically involving a complex of guar or hydroxypropyl guar with borate ions, showed that the chemical balance involving boric acid, borate ions free or associated with cis-diol sites of the polysaccharides chain, determines the number of links, and this balance is in function of temperature and pH [24]. Power et al. [25] studied the rheological behavior of the complex of HPG with borate ions, and the authors showed that despite the properties of guar and HPG being studied extensively by groups of rheology and industries of petroleum, a complete understanding of the behavior of these fluids
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Table 1 Composition and identification of the systems System 25S 25S/7T 5XG 5XG/7T 20S/5XG 20S/5XG/7T 50S/14T 40S/10XG/14T
Starch (g/l) 25 25 – 20 20 50 40
Xyloglucan (g/l) – – 5 5 5 5 – 10
Sodium borohydride (g/l) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Sodium tetraborate (g/l) – 7 – 7 – 7 14 14
and gels still is not clear. Rheological studies in the interaction of xyloglucan (XG) from H. courbaril seeds and borate ions were performed by Martin et al. [10], these authors observed that better viscoelasticity was related with greater Mw of XG. In the 11B NMR spectrum, it was found that the interaction of borate ions with xyloglucan was through galactose units. Recently, the critical strain for the gel formulated with konjac glucomannan was found to be independent of borax concentration, while the yield stress firstly increased with increasing borax concentration and then decreased [26]. Considering the rheological properties, the aim of this work was to characterize the behavior of systems of manioc starch, xyloglucan, and its blends in the presence of borate ions by dynamic viscosity curves, to obtain more plastic properties.
Table 2 Rheological parameters of the systems at 25 or 5 g/l evaluated by the Power law, Bingham and Herschel–Buckley models Sample a
25S 25S/7Ta 5XGa 5XG/7Ta 20S/5XGa 25Sb 25S/7Tb 5XGb 5XG/7Tb 20S/5XG/7Tb 20S/5XG/7Tc a b c
R2
χ2
τo (Pa)
K (Pa sn)
n
η (Pa s)
0.9979 0.9749 0.9998 0.9998 0.9870 0.9972 0.9871 0.9992 0.9998 0.9596 0.9993
0.0484 0.2873 0.0002 0.0002 0.1936 0.0127 0.0029 0.0004 0.0001 0.0702 0.1599
– – – – – 10.4894 10.7266 0.8266 0.7863 20.5533 27.7621
1.4384 2.0485 0.0615 0.0663 2.5776 – – – – – 23.1685
0.5359 0.3915 0.8562 0.8253 0.5496 – – – –
– – – – – 0.0642 0.0139 0.0230 0.0795 0.1184 –
0.5372
n
Power law (τ = Kγ ). Bingham (τ = τo + Kγn). Herschel–Buckley (τ − τo = ηγ).
(40%, w/v). Sodium tetraborate (T) was added to obtain the S/T, XG/T or S/XG/T system. All the systems formulated are present in Table 1. 2.5. Rheological properties
2. Materials and methods
Rotational experiments to the upward and downward curve had a duration of 2 min each one with shear rate ranging from 0 to 200 s− 1. The experimental data were mathematically evaluated and fitted according to the Herschel–Buckley model (Eq. (1)), Power law (Eq. (2)) or Bingham (Eq. (3)).
2.1. Materials
τ = τ o + Kγ n
The xyloglucan (XG) was obtained by exhaustive aqueous extraction from pooled and milled Hymenaea courbaril seeds acquired from EMBRAPA/Natal/RN. The viscous extracts were purified by centrifugation. The polymer was obtained after precipitation with two volumes of 96% ethanol and washed with acetone [15]. Starch (S) from Manihot utilissima industrially modified (cross bounded) by pre-gelatinization was supplied by Corn Products Brasil Ingredientes Industriais Ltda, Balsa Nova, State of Paraná, Brazil, in which amylose content was related as 18%. 2.2. Swelling and solubility of starch The swelling assay was made by the method of Leach et al. [27], using a 0.5% (w/v) solution. So, starting at 50 g/l, the solution was diluted with purified water to the correct concentration. Then it was centrifuged at 700 g for no more than 15 min. The volume of supernatant indicated the volume of water non-linked in the solution.
ð1Þ
where, τ is the shear stress (Pa), τo is the yield stress (Pa), K is the consistency coefficient (Pa sn), g is the shear rate (s− 1) and n is the flow behavior index of the fluid (dimensionless). τ = Kγn
ð2Þ
where, τ is the shear stress (Pa), K is the consistency coefficient (Pa sn), g is the shear rate (s− 1) and n is the flow behavior index of the fluid (dimensionless). τ−τ o = ηγ
ð3Þ
where, τ is the shear stress (Pa), τo is the yield stress (Pa), η is the Bingham plastic viscosity (Pa s) and g is the shear rate (s− 1).
2.3. Determination of XG critical concentration Solutions of starch (5 g/l) in water and XG (3 g/l) in 0.1 M sodium nitrite were prepared and diluted in water, which were utilized to determine, initially, the intrinsic viscosity [η], evaluated by extrapolation of reduced viscosity to the limit of zero concentration, where the linear coefficient is the [η]. Then, by slope of log specific viscosity versus log (c × [η]), the values of critical concentration were determined. 2.4. Preparation of the isolated polysaccharide solutions, blends and complexes with borate ions The starch (S) was solubilized in distilled water at 25 °C for 30 min and the xyloglucan (XG) in distilled water at 25 °C for 19 h. The S/XG blends, for example at 25 g/l, were obtained by adding the S solution (200 mg/5 ml) into the XG solution (50 mg/5 ml). The mixture was stirred for 20 min and sodium borohydrate was added, and then the pH of all solutions was adjusted for 12 with sodium hydroxide solution
Fig. 1. Frequency (f) dependence of systems 25S/7T (modulus (G′) — ●, (G″) — ○); and 20S/5XG/7T (modulus (G′) — ■, (G″) — □), in Haake RS 600 rheometer, spindle PP 35 mm, 1 Hz, at 25 °C.
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the dependence of log specific viscosity in function of log (concentration×[η]). To the XG sample the critical concentration (c⁎), limit of the ticket of the regimen diluted for the half-diluted one, found at 25 °C was 2.2 g/l and to XG was 1.2 g/l. These values indicate that above of these concentrations the samples will be more aggregated, therefore, its components have raised molar mass. 3.2. Swelling and solubility of the starch The value of swelling and solubility of the pre-gelatinized manioc starch was determined as 80%, showing a good correlation between the industrial process of modification (pre-gelatinization), which resulted in a good hydration of the sample. 3.3. Rheological properties: flow behavior of systems
Fig. 2. Temperature dependence of systems 25S/7T (modulus (G′) — ●; (G″) — ○) and 20S/5XG/7T (modulus (G′) — ■; (G″) — □) in Haake RS 600 rheometer, spindle C60/2° and PP 35 mm, at 1 Hz. (Y) shows the initial heating.
The dates were analyzed using the software Origin 7.0 (Origin Lab Corporation, MA, USA), in order to obtain the rheological (τo, n, K and η) and statistical parameters (R2 and χ2). The values of R2 and χ2 were obtained to evaluate the goodness of fit to the experimental results in the model applied. Non-oscillatory and oscillatory analyses were performed using Haake rheometers model Rheostress 600 or RS1 with the spindle C60/2° or PP20. A Haake water bath (DC 30) and a thermostatic UTC (universal thermostatic control) were used to control constant temperature at 25 °C or to temperature ramp (heating from 20 to 70 and cooling gradient from 70 to 20 °C), at a rate of 1.8 °C/min. Oil was applied to the exposed surfaces of the sample to prevent evaporation for experiments with gradients of temperature. The rheometer was interfaced to a microcomputer for control and data acquisition. Experiments were done on triplicate. The oscillatory measurements, in the viscoelastic region, were carried out in the frequency of 0.01–5 Hz, deformation of 2%, to all systems. 3. Results and discussion 3.1. Measurements of critical concentration of starch and xyloglucan samples With the intention to evaluate the concentrations where intermolecular interactions can be led, a convenient experiment is to examine
The starch systems at 25 g/l containing borate ions or not showed a pseudoplastic behavior. System 5XG/7T presented also a pseudoplastic behavior and a rupture pseudo-point [28] showing the formation of aggregates at low shear rates (b25 s− 1), which is a typical behavior of suspension (data not showed). The profile of the curves of viscosities of system 25S/7T showed a thixotropic behavior that indicated a reduction in the organization of the molecules in the system. On the other hand, system S/XG/T presented a rheopectic effect. In accordance with Barnes et al. [29], the rheopectic character could be described as a rearrangement of particles, producing one better organization of the system and the consequent increase of viscosity (data not shown). The better agreement of the rheological behavior of the generated systems was obtained when the mathematical models were applied (see Table 2) where the values of R2 were higher than 0.96 and the chisquares (χ2) were near zero that indicated a good adjust. For the systems 25S, 25S/7T and 5XG/7T two intervals of shear rate had been considered for the calculations, the Power law model between 5 and 100 s− 1, and the Bingham model for shear rate N100 s− 1. These treatments were effected since it could not apply the model of the Power law for shear rates b5 s− 1 or higher shear rates N100 s− 1 [30]. For the system 20S/5XG/7T the Power law model did not fit the data satisfactorily, and so the Herschel–Buckley model was used. The values of the index behavior (n) lesser than 1, prove that all systems have a pseudoplastic behavior. System 25S showed a low index of consistency (K) in relation to the 25S/7T and a lower pseudoplasticity. So, the presence of the salt modifies the flow behavior in the 25S/7T system, producing a material with better pseudoplasticity and higher consistency. These characteristics can be attributed to
Fig. 3. Frequency dependency of systems 50S/14T versus modulus (G′) — ●, (G″) — ○ (A) and tan δ— ×; η⁎ — ♦ (B) values, and 40S/10XG/14T versus modulus (G′) — ■; (G″) — □ (A), and tan δ— ◊; η⁎ — ★ (B) in Haake RS 1 rheometer, spindle PP 35 mm, deformation of 3%, at 25 °C.
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Fig. 4. Temperature dependence of: (A) systems 50S (modules (G′) — ▲, (G″) — Δ) and 50S/14T (modulus (G′) — ●, (G″— ○); (B) 50S/14T (tan δ— ×) and 40S/10XG/14T (tan δ— ♦) in Haake RS 1 rheometer, spindle C60/2° and PP 35 mm, respectively, at 20/70/20 °C, 0.02 Hz. (→) shows the initial heating program.
the removal of some water molecules by salt introduction, which promoted better interaction between polysaccharide chains. The system 20S/5XG/7T resulted in a material with a higher τo (27.7621 Pa), and greater K (23.1685 Pa s) in relation to the other systems. In this case, the cross linking that is present could be favored for the presence of the borate anions that complex with alcooxi groups in conditions of pH alkaline. Furthermore, the presence of the XG that promotes the complexation process (data of RMN spectra not shown), contributes to the increase of the values of τo and K parameters; also a superior value of plasticity for this system was observed. So, the presence of hydrocolloid and borate ions in mixture with starch resulted in a more plastic system. The measure of the interactions between systems S and XG, both in the presence of borate ions was evaluated by the following equations [31]: Interaction Index ðI1 Þ = η of
20S=5XG 20S + 5XG
ð4Þ
Interaction Index ðI2 Þ = η of
20S=5XG=7T 20S=7T + 5XG=7T
ð5Þ
The interaction index 0.91 by Eq. (4) indicated that the system lacks practical interest in terms of viscous synergism, when only starch and xyloglucan were mixed. But, in the presence of borate ions (Eq. (5)) the interaction index value is superior to 1 (1.27) that indicated a good synergism, because the viscosity of the mixture is greater than the sum of the viscosities of the isolated systems. 3.4. Viscoelastic properties of starch, starch/borate and starch/ xyloglucan/borate systems The performance of the storage and loss modules (G′ and G″, respectively) in function of the frequency (f) was made in the region of linear viscoelasticity (data not shown), to evaluate if the systems were solutions or gels. In Fig. 1 it is observed that for the 20S/5XG/7T system, G′ and G″ modules are higher than for 25S/7T, over a frequency range of 0.01– 7 Hz. However, although the storage modules are higher than the loss modules for both systems, the difference is not pronounced, indicating the presence of viscoelastic solution or weak gel formation. It is also showed that, in low frequency (b0.1 Hz), the solid character prevails for 25S/7T system, as expected by the presence of aggregates. However, in the higher frequencies, the solid character is more pronounced for 20S/5XG/7T sample, as a result of entanglement of the chains. It was calculated that the values of η⁎ for the mixture 20S/5XG/
7T were 9–17 fold greater than for 25S/7T, practically under all the frequencies studied, and it was observed that the viscosity decayed linearly (data not shown). The dependence of the modulus values G′ and G″ in function of the frequency sweep indicated that S/XG/T is a more viscoelastic solution, in comparison with the S/T system. A similar experiment depicted in Fig. 1 was carried out with 20S/ 5XG system, to confirm the influence of borate ions in the viscoelastic character (date not shown). In these analyses, both the modules showed total dependence of the frequency and the tan δ (G″/G′) values near 1 (indicated a liquid character) confirm the relevance of borate ions on the interaction process. The stability of systems 25S/7T and 20S/5XG/7T during heating and cooling was valuable by temperature sweep. The comparative analysis of the behavior between the systems showed the formation of a material with a higher solid character in the presence of XG once the G′ is higher than G″ for all temperatures. In the cooling the values of the modules were higher than in the heating process (Fig. 2). On this form, new interactions were obtained after heating (up 50 °C); and this performance resulted from the presence of borate ions, since, without these ions similar behaviors are not observed (data not shown). This comportment can be attributed to better solubilization of borate ions at ~60 °C and also changes in the starch conformation during heating; and both factors are favorable to better interaction and complexation. Since 5XG/7T did not form a viscoelastic solution it was not possible to observe the change on the hydrocolloid structure. To evaluate the effect of components (borate ions and polysaccharides) in the viscoelastic properties of the systems some assays were made with a variation of concentrations. For this, blends were formulated at concentrations of starch, xyloglucan and borate ions of 40, 10 and 14 g/l, respectively. According to the profile in Fig. 3A it is shown that there exists a lesser dependence of G′ and G″ in the frequency range for the system 40S/10XG/14 T. Furthermore, in this case the values of complex dynamic viscosity (η⁎) were significantly bigger than 50S/14T (Fig. 3B).
Table 3 Values of dynamic viscosity (η⁎) in the temperature range for different systems
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The lesser value of tan δ in the presence of xyloglucan indicates predominantly an elastic behavior, and the comparison of the values between the samples reflected differences in the organization of the structures (Fig. 3B), that can be reflected in the formation of a more rigid net [32,33]. In the literature the values of tan δ of 0.02 to 0.2 can classify the rheologic systems as an intermediate state between a weak gel and an elastic gel [34,35]. For mixture 40S/10XG/14T to obtain this scale the formation of the net is dependent of the high frequency, so this system showed a comportment of a gel, which presented a G′ four-fold higher than the G″. Data for G′ and G″ plotted as a function of frequency (Fig. 4A) can provide information on gel structure. The degree of frequency dependence can be determined by the well-known Power law equation [36]: GV= AωB where, G′ is the storage modulus, ω is the oscillation frequency (rad/s), and A is a constant. The exponent B is the slope in a log–log plot of G′ versus ω. Applying the Power law model, for systems 50S/14T and 40S/10XG/ 14T the A values were 47.51 and 146.54 Pa s, respectively and B were 0.4187 and 0.2061, respectively. By the B values it is possible to indicate that in the presence of xyloglucan more linkings were formed in such a way by physical as well as by chemical interaction. With this behavior, the XG/T functions as a good interlinking, especially during reptation process [37,38], which is described as the time for a given macromolecules to disengage by a snakelike motion from a tube made up of neighboring chains. The stability of system 50S/14T was analyzed by the variation of the temperature. During heating, the starch swell followed causing an increase in G′ to maximum value, which can be attributed to the formation of a network of swollen starch [39]. More heat and greater mobility or entropy might loosen the previous structure formed by breaking some hydrogen bonds or other possible interactions and rupturing the structure of swollen starch [40]. Consequently, the strength of network-like structure weakened and the modulus dropped [41]. After heating the swelling is completed and this is governed by the viscoelasticity of the particles (amylose and amylopectin) that act together as relatively rigid entities [42]. The rheogram depicted in Fig. 4A shows that in the presence of borate ions, the modulus values increase in relation to the starch system, and it was also observed that in the 50S/14T the gel melted as showed by the lowering of G′ and G″ values after heating, which is not observed for 50S that results in the system with a higher G′ indicating a change in the conformation. It was possible to see the very significant increase in the modulus after the heating of systems in the presence of xyloglucan (data not shown). For example at 30 °C, the modulus values were sixty two-fold higher with 14 g/l of tetraborate (Table 3), than the system containing half the concentration of components (Fig. 2). In the system S/XG/T it was observed that the arrangement of the phases contributed to the formation of the net. The values of G′ and G″ diminish with the increase of the temperature from 20 to 70 °C, a similarity with the behavior for other polymers [43], and increase when the temperature diminishes from 70 to 20 °C. This indicates that the net of the gel was strengthened during the heating and after that strong cross linkings are kept and weak linkings are converted into strong linkings during the cooling (see data of Table 3). This performance suggests that the main force that stabilizes the net is the hydrogen linking [44] and also the ionic ones. Raising the temperature increased the fluidity of the complex gel, indicating the dependence of cross-link density on temperature. Previous studies [45,46] have shown that complex formation in reactions between polymer and borate ions is exothermic and the complexation equilibrium constant decreases with increasing temperature.
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The lower tan δ values after heating (Fig. 4B) demonstrate the increase of the gel force in the S/SG/T system. So, these results also indicated the thermally reversible nature of S/T and S/XG/T gels. With the results obtained the whole chemical equilibrium involving dissociation of borax in water, formation of borate ions, and complexation between cis-diol sites of galactose units was present in the XG chains, and borate ions determine the number of cross linked chains, like those observed for PVA/borax [47], guar/borax [48] and konjac glucomannan [26]. To understand the mechanism of interaction between S/XG/T interactions, the systems composed of S, S/T, XG, XG/T, S/XG, S/XG/T, standards of amylose/T and of amylopectin/T, heated or not and also submitted or not to a shear stress were analyzed in the presence of lugol solutions. The results (data not showed) revealed that XG adopted a helical conformation, since a deep blue color was produced, similar to that of amylose, but only after the systems containing XG were submitted to shear together with heating. So, the process to produce a more solid system involves the unraveling of the starch/XG helix and a higher increase in the volume of the polysaccharides, and the water is imbibed and bound to the unraveled polymers by hydrogen bonding. By this comportment, it is supposed that the borate ions are arranged in the groove of the helices. Based on the importance of borate ions on cement the application of these composites on cement pastes is in progress. 4. Conclusion At higher oscillation frequencies the complex gels S/XG/T relax like true gel materials with a predominant elastic relaxation response. Results also indicated the thermally reversible nature of S/T and S/XG/T gels. The rheological comportment of the composites formulated revealed that on shear and heating processes the xyloglucans likely adopt a twisted conformation, that is complementary to double helices of amylose/amylopectin, and in the presence of tetraborate ions they provide a better viscoelastic property and higher solid character when compared to that in the absence of the hydrocolloid. Acknowledgments The authors gratefully acknowledge CNPq, Fundação Araucária for the financial support, Corn Products do Brasil for the gift of modified cassava starch, and also the FINEP/LACTEC for allowing access to Haake RS 600 rheometer. References [1] G.J.S. Reid, in: P.M. Dey, R.A. Dixon (Eds.), Biochemistry of Storage Carbohydrates, Academic Press, London, 1985. [2] P.S. Rao, H.C. Srivastava, Industrial Gums: Polysaccharides and Their Derivatives, 1973. [3] S. Miyazaki, F. Suisha, N. Kawasaki, M. Shirakawa, K. Yamatoya, D. Attwood, J. Control. Release 56 (1998) 75. [4] S. Yamanaka, Y. Yuguchi, H. Urakawa, K. Kajiwara, M. Shirakawa, K. Yamatoya, Food Hydrocoll. 14 (2000) 125. [5] T. Hayashi, Ann. Rev. Plant Biol. 40 (1989) 139. [6] N.N. Lima, F. Reicher, J.B.C. Corrêa, J.L.M.S. Ganter, M.R. Sierakowski, Ciênc. Cult. 45 (1993) 22. [7] N.N. Lima, F. Reicher, J.B.C. Corrêa, J.L.M.S. Ganter, M.-R. Sierakowski, Int. J. Biol. Macromol. 17 (1995) 413. [8] C. Vargas-Rechia, F. Reicher, M.R. Sierakowski, A. Heyraud, H. Driguez, Y. Liénart, Plant Physiol. 116 (1998) 1013. [9] N. Lima-Nishimura, M. Quoirin, Y.G. Naddaf, M. Wilhelm, L.L.F. Ribas, M.-R. Sierakowski, Cell Biol. Morphogenesis 21 (2003) 402. [10] S. Martin, R.A. Freitas, E. Obayashi, M.-R. Sierakowski, Carbohydr. Polymers 54 (2003) 287. [11] R.A. Freitas, S. Martin, G.L. Santos, F. Valenga, M.S. Buckeridge, F. Reicher, M.-R. Sierakowski, Carbohydr. Polym. 60 (2005) 507. [12] J.M.V. Blanshard, Starch: Properties and Potential, 1987, p. 16. [13] H.F. Zobel, A.M. Stephen, Food Polysaccharides and their Applications, 1995, p. 19. [14] T. Funami, Y. Kataoka, S. Noda, M. Hiroe, S. Ishihara, I. Asai, R. Takahashic, N. Inouchid, K. Nishinar, Food Hydrocol. 22 (2008) 777.
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Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m s e c
Rheological behavior of borate complex and polysaccharides M.R. Gouvêa, C. Ribeiro, C.F. de Souza, I. Marvila-Oliveira, N. Lucyszyn, M.-R. Sierakowski ⁎ Laboratório de Biopolímeros, Departamento de Química, Universidade Federal do Paraná, P.O. Box 19081, 81531-990 Curitiba, Paraná, Brazil
a r t i c l e
i n f o
Article history: Received 31 May 2008 Accepted 8 October 2008 Available online 1 November 2008 Keywords: Starch Xyloglucan Rheology Borate ions Viscoelasticity Gel
a b s t r a c t In this work the rheological behavior of manioc starch (S) industrially modified and its blends with a xyloglucan (XG) in the presence of tetraborate (T) ions at pH 12 was described. At rotational measurements the viscosity values showed a good interaction between polysaccharides (20/5 g/l, respectively, to S and XG), which were highly modified by the presence of tetraborate (7 g/l) resulting in better pseudo or plasticity. To system S/XG/T at 20/5/7 or 40/10/14 g/l the rheological properties were dependent of polysaccharides/T concentration. Mixtures at 25/7 g/l performed a viscoelastic solution, and at 50/14 g/l a weak gel. After the temperature sweep analyses (heating and cooling), a more solid character was obtained. This performance could be explained as a result of total gelatinization process that benefits the structural reorganization and better interaction between polysaccharides and the tetraborate complex formed with the hydrocolloid. Also, it was observed that the S/XG/T system, after heating/cooling together with a shear stress, adopted a helical conformation similar to that obtained with amylose standard, since it was colored in blue with lugol. So, the interactions are related with the conformational change of S and XG and also with shear processes, which aid the reptation phenomena and improvement of the solid character. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The polysaccharides are utilized in several industries as functional ingredients for controlling stability, structure, and texture of products. Xyloglucan is a water-soluble neutral polysaccharide, found in the primary cell walls of non-graminaceous (monocotyledons) and in the cotyledon of some dicotyledonous seeds, where it has functioned as a storage [1]. It is the commercial form extracted from Tamarindus indica (tamarindo) seeds which is used in cosmetic, biomedical and food applications [2–4]. One of the properties of xyloglucan is its high viscosity in aqueous solutions. This biopolymer shows a main chain with (1→4)-linked β-D-glucan, and in the side chains xylose units are linked to the glucose units in the C-6 position. Some of the xylose units are also substituted at C-2 by β-D-galactose units [5]. In the Federal University of Paraná (UFPR) considerable attention has been paid to the xyloglucan extracted from H. courbaril (jatobá) seeds obtained at different Brazilian locations, whose partial fine chemical structure and properties have been determined [6–11]. Other polysaccharide widely used is the starch, that is composed of anhydrous glucose units linked primarily through α-D-(1→4) glycosidic bonds. While the detailed fine structure has not been fully elucidated, it has been firmly established that starch is a heterogeneous material consisting of varying proportions of amylose and amylopectin ⁎ Corresponding author. Present address: Laboratório de Biopolímeros (BIOPOL), Departamento de Química, Universidade Federal do Paraná, C.P. 19081, 81531-990 Curitiba, PR, Brazil. Tel.: +55 41 33613260; fax: +55 41 33613186. E-mail address: [email protected] (M.-R. Sierakowski). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.10.019
[12,13]. Some properties of starch can be modified by the presence of hydrocolloids, to improve their rheological characters, since that mixture of polysaccharides in solution is different from pure systems [13,14]. The synergic interactions between starch and hydrocolloids are of great interest commercially, since they resulted in systems with different functionalities, new rheological characteristics or better textures using minor quantity of polysaccharides, properties which can possibly be used for other applications also [11,15]. Regarding the industrial potential in the processing of polysaccharides, the knowledge of the rheological behavior of starches and other biopolymers is very useful in quality/process control and equipment selection. It is known that at alkaline pH, polysaccharides complex with borate ions forming some heterocyclic boron compounds [16–20]. In the literature it was related that the presence of borate ions in xyloglucan (tamarindo seeds) made up viscous and elastic solutions [21]. This effect was demonstrated previously by Ghose and Krisna [22]. The studies involving guar gum and hydroxypropyl guar (HPG) in high concentrations and pH variation related that the behavior of relaxation of a gel depends on the nature of links due to borate ions [23]. Other studies rheologically involving a complex of guar or hydroxypropyl guar with borate ions, showed that the chemical balance involving boric acid, borate ions free or associated with cis-diol sites of the polysaccharides chain, determines the number of links, and this balance is in function of temperature and pH [24]. Power et al. [25] studied the rheological behavior of the complex of HPG with borate ions, and the authors showed that despite the properties of guar and HPG being studied extensively by groups of rheology and industries of petroleum, a complete understanding of the behavior of these fluids
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Table 1 Composition and identification of the systems System 25S 25S/7T 5XG 5XG/7T 20S/5XG 20S/5XG/7T 50S/14T 40S/10XG/14T
Starch (g/l) 25 25 – 20 20 50 40
Xyloglucan (g/l) – – 5 5 5 5 – 10
Sodium borohydride (g/l) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Sodium tetraborate (g/l) – 7 – 7 – 7 14 14
and gels still is not clear. Rheological studies in the interaction of xyloglucan (XG) from H. courbaril seeds and borate ions were performed by Martin et al. [10], these authors observed that better viscoelasticity was related with greater Mw of XG. In the 11B NMR spectrum, it was found that the interaction of borate ions with xyloglucan was through galactose units. Recently, the critical strain for the gel formulated with konjac glucomannan was found to be independent of borax concentration, while the yield stress firstly increased with increasing borax concentration and then decreased [26]. Considering the rheological properties, the aim of this work was to characterize the behavior of systems of manioc starch, xyloglucan, and its blends in the presence of borate ions by dynamic viscosity curves, to obtain more plastic properties.
Table 2 Rheological parameters of the systems at 25 or 5 g/l evaluated by the Power law, Bingham and Herschel–Buckley models Sample a
25S 25S/7Ta 5XGa 5XG/7Ta 20S/5XGa 25Sb 25S/7Tb 5XGb 5XG/7Tb 20S/5XG/7Tb 20S/5XG/7Tc a b c
R2
χ2
τo (Pa)
K (Pa sn)
n
η (Pa s)
0.9979 0.9749 0.9998 0.9998 0.9870 0.9972 0.9871 0.9992 0.9998 0.9596 0.9993
0.0484 0.2873 0.0002 0.0002 0.1936 0.0127 0.0029 0.0004 0.0001 0.0702 0.1599
– – – – – 10.4894 10.7266 0.8266 0.7863 20.5533 27.7621
1.4384 2.0485 0.0615 0.0663 2.5776 – – – – – 23.1685
0.5359 0.3915 0.8562 0.8253 0.5496 – – – –
– – – – – 0.0642 0.0139 0.0230 0.0795 0.1184 –
0.5372
n
Power law (τ = Kγ ). Bingham (τ = τo + Kγn). Herschel–Buckley (τ − τo = ηγ).
(40%, w/v). Sodium tetraborate (T) was added to obtain the S/T, XG/T or S/XG/T system. All the systems formulated are present in Table 1. 2.5. Rheological properties
2. Materials and methods
Rotational experiments to the upward and downward curve had a duration of 2 min each one with shear rate ranging from 0 to 200 s− 1. The experimental data were mathematically evaluated and fitted according to the Herschel–Buckley model (Eq. (1)), Power law (Eq. (2)) or Bingham (Eq. (3)).
2.1. Materials
τ = τ o + Kγ n
The xyloglucan (XG) was obtained by exhaustive aqueous extraction from pooled and milled Hymenaea courbaril seeds acquired from EMBRAPA/Natal/RN. The viscous extracts were purified by centrifugation. The polymer was obtained after precipitation with two volumes of 96% ethanol and washed with acetone [15]. Starch (S) from Manihot utilissima industrially modified (cross bounded) by pre-gelatinization was supplied by Corn Products Brasil Ingredientes Industriais Ltda, Balsa Nova, State of Paraná, Brazil, in which amylose content was related as 18%. 2.2. Swelling and solubility of starch The swelling assay was made by the method of Leach et al. [27], using a 0.5% (w/v) solution. So, starting at 50 g/l, the solution was diluted with purified water to the correct concentration. Then it was centrifuged at 700 g for no more than 15 min. The volume of supernatant indicated the volume of water non-linked in the solution.
ð1Þ
where, τ is the shear stress (Pa), τo is the yield stress (Pa), K is the consistency coefficient (Pa sn), g is the shear rate (s− 1) and n is the flow behavior index of the fluid (dimensionless). τ = Kγn
ð2Þ
where, τ is the shear stress (Pa), K is the consistency coefficient (Pa sn), g is the shear rate (s− 1) and n is the flow behavior index of the fluid (dimensionless). τ−τ o = ηγ
ð3Þ
where, τ is the shear stress (Pa), τo is the yield stress (Pa), η is the Bingham plastic viscosity (Pa s) and g is the shear rate (s− 1).
2.3. Determination of XG critical concentration Solutions of starch (5 g/l) in water and XG (3 g/l) in 0.1 M sodium nitrite were prepared and diluted in water, which were utilized to determine, initially, the intrinsic viscosity [η], evaluated by extrapolation of reduced viscosity to the limit of zero concentration, where the linear coefficient is the [η]. Then, by slope of log specific viscosity versus log (c × [η]), the values of critical concentration were determined. 2.4. Preparation of the isolated polysaccharide solutions, blends and complexes with borate ions The starch (S) was solubilized in distilled water at 25 °C for 30 min and the xyloglucan (XG) in distilled water at 25 °C for 19 h. The S/XG blends, for example at 25 g/l, were obtained by adding the S solution (200 mg/5 ml) into the XG solution (50 mg/5 ml). The mixture was stirred for 20 min and sodium borohydrate was added, and then the pH of all solutions was adjusted for 12 with sodium hydroxide solution
Fig. 1. Frequency (f) dependence of systems 25S/7T (modulus (G′) — ●, (G″) — ○); and 20S/5XG/7T (modulus (G′) — ■, (G″) — □), in Haake RS 600 rheometer, spindle PP 35 mm, 1 Hz, at 25 °C.
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the dependence of log specific viscosity in function of log (concentration×[η]). To the XG sample the critical concentration (c⁎), limit of the ticket of the regimen diluted for the half-diluted one, found at 25 °C was 2.2 g/l and to XG was 1.2 g/l. These values indicate that above of these concentrations the samples will be more aggregated, therefore, its components have raised molar mass. 3.2. Swelling and solubility of the starch The value of swelling and solubility of the pre-gelatinized manioc starch was determined as 80%, showing a good correlation between the industrial process of modification (pre-gelatinization), which resulted in a good hydration of the sample. 3.3. Rheological properties: flow behavior of systems
Fig. 2. Temperature dependence of systems 25S/7T (modulus (G′) — ●; (G″) — ○) and 20S/5XG/7T (modulus (G′) — ■; (G″) — □) in Haake RS 600 rheometer, spindle C60/2° and PP 35 mm, at 1 Hz. (Y) shows the initial heating.
The dates were analyzed using the software Origin 7.0 (Origin Lab Corporation, MA, USA), in order to obtain the rheological (τo, n, K and η) and statistical parameters (R2 and χ2). The values of R2 and χ2 were obtained to evaluate the goodness of fit to the experimental results in the model applied. Non-oscillatory and oscillatory analyses were performed using Haake rheometers model Rheostress 600 or RS1 with the spindle C60/2° or PP20. A Haake water bath (DC 30) and a thermostatic UTC (universal thermostatic control) were used to control constant temperature at 25 °C or to temperature ramp (heating from 20 to 70 and cooling gradient from 70 to 20 °C), at a rate of 1.8 °C/min. Oil was applied to the exposed surfaces of the sample to prevent evaporation for experiments with gradients of temperature. The rheometer was interfaced to a microcomputer for control and data acquisition. Experiments were done on triplicate. The oscillatory measurements, in the viscoelastic region, were carried out in the frequency of 0.01–5 Hz, deformation of 2%, to all systems. 3. Results and discussion 3.1. Measurements of critical concentration of starch and xyloglucan samples With the intention to evaluate the concentrations where intermolecular interactions can be led, a convenient experiment is to examine
The starch systems at 25 g/l containing borate ions or not showed a pseudoplastic behavior. System 5XG/7T presented also a pseudoplastic behavior and a rupture pseudo-point [28] showing the formation of aggregates at low shear rates (b25 s− 1), which is a typical behavior of suspension (data not showed). The profile of the curves of viscosities of system 25S/7T showed a thixotropic behavior that indicated a reduction in the organization of the molecules in the system. On the other hand, system S/XG/T presented a rheopectic effect. In accordance with Barnes et al. [29], the rheopectic character could be described as a rearrangement of particles, producing one better organization of the system and the consequent increase of viscosity (data not shown). The better agreement of the rheological behavior of the generated systems was obtained when the mathematical models were applied (see Table 2) where the values of R2 were higher than 0.96 and the chisquares (χ2) were near zero that indicated a good adjust. For the systems 25S, 25S/7T and 5XG/7T two intervals of shear rate had been considered for the calculations, the Power law model between 5 and 100 s− 1, and the Bingham model for shear rate N100 s− 1. These treatments were effected since it could not apply the model of the Power law for shear rates b5 s− 1 or higher shear rates N100 s− 1 [30]. For the system 20S/5XG/7T the Power law model did not fit the data satisfactorily, and so the Herschel–Buckley model was used. The values of the index behavior (n) lesser than 1, prove that all systems have a pseudoplastic behavior. System 25S showed a low index of consistency (K) in relation to the 25S/7T and a lower pseudoplasticity. So, the presence of the salt modifies the flow behavior in the 25S/7T system, producing a material with better pseudoplasticity and higher consistency. These characteristics can be attributed to
Fig. 3. Frequency dependency of systems 50S/14T versus modulus (G′) — ●, (G″) — ○ (A) and tan δ— ×; η⁎ — ♦ (B) values, and 40S/10XG/14T versus modulus (G′) — ■; (G″) — □ (A), and tan δ— ◊; η⁎ — ★ (B) in Haake RS 1 rheometer, spindle PP 35 mm, deformation of 3%, at 25 °C.
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Fig. 4. Temperature dependence of: (A) systems 50S (modules (G′) — ▲, (G″) — Δ) and 50S/14T (modulus (G′) — ●, (G″— ○); (B) 50S/14T (tan δ— ×) and 40S/10XG/14T (tan δ— ♦) in Haake RS 1 rheometer, spindle C60/2° and PP 35 mm, respectively, at 20/70/20 °C, 0.02 Hz. (→) shows the initial heating program.
the removal of some water molecules by salt introduction, which promoted better interaction between polysaccharide chains. The system 20S/5XG/7T resulted in a material with a higher τo (27.7621 Pa), and greater K (23.1685 Pa s) in relation to the other systems. In this case, the cross linking that is present could be favored for the presence of the borate anions that complex with alcooxi groups in conditions of pH alkaline. Furthermore, the presence of the XG that promotes the complexation process (data of RMN spectra not shown), contributes to the increase of the values of τo and K parameters; also a superior value of plasticity for this system was observed. So, the presence of hydrocolloid and borate ions in mixture with starch resulted in a more plastic system. The measure of the interactions between systems S and XG, both in the presence of borate ions was evaluated by the following equations [31]: Interaction Index ðI1 Þ = η of
20S=5XG 20S + 5XG
ð4Þ
Interaction Index ðI2 Þ = η of
20S=5XG=7T 20S=7T + 5XG=7T
ð5Þ
The interaction index 0.91 by Eq. (4) indicated that the system lacks practical interest in terms of viscous synergism, when only starch and xyloglucan were mixed. But, in the presence of borate ions (Eq. (5)) the interaction index value is superior to 1 (1.27) that indicated a good synergism, because the viscosity of the mixture is greater than the sum of the viscosities of the isolated systems. 3.4. Viscoelastic properties of starch, starch/borate and starch/ xyloglucan/borate systems The performance of the storage and loss modules (G′ and G″, respectively) in function of the frequency (f) was made in the region of linear viscoelasticity (data not shown), to evaluate if the systems were solutions or gels. In Fig. 1 it is observed that for the 20S/5XG/7T system, G′ and G″ modules are higher than for 25S/7T, over a frequency range of 0.01– 7 Hz. However, although the storage modules are higher than the loss modules for both systems, the difference is not pronounced, indicating the presence of viscoelastic solution or weak gel formation. It is also showed that, in low frequency (b0.1 Hz), the solid character prevails for 25S/7T system, as expected by the presence of aggregates. However, in the higher frequencies, the solid character is more pronounced for 20S/5XG/7T sample, as a result of entanglement of the chains. It was calculated that the values of η⁎ for the mixture 20S/5XG/
7T were 9–17 fold greater than for 25S/7T, practically under all the frequencies studied, and it was observed that the viscosity decayed linearly (data not shown). The dependence of the modulus values G′ and G″ in function of the frequency sweep indicated that S/XG/T is a more viscoelastic solution, in comparison with the S/T system. A similar experiment depicted in Fig. 1 was carried out with 20S/ 5XG system, to confirm the influence of borate ions in the viscoelastic character (date not shown). In these analyses, both the modules showed total dependence of the frequency and the tan δ (G″/G′) values near 1 (indicated a liquid character) confirm the relevance of borate ions on the interaction process. The stability of systems 25S/7T and 20S/5XG/7T during heating and cooling was valuable by temperature sweep. The comparative analysis of the behavior between the systems showed the formation of a material with a higher solid character in the presence of XG once the G′ is higher than G″ for all temperatures. In the cooling the values of the modules were higher than in the heating process (Fig. 2). On this form, new interactions were obtained after heating (up 50 °C); and this performance resulted from the presence of borate ions, since, without these ions similar behaviors are not observed (data not shown). This comportment can be attributed to better solubilization of borate ions at ~60 °C and also changes in the starch conformation during heating; and both factors are favorable to better interaction and complexation. Since 5XG/7T did not form a viscoelastic solution it was not possible to observe the change on the hydrocolloid structure. To evaluate the effect of components (borate ions and polysaccharides) in the viscoelastic properties of the systems some assays were made with a variation of concentrations. For this, blends were formulated at concentrations of starch, xyloglucan and borate ions of 40, 10 and 14 g/l, respectively. According to the profile in Fig. 3A it is shown that there exists a lesser dependence of G′ and G″ in the frequency range for the system 40S/10XG/14 T. Furthermore, in this case the values of complex dynamic viscosity (η⁎) were significantly bigger than 50S/14T (Fig. 3B).
Table 3 Values of dynamic viscosity (η⁎) in the temperature range for different systems
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The lesser value of tan δ in the presence of xyloglucan indicates predominantly an elastic behavior, and the comparison of the values between the samples reflected differences in the organization of the structures (Fig. 3B), that can be reflected in the formation of a more rigid net [32,33]. In the literature the values of tan δ of 0.02 to 0.2 can classify the rheologic systems as an intermediate state between a weak gel and an elastic gel [34,35]. For mixture 40S/10XG/14T to obtain this scale the formation of the net is dependent of the high frequency, so this system showed a comportment of a gel, which presented a G′ four-fold higher than the G″. Data for G′ and G″ plotted as a function of frequency (Fig. 4A) can provide information on gel structure. The degree of frequency dependence can be determined by the well-known Power law equation [36]: GV= AωB where, G′ is the storage modulus, ω is the oscillation frequency (rad/s), and A is a constant. The exponent B is the slope in a log–log plot of G′ versus ω. Applying the Power law model, for systems 50S/14T and 40S/10XG/ 14T the A values were 47.51 and 146.54 Pa s, respectively and B were 0.4187 and 0.2061, respectively. By the B values it is possible to indicate that in the presence of xyloglucan more linkings were formed in such a way by physical as well as by chemical interaction. With this behavior, the XG/T functions as a good interlinking, especially during reptation process [37,38], which is described as the time for a given macromolecules to disengage by a snakelike motion from a tube made up of neighboring chains. The stability of system 50S/14T was analyzed by the variation of the temperature. During heating, the starch swell followed causing an increase in G′ to maximum value, which can be attributed to the formation of a network of swollen starch [39]. More heat and greater mobility or entropy might loosen the previous structure formed by breaking some hydrogen bonds or other possible interactions and rupturing the structure of swollen starch [40]. Consequently, the strength of network-like structure weakened and the modulus dropped [41]. After heating the swelling is completed and this is governed by the viscoelasticity of the particles (amylose and amylopectin) that act together as relatively rigid entities [42]. The rheogram depicted in Fig. 4A shows that in the presence of borate ions, the modulus values increase in relation to the starch system, and it was also observed that in the 50S/14T the gel melted as showed by the lowering of G′ and G″ values after heating, which is not observed for 50S that results in the system with a higher G′ indicating a change in the conformation. It was possible to see the very significant increase in the modulus after the heating of systems in the presence of xyloglucan (data not shown). For example at 30 °C, the modulus values were sixty two-fold higher with 14 g/l of tetraborate (Table 3), than the system containing half the concentration of components (Fig. 2). In the system S/XG/T it was observed that the arrangement of the phases contributed to the formation of the net. The values of G′ and G″ diminish with the increase of the temperature from 20 to 70 °C, a similarity with the behavior for other polymers [43], and increase when the temperature diminishes from 70 to 20 °C. This indicates that the net of the gel was strengthened during the heating and after that strong cross linkings are kept and weak linkings are converted into strong linkings during the cooling (see data of Table 3). This performance suggests that the main force that stabilizes the net is the hydrogen linking [44] and also the ionic ones. Raising the temperature increased the fluidity of the complex gel, indicating the dependence of cross-link density on temperature. Previous studies [45,46] have shown that complex formation in reactions between polymer and borate ions is exothermic and the complexation equilibrium constant decreases with increasing temperature.
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The lower tan δ values after heating (Fig. 4B) demonstrate the increase of the gel force in the S/SG/T system. So, these results also indicated the thermally reversible nature of S/T and S/XG/T gels. With the results obtained the whole chemical equilibrium involving dissociation of borax in water, formation of borate ions, and complexation between cis-diol sites of galactose units was present in the XG chains, and borate ions determine the number of cross linked chains, like those observed for PVA/borax [47], guar/borax [48] and konjac glucomannan [26]. To understand the mechanism of interaction between S/XG/T interactions, the systems composed of S, S/T, XG, XG/T, S/XG, S/XG/T, standards of amylose/T and of amylopectin/T, heated or not and also submitted or not to a shear stress were analyzed in the presence of lugol solutions. The results (data not showed) revealed that XG adopted a helical conformation, since a deep blue color was produced, similar to that of amylose, but only after the systems containing XG were submitted to shear together with heating. So, the process to produce a more solid system involves the unraveling of the starch/XG helix and a higher increase in the volume of the polysaccharides, and the water is imbibed and bound to the unraveled polymers by hydrogen bonding. By this comportment, it is supposed that the borate ions are arranged in the groove of the helices. Based on the importance of borate ions on cement the application of these composites on cement pastes is in progress. 4. Conclusion At higher oscillation frequencies the complex gels S/XG/T relax like true gel materials with a predominant elastic relaxation response. Results also indicated the thermally reversible nature of S/T and S/XG/T gels. The rheological comportment of the composites formulated revealed that on shear and heating processes the xyloglucans likely adopt a twisted conformation, that is complementary to double helices of amylose/amylopectin, and in the presence of tetraborate ions they provide a better viscoelastic property and higher solid character when compared to that in the absence of the hydrocolloid. Acknowledgments The authors gratefully acknowledge CNPq, Fundação Araucária for the financial support, Corn Products do Brasil for the gift of modified cassava starch, and also the FINEP/LACTEC for allowing access to Haake RS 600 rheometer. References [1] G.J.S. Reid, in: P.M. Dey, R.A. Dixon (Eds.), Biochemistry of Storage Carbohydrates, Academic Press, London, 1985. [2] P.S. Rao, H.C. Srivastava, Industrial Gums: Polysaccharides and Their Derivatives, 1973. [3] S. Miyazaki, F. Suisha, N. Kawasaki, M. Shirakawa, K. Yamatoya, D. Attwood, J. Control. Release 56 (1998) 75. [4] S. Yamanaka, Y. Yuguchi, H. Urakawa, K. Kajiwara, M. Shirakawa, K. Yamatoya, Food Hydrocoll. 14 (2000) 125. [5] T. Hayashi, Ann. Rev. Plant Biol. 40 (1989) 139. [6] N.N. Lima, F. Reicher, J.B.C. Corrêa, J.L.M.S. Ganter, M.R. Sierakowski, Ciênc. Cult. 45 (1993) 22. [7] N.N. Lima, F. Reicher, J.B.C. Corrêa, J.L.M.S. Ganter, M.-R. Sierakowski, Int. J. Biol. Macromol. 17 (1995) 413. [8] C. Vargas-Rechia, F. Reicher, M.R. Sierakowski, A. Heyraud, H. Driguez, Y. Liénart, Plant Physiol. 116 (1998) 1013. [9] N. Lima-Nishimura, M. Quoirin, Y.G. Naddaf, M. Wilhelm, L.L.F. Ribas, M.-R. Sierakowski, Cell Biol. Morphogenesis 21 (2003) 402. [10] S. Martin, R.A. Freitas, E. Obayashi, M.-R. Sierakowski, Carbohydr. Polymers 54 (2003) 287. [11] R.A. Freitas, S. Martin, G.L. Santos, F. Valenga, M.S. Buckeridge, F. Reicher, M.-R. Sierakowski, Carbohydr. Polym. 60 (2005) 507. [12] J.M.V. Blanshard, Starch: Properties and Potential, 1987, p. 16. [13] H.F. Zobel, A.M. Stephen, Food Polysaccharides and their Applications, 1995, p. 19. [14] T. Funami, Y. Kataoka, S. Noda, M. Hiroe, S. Ishihara, I. Asai, R. Takahashic, N. Inouchid, K. Nishinar, Food Hydrocol. 22 (2008) 777.
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