Steady and Dynamic Shear Rheology of Rice Starch-Galactomannan Mixtures

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1 310 DOI /star Starch/Stärke 57 (2005) Dongryel Yoo Chion Kim Byoungseung Yoo Department of Food Science and Technology, Dongguk University, Seoul, South Korea Steady and Dynamic Shear Rheology of Rice Starch-Galactomannan Mixtures Rheological properties of rice starch-galactomannan mixtures (5%, w/w) at different concentrations (0, 0.2, 0.4, 0.6 and 0.8%, w/w) of guar gum and locust bean gum (LBG) were investigated in steady and dynamic shear. Rice starch-galactomannan mixtures showed high shear-thinning flow behaviors with high Casson yield stress. Consistency index (K), apparent viscosity (Z a,100 ) and yield stress (s oc ) increased with the increase in gum concentration. Over the temperature range of C, the effect of temperature on apparent viscosity (Z a,100 ) was described by the Arrhenius equation. The activation energy values (E a = kj/mol) of rice starch-galactomannan mixtures ( % gum concentration) were much lower than that (E a = 12.8 kj/mol) of rice starch dispersion with no added gum. E a values of rice starch-lbg mixtures were lower in comparison to rice starch-guar gum mixtures. Storage (G 0 ) and loss (G 00 ) moduli of rice starch-galactomannan mixtures increased with the increase in frequency (o), while complex viscosity (Z*) decreased. The magnitudes of G 0 and G 00 increased with the increase in gum concentration. Dynamic rheological data of ln (G 0, G 00 ) versus ln frequency (o) of rice starch-galactomannan mixtures have positive slopes with G 0 greater than G 00 over most of the frequency range, indicating that their dynamic rheological behavior seems to be a weak gel-like behavior. Keywords: Rice starch; Guar gum; Locust bean gum; Rheological properties; Dynamic modulus Research Paper 1 Introduction The addition of hydrocolloids to starch in food system is widely used to modify and control the rheological properties of starch. The specific adjustment of the rheological properties of starch is of significance in order to regulate production processes and to optimize applicability, stability and sensory properties of food products [1]. In particular, since both ingredients exist together in many food systems, knowledge about the rheological properties of starch-hydrocolloid mixtures is important for understanding molecular interactions between starches and hydrocolloids [2], and for obtaining some practical information on the role and potential usefulness of hydrocolloids in starch-based products [3]. Many researchers have studied the effect of hydrocolloids on rheological properties of various starches and found that the addition of hydrocolloids influenced the viscosity and retrogradation of starch dispersions, as well as the syneresis of starch gels [4 9]. Correspondence: Byoungseung Yoo, Department of Food Science and Technology, Dongguk University, 3 Pil-dong, Chung-gu, Seoul , Korea. Phone: , Fax: , bsyoo@dongguk.edu. Galactomannans (e.g. guar gum and locust bean gum) are water-soluble polysaccharides from the ground endosperm of seeds. The galactomannans obtained from guar (Cyamopsis tetragonolobus) andcarob(ceratonia siliqua) seeds are important thickening and gelling agents in food systems. Galactomannans have a main chain of (1?4)-linked b-dmannopyranosyl units, bearing single a-d-galactopyranosyl units attached to O-6 of about 56% of the main-chain units [10]. Guar gum and locust bean gum (LBG) differ in the ratio of D-mannosyl to D-galactosyl unit which is about 1.8:1 for guar gum and 3.9:1 for LBG [11]. Both galactomannans are composed of long, rather rigid chains that have a large hydrodynamic volume, thus providing high solution viscosity. Guar gum, especially, produces high viscosity with pseudoplastic rheology at low concentrations [10]. Many researchers have studied the effect of galactomannans (guar gum and LBG) on rheological properties of various starches, such as wheat starch [3 5, 12, 13], corn starch [5, 13, 14], waxy corn starch [13], tapioca starch [13], yam starch [15] and waxy rice starch [16], by using Brabender Viscoamylograph, Rapid Visco Analyser and rotational viscometer. They found that the viscosity of mixtures depended on the type and concentration of gum, temperature and the type of starch. However, a

2 Starch/Stärke 57 (2005) Rheology of Rice Starch-Galactomannan Mixtures 311 study on the rheological properties of rice starch dispersion in the presence of galactomannans has not been published. Most rheological studies concerning starchgalactomannan mixtures have also been conducted under steady shear except for the studies of Alloncle and Doublier [17], Korus et al. [18] and Kulicke et al. [16], who examined the effect of galactomannans (guar gum or LBG) on dynamic shear rheological properties of corn, triticale and waxy rice starches, respectively. The dynamic shear rheological tests for small-deformation oscillatory measurements, in general, have been used to obtain valuable information on the viscoelastic properties of starch-gum mixtures without breaking their structural elements. These tests allow researchers to relate dynamic rheological parameters to the sample s molecular structure [19]. Therefore, studies on dynamic rheological properties are very important in providing the insight to understand the component interactions in starch-gum mixture system. However, no attempt has yet been made to study rheological properties of rice starch in both steady and dynamic shear as affected by the addition of guar gum and LBG. No detailed studies have also been reported for comparing rheological differences between rice starch-guar gum and rice starch-lbg mixtures at various gum concentrations, although only a few studies have been conducted for waxy rice [16] and corn starches [17]. Rheological studies of rice starch-galactomannan mixtures with different gum concentrations can provide valuable information on the synergistic interactions of galactomannans with rice starch dispersion in a molecular interpretation. Therefore, the objectives of the present study were: (1) to determine the steady and dynamic shear rheological properties of rice starch-galactomannan mixtures, (2) to compare the rheological properties of rice starch-guar gum mixtures and those of rice starch-lbg mixtures, and (3) to examine the effect of gum concentration and temperature on rheological properties of rice starch-galactomannan mixtures. 2 Materials and Methods 2.1 Materials and preparation of rice starch dispersions Rice starch and galactomannans (guar gum and locust bean gum) were purchased from Sigma Co. (St. Louis, MO, USA). Rice starch-galactomannan mixtures (5%, w/ w) were prepared by mixing rice starch with distilled water, and guar gum and locust bean gum (LBG) to obtain 0.2, 0.4, 0.6, and 0.8% (weight basis) gum levels, as described by Kim and Yoo [20]. A starch dispersion (control) with no added gum was also prepared. The mixture was moderately stirred for 1 h at room temperature, and then was heated at 957C in a water bath for 30 min with mild agitation provided by a magnetic stirrer. At the end of the heating period, the hot sample mixture was immediately transferred to the rheometer plate for the measurements of rheological properties. 2.2 Rheological measurements Steady and dynamic shear rheological properties were obtained with a rheometer (AR 1000, TA Instruments, New Castle, DE, USA) at 257C, using a parallel plate system (4 cm dia.) at a gap of 500 mm. For rheological measurements, the sample was loaded onto the platen of the rheometer and the exposed sample edge was covered with a thin layer of light paraffin oil to prevent evaporation during measurements. The sample was sheared continuously from 0.1 to 1000 s 21. In order to describe the variation in the rheological properties of samples under steady shear, the data were fitted to the well-known power law (Eq. 1) and Casson (Eq. 2) models: s ¼ K g n (1) s 0:5 ¼ K oc þ K c g 0:5 (2) where s is the shear stress (Pa), g is the shear rate (s 21 ), K the consistency index (Pa?s n ), n and n H are the flow behavior indexes (dimensionless), and (K c ) 2 is the Casson plastic viscosity (Z c ). Casson yield stress (s oc ) according to the Casson model (Eq. 2) was determined as the square of the intercept (K oc ), that was obtained from linear regression of the square roots of shear rate-shear stress data. Using the values of K and n, the apparent viscosity (Z a,100 ) at 100 s 21 was calculated. The steady shear rheological measurements were also conducted at various temperatures in the range of C in order to investigate the effect of temperature on Z a,100. Dynamic shear properties were obtained from frequency sweeps over the range of rad/s at 3% strain. The 3% strain was in the linear viscoelastic region. Frequency sweep tests were also performed at 207C. TA rheometer Data Analysis software (version VI. 1.76) was used to obtain the experimental data and to calculate storage modulus (G 0 ), loss modulus (G 00 ) and complex viscosity (Z*). All rheological measurements in steady and dynamic shear were performed in triplicate. Results reported were an average of the three measurements.

3 312 D. Yoo et al. Starch/Stärke 57 (2005) Results and Discussion 3.1 Steady shear properties Rheograms of rice starch-galactomannan (guar gum and locust bean gum) mixtures with different gum concentrations (0, 0.2, 0.4, 0.6 and 0.8%) at 207C indicated the non-newtonian (pseudoplastic) nature of this mixture with yield stress (Fig. 1). The flow behavior index (n), which indicates the extent of shear-thinning behavior as it deviates from 1, was in the range of (Tab. 1). The n values did not change much with the increase in galactomannan concentration ( % gum) except for 0.2% LBG. Rice starch-guar gum mixtures had lower n values ( ) than the control with no added gum (0.24) and rice starch-lbg mixtures ( ), indicating a more pseudoplastic behavior of the guar gum mixtures. This result is in good agreement with those found in wheat starch-galactomannan mixtures [12]. The degree of pseudoplasticity of starch-galactomannan mixtures can be explained in terms of gum structure. According to Sajjan and Rao [12] and Sudhakar et al. [14], guar gum consists of a mannan backbone with alternate galactose branches, which inhibit the formation of intramolecular hydrogen bondings. This keeps the molecule in an extended form, which can readily interact with amylose molecules through non-covalent hydrogen bondings, resulting in an extended conformation that, in turn, increases the degree of pseudoplasticity. In contrast, locust bean gum consists of a mannan backbone with irregular galactose branches, which tends to coil with the formation of intramolecular hydrogen bondings and hence interacts less with the linear amylose molecules due to the decrease in the number of hydroxyl groups available to form intermolecular hydrogen bondings with amylose molecules. Therefore, a much higher Fig. 1. Shear rate-shear stress plots for rice starchgalactomannan mixtures at different gum concentrations at 207C: (m) 0% (control), (u) 0.2%, (n) 0.4%, (s) 0.6%, (d) 0.8%. (a) Guar gum and (b) Locust bean gum. Tab. 1. Effect of guar gum and LBG concentration on steady shear rheological properties of rice starch dispersions at 207C. a Gum type Concentration [%] Apparent viscosity Z a,100 [P?s n ] Consistency index K [Pa] Flow behavior index n [-] Yield stress s oc [Pa] Control 0 (no gum) Guar gum LBG a Values are mean6standard deviation for triplicate measurements.

4 Starch/Stärke 57 (2005) Rheology of Rice Starch-Galactomannan Mixtures 313 degree of pseudoplasticity can be observed for the rice starch-guar gum mixtures compared to rice starch-lbg mixtures. The values of apparent viscosity (Z a,100 ), consistency index (K) and Casson yield stress (s oc ) obtained from the power law and Casson models increased with the increase in concentrations of galactomannans. Compared to the control (0% gum), rice starch dispersions at different gum concentrations also showed much higher values of s oc in the range of From these observations, it was found that the rice starch-galactomannan mixtures ( % gum) were highly shear-thinning fluids with high yield stresses. The magnitudes of Z a,100 and K of rice starch-galactomannan mixtures also were much higher than those of the control, indicating that there was a higher synergism with galactomannans due to starch-galactomannan interaction. According to Alloncle and Doublier [17] and Morris [21], starch dispersions may be regarded as composite materials consisting of swollen granules (amylopectin) dispersed in a continuous biopolymer matrix (amylose). Alloncle et al. [5] indicated that galactomannans are located within the continuous phase (amylose), and thus the volume of this phase is reduced, which causes a dramatic increase in the concentration of galactomannans in the continuous phase, thereby resulting in a very high viscosity. This result is in good agreements with those found for the mixtures of galactomannan with wheat flour [22] and other starches, such as corn starch [2, 13, 14] wheat starch [2, 13], triticale starch [18], rice starch [2, 23] and yam starch [15]. From these results, rice starch-galactomannan mixtures at gum concentrations of % show a strong influence of galactomannan concentration on steady shear properties. In the comparison of K values of rice starch-guar gum and rice starch-lbg mixtures, rice starch-guar gum mixtures at high concentrations (. 0.2%) showed higher K values than rice starch-lbg mixtures (Tab. 1). This indicates that there was a higher synergism with guar gum due to its greater hydration capacity as compared to locust bean gum, as suggested by Sudhakar et al. [14]. In general, it has been known that the effect of temperature on rheological properties needs to be documented because a wide range of temperatures is encountered during processing and storage of fluid foods [24]. The effect of temperature (20 657C) on apparent viscosity (Z a,100 )ata specified shear rate of rice starch-galactomannan mixtures can be described by the Arrhenius relationship (Eq. 3), in which the apparent viscosity decreases with temperature following an exponential function. The Arrhenius temperature relationship has been confirmed experimentally in previous studies of native corn starch [25], modified corn starch [26], rice flour dispersion [27], acorn flour-guar gum mixture [28] and rice starch-xanthan gum mixture [20]. Z a,100 = A?exp(E a /RT) (3) Tab. 2. Activation energies (E a ) of rice starch dispersions with addition of guar gum and LBG at different concentrations. Gum type Concentration [%] A [ Pa?s] E a [kj/mol] Control 0 (no gum) Guar gum LBG where, Z a,100 is the apparent viscosity (Pa?s) at 100 s 21, A is a constant (Pa?s), T is the absolute temperature (K), R is the gas constant ( J mol 21 K 21 ) and E a is the activation energy (kj/mol). The magnitudes of E a and A were determined from regression analysis of 1/T versus ln h a,100. The calculated values of E a and constant A were in the range of kj/mol and Pa? s, respectively, with high determination coefficients (R 2 = ) (Tab. 2). The activation energy decreased and constant A increased with the increase in gum concentration except for 0.2% guar gum. E a values ( kj/mol) of rice starch-galactomannan mixtures were much lower than that (12.8 kj/mol) of the control (0% gum). These observed results indicate that the decrease in apparent viscosity with temperature is less pronounced at rice starch-galactomannan mixtures in comparison with the control. The E a values ( kj/mol) of rice starch-lbg mixtures were much lower than those ( kj/mol) of the rice starch-guar gum mixtures. They also changed considerably with the increase in gum concentration in comparison to rice starch-guar gum mixtures. From these observations, it could be concluded that the rheological behavior of rice starch-lbg mixtures in the gum concentration range of % was less temperature-dependent and greater gum concentrationdependent in comparison to rice starch-guar gum mixtures. 3.2 Dynamic shear properties Fig. 2 shows changes in storage modulus (G 0 ), loss modulus (G 00 ) and complex viscosity (Z*) as a function of the frequency (o) for a typical rice starch dispersion (i.e. 0% gum) at 207C. The storage modulus G 0 is a measure of the energy that is stored in the material or recoverable per cycle of deformation, G 00 is a measure of the energy that is lost as viscous dissipation per cycle of deformation and R 2

5 314 D. Yoo et al. Starch/Stärke 57 (2005) Z* is a measure of the overall resistance to flow. The magnitudes of G 0 and G 00 increased with the increase in o and G 0 was much higher than G 00 at all values of o with a small frequency dependency. An ln Z* versus ln o plot shows shear-thinning behavior following the power law model. Such behavior is in good agreement with those found in other starch dispersions [1, 16 18, 20, 25]. Tab. 3 shows G 0, G 00 and complex viscosity (Z*) at 6.3 rad/s of rice starch-galactomannan mixtures at 207C. The G 0, G 00 and Z* values of rice starch-galactomannan mixtures ( % gum) increased with the increase in gum concentration. Such increase of dynamic moduli can be attributed to the increase in viscoelastic properties of added gum, which is concentrated within the continuous phase in the starch-gum mixture systems, due to its thickening properties, as indicated by Alloncle and Doublier [17]. However, G 0 values of mixtures at 0.2% gum concentration were lower than that of the control, showing that the elastic properties can be decreased at low gum concentrations (,0.4%). Fig. 2. Plot of ln o vs. ln G 0 (s), ln G 0 (u), and ln Z*(n) for a typical rice starch dispersion (0% gum) at 207C. One characteristic value for evaluation of the viscoelastic behavior can be described as tan d stating directly the ratio of G 00 /G 0 value. A tan d, 1 indicates predominantly elastic behavior while a tan d. 1 indicates predominantly viscous behavior. As shown in Tab. 3, the tan d values of rice starch-galactomannan mixtures were lower than one, indicating that the mixtures are more elastic than viscous, in good agreement with the results found in other starchgalactomannan mixtures [16 18, 20]. The tan d values of rice starch-galactomannan mixtures also were higher than the control, and increased with the increase in gum concentration. This means that G 00 increases much more than G 0 after adding galactomannans to rice starch dispersions. The higher growth of G 00 than of G 0 was more pronounced at rice starch-lbg mixtures (tan d = ) in comparison to rice starch-guar gum mixtures (tan d = ), as shown in Figs. 3 and 4. The magnitudes (0.20 for guar gum; 0.23 for LBG) of tan d at 0.2% gum concentration also were much higher than that (0.12) of the control, indicating that there was a more pronounced effect of galactomannans on the viscous properties of rice starch dispersions. From these observations it was concluded that the viscous properties of rice starchgalactomannan mixtures were affected by the addition of gum and depended on the type of galactomannan and gum concentration. In general, the viscous properties in dependence on gum concentration can be explained by the thermodynamic incompatibility phenomena between the amylose component and the galactomannans, as noted by Alloncle and Doublier [17] and Eidam and Kulicke [29]. Therefore, the rheological behavior in rice starch-galactomannan mixture is determined by the Tab. 3. Storage (G 0 ) and loss (G 00 ) moduli, and complex viscosity (Z*) at 6.3 rad/s of rice starchgalactomannans mixture at 207C a. Gum type Concentration [%] G 0 [Pa] G 00 [Pa] Z*[Pa?s] tan d Control 0 (no gum) Guar gum LBG a Values are mean6standard deviation for triplicate measurements.

6 Starch/Stärke 57 (2005) Rheology of Rice Starch-Galactomannan Mixtures 315 Fig. 3. Plots of ln G 0 (d), ln G 00 (s) vs. ln o for rice starch-guar gum mixtures with different gum concentrations: (a) 0.2%, (b) 0.4%, (c) 0.6%, (d) 0.8%. Fig. 4. Plots of ln G 0 (d), ln G 00 (s) vs. ln o for rice starch-lbg mixtures with different gum concentrations: (a) 0.2%, (b) 0.4%, (c) 0.6%, (d) 0.8%.

7 316 D. Yoo et al. Starch/Stärke 57 (2005) characteristics of the rice starch (permanent junction zones in the network) and the characteristics of the galactomannans (temporary entanglements in the network), as desccribed by Eidam and Kulicke [29] and Kulicke et al. [16], who studied the viscoelastic properties of waxy rice starch-galactomannan and corn starchgalactomannan mixtures, respectively. According to Eidam and Kulicke [29], in the three-dimensional network the hydrocolloid molecules function as disruptive sites, resulting in network defects. The number of permanent junction zones in the starch is reduced and the overall elasticity decreases. Therefore, it was concluded that the decrease in junction zones of rice starch molecules with increasing galactomannan concentration results in an increase of the viscous properties. The dynamic rheological data of ln (G 0, G 00 ) versus ln o were also subjected to linear regression; the magnitudes of slopes and determination coefficients (R 2 ) are summarized in Tab. 3. The slopes of G 0 ( ) and G 00 ( ) were positive with high R 2 ( ). The slopes of G 00 at the control and 0.2% gum concentration were relatively much higher than those of G 0, suggesting that rice starchgalactomannan mixtures at low concentrations (,0.4%) are more elastic than viscous. While the slope values of G 0 increased with the increase in gum concentration, those of G 00 decreased. There were also differences between slope values for the control (0% gum) and rice starch-galactomannan mixtures. These observed results show that the elastic properties of rice starch dispersion can be decreased by the addition of galactomannans. The slopes (G 0 = ; G 00 = ) for rice starch-lbg mixtures were relatively higher than those (G 0 = ; G 00 = ) for rice starch-guar gum mixtures, indicating that dynamic moduli of rice starch-lbg mixtures showed the greater dependency on frequency than those of rice starch-guar gum mixtures (Tab. 4). From a structure point of view, for true gels ln (G 0, G 00 ) versus ln o plots have zero slope, while for weak gels and highly concentrated dispersions such plots have positive slopes and G 0 has a higher magnitude than G 00 with the frequence dependency as described by Ross-Murphy [30] and Clark and Ross-Murphy [31]. From these dynamic rheological data (Tabs. 3 and 4), it was found that the dynamic rheological behavior of the rice starchgalactomannan mixtures ( % gum) seems to be similar to that of weak gels because the slopes ( Pa?s) were positive and the values ( Pa) of G 0 were higher than those ( Pa) of G 00. Such dynamic rheological behaviors of rice starch-galactomannan mixtures can merely be related to the viscoelastic properties of galactomannans which give rise to a weak three-dimensional network due to associations of ordered chain segments [32]. A similar trend was reported with other starch dispersions containing guar gum [16 18], LBG [16, 17] and xanthan gum [16, 17, 20, 33]. 4 Conclusions Rice starch-galactomannan (guar gum and locust bean gum) mixtures with different gum concentrations (0, 0.2, 0.4, 0.6 and 0.8%) at 207C exhibited a high shear-thinning behavior with high Casson yield stress values (s oc ). Magnitudes of consistency index (K) and apparent viscosity (Z a,100 ) for the mixtures were influenced strongly by temperature and gum concentration. The effect of temperature on Z a,100 of rice starch-galactomannan mixtures is described well by the Arrhenius relationship with high correlations (R 2 ). E a values of rice starch-lbg mixtures were much lower than those of the control (0% gum concentration) and rice starch-guar gum mixtures, indicating that the rheological behaviors of rice starch-lbg mixtures in the gum concentration range of % are less Tab. 4. Slope values of ln G 0 and ln G 00 versus ln o curve of rice starch-galactomannan mixtures with different gum concentrations at 207C. a Gum type Concentration [%] Slope of G 0 [Pa?s] R 2 Slope of G 00 [Pa?s] R 2 Control 0 (no gum) Guar gum LBG a Values are mean6standard deviation for triplicate measurements.

8 Starch/Stärke 57 (2005) Rheology of Rice Starch-Galactomannan Mixtures 317 temperature-dependent. Based on dynamic rheological data of storage (G 0 ) and loss moduli (G 00 ) as well as complex viscosity (Z*) as a function of frequency (o) at 207C, the rice starch-galactomannan mixtures at gum concentration of % displayed the rheological behavior similar to that of weak gels because the slopes were positive and the magnitudes of G 0 were higher than those of G 00 at all values of w applied. The values of dynamic moduli (G 0, G 00 and Z*) were influenced by the addition of galactomannans, increased with the increase in gum concentration. The effect of galactomannans on the viscoelastic properties in the rice starch-galactomannan mixture systems appeared to be related to the thermodynamic incompatibility of galactomannans with the amylose component in starch. 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