Rheological Characterization of Binary Combination of Gleditsia…

Abstract

In the current study, Gleditsia triacanthos gum and tapioca starch were combined and rheological characterization of the binary mixture was performed. Gleditsia triacanthos gum at four different concentrations (0.1, 0.4, 0.7, and 1.0%, w/v) was mixed with tapioca starch at a constant level (2.5%, w/v). Steady shear and dynamic shear rheological properties of samples were determined using a stress/strain controlled rheometer at a constant temperature (25ºC). Oswald de Waele model was used to describe the shear rate effect on apparent viscosity values of the samples. Increase in the Gleditsia triacanthos level increased the apparent viscosity and consistency coefficient of the samples. Similarly, dynamic shear rheological parameters of the samples increased with the increase of Gleditsia triacanthos gum. The concentration of 0.7% for Gleditsia triacanthos gum was determined to be cross point for the liquid–solid phase. From this level, the elastic behavior became more dominant compared to viscous behavior because tangent delta was equal to one in this concentration. To describe the effect of Gleditsia triacanthos level on the studied rheological parameters, the Power law and exponential type models were used.

Keywords:

Tapioca starch which is produced with the extraction from cassava (Manihot esculenta) roots found in equatorial regions between the Tropic of Cancer and the Tropic of Capricorn, is often used in certain food products because it is cheap, safe, and natural. Tapioca starch is different from other starches because it contains a low level of residual materials (fat, protein, and ash), lower amylose content and high molecular weights of amylose and amylopectin. As a dry, white, and tasteless ingredient in foods, native, and modified tapioca starches have been widely used as a thickening and gelling agent.[Citation1]

Galactomannans which are mostly found in seed endosperm of Leguminosae composed of a linear mannan backbone having side chains of a single galactose unit.[Citation2] According to botanical origin and mannose (M): galactose (G) ratio, galactomannans differ from each other just like guar gum (M:G, 2:1),[Citation3] tara gum (M:G, 3:1),[Citation4] and locust bean gum (M:G, 4:1).[Citation5] They are widely used in the textile, pharmaceutical, biomedical, cosmetics, and food industries due to their different techno-functional properties mainly as thickening and stabilizing in a range of applications.[Citation6,Citation7] Mazzini[Citation8] reported the presence of galactomannans in the seeds of Gleditsia triacanthos (Gt). Sciarini et al.[Citation9] investigated the effect of three extraction procedures on the chemical composition and functional properties of Gt gum. The rheological behavior, under steady and dynamic shear and extensional conditions of galactomannans isolated from Gt was determined by Bourbon et al.[Citation10] Their results showed that by increasing the polymer concentration and decreasing the temperature, the relaxation times, elastic modulus, and rupture times increased. Cengiz et al.[Citation11] investigated the synergistic interactions between Gt gum and some commonly used hydrocolloids (xanthan, κ-carrageenan, carboxymethyl cellulose, and alginate) with two different hydration temperatures (25 and 80°C) and they showed the there was a good synergistic interactions between Gt gum and other selected hydrocolloids and the best synergism was in the binary mixture of Gt:xanthan gum.

Galactomannans are widely used in starch-based food formulations to improve final quality of foods. Starch-galactomannan mixtures exhibit many functions to control rheological and textural properties of foods and to improve moisture retention and reduce cost of production. Chaisawang and Suphantharika[Citation12] studied effects of guar and xanthan gums on pasting and rheological properties of native and anionic tapioca starches. They reported that addition of guar and xanthan gums increased peak, breakdown, and final viscosities of native tapioca starch. Though xanthan gum shows negative effect on addition of anionic tapioca starches, guar gum had similar results. Adamu and Jin[Citation13] investigated the effect of some chemical agents on some physical and rheological properties of starch-guar gum mixtures and they reported that the added agents had significant effect on the physical and rheological properties of the extrudates. Ahmed et al.[Citation14] reported that the effect of guar gum which is a common galactomannan on the mixture of Arabic gum and they reported that the gum concentration had significant effect on the rheological parameters of the solution. Karaman et al.[Citation15] reported that the synergism of the galactomannan based guar gum and other gums was related to the structure of the gum. Chantaro and Pongsawatmanit[Citation16] investigated the changes in pasting and gelatinization behavior of tapioca starch on addition of xanthan gum. During the gelatinization process, xanthan affects storage modulus (G′) and loss modulus (G″) of tapioca starch dispersions. As well increasing both, a more solid-like behavior occurred because of higher storage modulus (G′) compared to loss modulus (G″). Additionally, the final viscosity, peak viscosity, and breakdown values of mixtures increased with increasing xanthan gum substance. Viscoelasticity and texture stability of Chinese shrimp dumpling dough during storage at 4°C were studied by Seetapan et al.[Citation17] They reported that addition of modified tapioca starch and xanthan gum on dough formulations could be beneficial for the shrimp dumpling dough, during the storage process having frozen/chilled. The main aim of the present study was to characterize the steady and dynamic shear rheological properties of Gt galactomannan and tapioca starch mixtures and to determine and model the effect of Gt concentration using Power law and exponential-type models.

Tapioca starch was purchased from a local market in Turkey. Gt pods were collected from the trees in Kayseri, Turkey.

Seeds of Gt were separated from the pods manually and washed until the impurities are removed. Washed seeds were dried and stored at a room temperature until use. Gt galactomannan was extracted from the seeds according to procedure described by Cerqueira et al.[Citation18] First, the seeds were milled using a laboratory type grinder (Herzog-Milling, HPF, Germany). Ground seeds were mixed with the distilled water (1:40 w/v) and boiled for 30 min. The mixture was filtered using a layer of gauze and upper residue was repeatedly boiled to achieve complete extraction. Filtrate was centrifuged for 20 min (Nüve, 800R, Turkey) at 2000 × g to remove impurities. After precipitation of supernatants with ethanol (96% v/v), the Gt galactomannan was dried in an oven (Nüve, ES120, Turkey) at 30°C for 12 h. The dried material was milled using a grinder (Herzog-Milling, HPF, Germany) and the Gt galactomannan powder was stored at room condition before analysis. With different concentrations of Gt (0, 0.1, 0.4, 0.7, 1%) and tapioca starch (2.5%), mixtures were prepared. Weighed samples were dissolved in cold water and the solution temperature was rapidly increased to 85°C. The solution was kept at 85°C for 15 min and cooled at room temperature. After waiting 24 h for complete hydration, rheological measurements were performed.

Rheological analyses were carried out using a strain/stress controlled rheometer (Thermo-Haake MARS III, Karlsruhe, Germany) equipped with a Peltier heating system. The samples were sheared using a plate–plate configuration (diameter 35 mm and gap 0.500 mm). The apparent viscosity of solutions was recorded in the range of 1–100 s−1 shear rate at 25°C. During the shearing, a total of 24 data points were recorded at 10 s intervals. Each measurement was replicated with two repetitions. The Oswald de Waele model was used for the calculations of consistency coefficient and flow behavior index as following.

(1) η=K(γ˙)n−1(1)

where ηa is the apparent viscosity (Pa s), K is the consistency coefficient (Pa sn), γ˙ is shear rate (1/s), and n is flow behavior index.

Dynamic shear rheological characteristics of Gt-tapioca starch solutions were determined using a strain/stress controlled rheometer (Thermo-HAAKE, Rheostress 1, Germany) equipped with a temperature-control unit (Thermo-HAAKE, Karlsruhe K15 Germany). The measurements were carried out using a plate–plate configuration with a plate diameter of 35 mm and a gap of 0.500 mm. Sample was placed between the plate–plate geometry and the measurement was started immediately.

Before starting to the frequency sweep tests, stress sweep test was applied to see the linear viscoelastic region (LVR) of sample solution. Then, the frequency sweep test was conducted for all samples using a dynamic oscillatory shear rheometer. Dynamic shear measurements were performed in the frequency range of 0.1–10 Hz at a constant shear stress (0.2 Pa, in LVR) in the LVR and constant temperature (25°C). Dynamic shear values recorded at the frequency values ranging between 0.1–10 Hz were used to evaluate the dynamic mechanical spectra of the samples. Each measurement was replicated three times with two repetitions.The dynamic mechanical spectra parameters of G′ (storage modulus) and G′′ (loss modulus) were calculated using the following equations:[Citation19]

(2) G′’=’G’×’cosδ(2)

(3) G′′’=’G’×’sinδ(3)

Loss tangent, which is a dimensionless number giving a clear indication of whether the material behavior is solid-like or liquid-like, was determined using the following equation.[Citation20]

(4) tanδ’=’G′′/G′(4)

Equations of complex modulus G* and complex viscosity η* as following can be used to characterize the overall response of the sample against to the sinusoidal strain:

(5) η∗=’G∗/ω(5)

The variation of apparent viscosity with concentration (C) can be described by several models.[Citation21,Citation22] In our study, the variations of the other parameters with concentration were described by the following models. In this respect, the most common used models are generally Power law[Citation23] and exponential-type (Exp) models as follows:

(6) f1,PLC=η50=η1Ca1→γ˙=50s−1(6)

(7) f2,expC=η50=η2expa2C→γ˙=50s−1(7)

All statistical calculations were carried out using the Statistical Analysis System (SAS) Software. One-way analysis of variance (ANOVA) was applied using the general linear model procedure. Duncan multiple range test was used to show the differences among mean values with the significance level of 0.05.

Changes in apparent viscosity and shear stress versus shear rate of Gt-tapioca starch mixed solutions with different concentrations at constant temperature (25°C) are shown in . As the apparent viscosity decreased with the increase in shear rate, all the Gt-tapioca starch mixtures showed non-Newtonian shear-thinning behavior. Choi and Chang[Citation24] studied steady and dynamic shear rheological properties of buckwheat starch-galactomannan mixtures and they reported that all the mixtures showed shear-thinning behavior. Rice starch-galactomannan mixtures showed high shear-thinning flow behaviors with high Casson yield stress.[Citation25] shows the steady shear rheological parameters of Gt-tapioca starch mixed solutions. As shown in , apparent viscosities of mixtures increased with the increase in Gt gum concentrations. Highest apparent viscosity value (1.556 Pa) at 50 s–1 was determined in 1% Gt + 2.5% tapioca starch mixtures while the apparent viscosity of Gt free tapioca starch solution (2.5% w/v) was 0.176 Pa s. Kim and Yoo[Citation26] studied rheological and thermal effects of galactomannan addition to a corn starch paste. Dynamic shear modulus (G′, G″), and complex viscosity (η*) of a corn starch-galactomannan mixture increased with the increase of gum concentrations. All samples with a Gt gum revealed higher shear stress value than tapioca starch solution and they increased with the increase of the amount of Gt gum. This is confirmed by the consistency coefficient (K) for the Power law model calculated with for tapioca starch-Gt gum mixtures, which are clearly higher than is the case for tapioca starch. The smaller values of the flow behavior index found for all samples with Gt gum compared to tapioca starch mixtures () mean that they are having tendency to shear-thinning behavior.

TABLE 1 Steady shear rheological parameters of Gt-tapioca starch mixed solutions

FIGURE 1 Apparent viscosity and shear stress changes versus shear rate of Gt-tapioca starch mixed solutions.

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Starch-galactomannan influences have been reported by many authors[Citation25–Citation29] and they concluded that the galactomannans produce high viscosity at a low concentration because of having long, soluble, rather rigid chains that have a large hydrodynamic volume.[Citation30] Interactions between tapioca starch exudates (amylose) and Gt gum can be explained by the mechanisms of influence of gum on the physical properties of starch granules, such as size, shape, and granule integrity, as well as the amount of exudate from the starch granules.[Citation31] Flow behavior index (n) of tapioca starch was determined as 0.519 to be highest compared to all Gt added mixture samples. Because their flow behavior index values were calculated to be lower than unity, all mixtures showed shear thinning behavior. Depending on the gum concentrations in the mixtures, the consistency coefficient of samples changed to a great extent. Consistency coefficient value of Gt-tapioca starch mixtures changed between 1.093 and 17.070 Pa sn. This result proved the viscosity increasing of tapioca starch, going on Gt gum addition, with high coefficient of determinations.

shows dynamic mechanical spectra values of Gt-tapioca starch mixed solutions at different concentrations. As is seen in the table, dynamic modulus (G′ and G″) of tapioca starch-Gt galactomannan mixtures were much higher than those of the control sample which is sole starch dispersion (). Storage modulus of sole starch dispersion was 0.417 Pa while the storage modulus value of 1% Gt+2.5% tapioca starch was 11.800 Pa. Increase in the Gt provided a tremendous increase in the storage modulus value of the sample. Similar increase was observed in loss modulus depending on Gt concentration. With a 0.129 Pa s, the lowest complex viscosity was measured in 2.5% tapioca starch sample. As is shown in , addition of Gt gum to the tapioca starch mixtures considerably affected the complex viscosity of the mixed solutions. Increasing the Gt galactomannan concentration in tapioca starch mixtures comparatively causes the complex viscosity rises as an example of 1% Gt galactomannan + 2.5% tapioca starch solutions in which complex viscosity (the highest one) was measured as 2.423 Pa s. Similar results were also evaluated with other starches mixed with galactomannans in studies.[Citation24,Citation25,Citation28,Citation29] As can be seen from the tangent delta values, the concentration of 0.7% for Gt gum was determined to be cross point. From this point, the elastic behavior started to be dominant and solid like behavior started to occur due to the increase in gel-like structure in the Gt-tapioca starch mixture ().

TABLE 2 Dynamic mechanical spectra values of Gt-tapioca starch mixed solutions

FIGURE 2 Changes in dynamic rheological properties of tapioca starch-Gt gum gels at different concentrations.

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FIGURE 3 Complex viscosities of tapioca starch-Gt gum gels at different concentrations.

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FIGURE 4 Changes in G′ and G″ for tapioca starch-Gt galactomannan mixtures as a function of gum concentrations.

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The Power law and exponential models were used to determine the effect of tapioca starch concentration on steady and dynamic shear rheological parameters. Values of the parameters of Power law and exponential-type relationships are also shown in . As shown in the table, determination coefficients (R2) were obtained between (0.753–0.998) for exponential and (0.728–0.997) for Power law models. It could be said Power law model was slightly better for the fitting of the data to estimate the Gt gum concentration compared to exponential one.

TABLE 3 Effect of tapioca starch concentration on steady and dynamic rheological parameters

It was concluded that Gt gum and tapioca starch showed good interaction and synergism. An increase in the Gt level provided an increase in the apparent viscosity and consistency coefficient of mixture samples. Also, Gt caused an increased in the viscoelastic properties of mixture samples. The concentration of 0.7% for Gt gum was determined to be cross point. From this level, the elastic behavior became more dominant than viscous behavior because tangent delta was equal to one in this concentration. To describe the effect of Gt level on rheological parameters, Power law and exponential-type models were used and they described the recorded data effectively.

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