Dynamic rheological properties of sesame protein dispersions

Interest in utilizing new sustainable protein sources is increasing, and understanding the rheological behavior of sesame protein can be useful in further application. Small amplitude oscillatory shear measurements of sesame protein dispersions at varying concentrations of 5.0%, 7.5%, and 10.0% were examined at both linear and nonlinear regions. Although the sesame protein dispersions showed pseudoplastic behavior, the shear‐thickening effect of sesame proteins at higher concentrations makes it possible to apply in beverages without increasing viscosity. The low values of the yield point of the sesame protein explained its feasibility for utilization in varying products such as high‐protein‐enriched beverages without the adverse effect of the high viscosity. The fracture stress and strain indicated the high strength of the sesame protein to the mechanical changes. The mechanical properties of the sesame proteins also confirmed typical strong gel behavior. The complex viscosity (η*) was decreased linearly with frequency demonstrating the shear thinning phenomenon. The frequency dependency of the protein was shown a low n value that explains a relatively elastic gel structure. These rheological characteristics of the sesame proteins might be more reliable than the previous works on the static rheological behavior, which provides a new horizon in the application of a sustainable protein in food industries.


| INTRODUCTION
The importance of food security and the growth of the global population has increased the research on the utilization of sustainable food protein sources (Amagliani et al., 2016). The protein demands require alternative protein supplements that should be inexpensive and more functional. Plant proteins have an increasing trend because of their low cost, unlimited supply chain, sustainability, functional properties, and health benefits (Sá et al., 2020;Shen et al., 2021). As a result, plant proteins find many applications in food products such as meat products, bars, bakeries, snacks, candies, and beverages (Curtain & Grafenauer, 2019). Furthermore, plant proteins have a great supplement market size of 4.79 billion $ in 2019, and it would be expected to reach 7 billion $ by 2027 (Fortune Business Insights, 2021). In contrast, agro-industrial wastes and by-products can be considered sustainable and convenient ingredients in plant protein production such as cereals, seeds, and legumes (Sá et al., 2020). Sesame (Seamum indicum L.) is a key oilseed with high potential for nutritional and medical purposes. It has several bioactives, that is, protein, oil, lignans, and tocopherols, which are all benefits in nutritional health. However, sesame is rich in oil and fatty acids (37%-50%) but also contains a high amount of proteins (33%-40%) varying on its cultivars (Morris et al., 2021). Sesame proteins are mainly globulins (67.3%), albumins (8.6%), prolamines (1.4%), and glutenins (6.9%), which can be considered promising sustainable protein sources that have functional behaviors such as high water solubility, emulsifying, and foaming properties that increased its practical application (Özdemir et al., 2022).
The primary sesame protein is the insoluble 11S, which accounts to 60%-70% of the total proteins (G omez-Arellano et al., 2017). Furthermore, sesame protein contains various endo-and exo-peptidase with a high activity that mainly exists in the water-soluble protein fraction (Chen et al., 2021). Sesame proteins are also considered complete proteins because of their richness in essential amino acids as well as high methionine and tryptophan content that can easily be digested and absorbed by the gastrointestinal tract, which accounts for 30% of the amino acids and making it an ideal plant protein source (Achouri et al., 2012;Di et al., 2022;Görgüç et al., 2020;Sá et al., 2020).
The rheological and textural behavior of food can be improved by using the sesame protein because of its polymeric nature and making it acceptable for consumers (Escamilla-Silva et al., 2003). The viscoelastic properties of protein dispersion mainly depend on the intrinsic factors, that is, molecular size, shape, ionic strength, and surface charge as well as extrinsic factors such as pH, temperature, and shear history (Kinsella, 1982). Protein dispersions are mainly used at high concentrations, because the protein-protein interactions are the most critical factor in protein viscosity, which increases exponentially with the protein concentrations (Damodaran et al., 2007). Furthermore, the rheological properties of proteins at high concentrations are a challenge, and different techniques such as cavity rheometer were provided to measure the plant proteins' rheological behavior (Wittek et al., 2021).
Although the rheological behaviors of protein dispersions can be determined by conventional geometries, the sesame proteins, because of their large particle size, cannot be presumed as homogenous, and therefore, several researchers have used vane and impeller geometries (Bbosa et al., 2017;Gögus et al., 2006;G omez-Arellano et al., 2017).
However, the functional and physicochemical properties of sesame protein concentrate and isolate have been investigated (Achouri et al., 2012;Saini et al., 2018); the research on its rheological behavior was rarely addressed (G omez-Arellano et al., 2017). Although the effect of pH, concentration, and ionic strength on the viscoelastic properties of sesame proteins with a rotational rheometer with a special helical agitation system has been determined, the dynamic rheological properties of sesame proteins that are critical in food processing and formulation did not consider. To the best of our knowledge, nobody evaluates the dynamic rheological behavior of sesame proteins. Therefore, the aim of the current work was to evaluate the mechanical and rheological properties of sesame proteins at dynamic conditions.

| Materials
Sesame seed was purchased from a local market in Mashhad County (Iran) and transferred to the Research Institute of Food Science and Technology (RIFT). Sodium hydroxide, hydrochloric acid, and hexane were obtained from Merck, Germany (Merck KgaA, Darmstadt, Germany). All laboratory materials used in the experiment were of laboratory grade.

| Sesame protein extraction
Sesame protein was extracted from sesame seed according to the previous methods with some slight modifications (Di et al., 2022;Mir et al., 2020). The sesame seed was initially dedulled and powdered by Moulinex miller (Model depose 00022, France), and then the sesame powder was defatted using hexane at a ratio (1:10 w/v) along with stirring for 3 h. Defatted sesame was filtered twice, dried, and passed through an 80-mesh sieve (US Standard sieve). The sesame protein was obtained by the alkaline method from the defatted sesame powder (Deng et al., 2019). The samples were diluted with distilled water at a ratio of 1:10 (w/v), and the pH was adjusted to 9.5. Then, the mixture was unremittingly stirred in a bath shaker with the speed of 360 rpm for 3 h at 50 C and centrifuged (Orto Alersa brand, Digicen 21 model, USA) at 4000 g for 30 min at ambient temperature ($23 C). The supernatant was removed, and the pH was adjusted to the isoelectric point of the protein (4.0). The samples were recentrifuged at 8000 g for 30 min; the precipitate was suspended with distilled water, neutralized with 1 M NaOH, and finally freeze-dried (Freeze dryer FDU-8624, Operon, Gimpo City, Korea) and stored at À5 C.

| Chemical composition and protein content
The protein content of sesame protein dispersions was determined by the Kjeldahl method (AOAC, 2002). The Kjeldahl digestion and distilling system (B-322, BÜCHI, Co., Switzerland) was used to the determine nitrogen content of the protein samples through the use of the conversion factor of 6.25 (AOAC, 2002).
The moisture, ash, crude fiber, and fat content were determined by the Association of Official Analytical Chemists methods (AOAC, 2002). The carbohydrate content was also calculated by subtracting the amount of other compounds from 100.

| Rheological experiments
The rheological properties of the protein dispersions were measured by using a stress-controlled rheometer (Model 301 MCR, Anton Paar Co., Germany), which was fitted with a cone and plate geometry (CP50-2) and a gap size of 0.21 mm. The rheometer was also equipped with a thermostat bath controller that permits to control of the temperature precisely. The sesame protein dispersions at different concentrations of 5%, 7.5%, and 10% were prepared with distilled water and mixed thoroughly by a magnetic stirrer (Copens, Model Ms-300 Hs, South Korea) with a speed of 360 rpm for 60 min at 25 C.
The dispersions were kept overnight in a refrigerator (4 C) to complete the hydration prior to the rheological measurements.
In order to determine the linear viscoelastic region (LVE) of the dispersions, the changes in elastic (G 0 ) and viscous (G 00 ) moduli as a function of strain (0.1% to 100%) were measured at constant frequency 1 Hz and 20 C.
Small amplitude oscillatory shear (SAOS) measurements were also used to determine the mechanical properties of the protein dispersions. The viscoelastic properties of sesame protein dispersions were determined by using the frequency range of 0.01 to 100 Hz in the linear viscoelastic region. A frequency sweep test of the viscous and elastic behavior of the material at 0.5% strain was selected for all concentrations of sesame proteins. The experiments were performed at an ambient temperature of 20 C at a constant strain of 0.5%. The degree of frequency dependence of G 0 is considered an indication of the viscoelastic nature of a gel. Thus, the degree of frequency dependence of elastic modulus (Equation 1) was determined by a Power-law model as follows (Rafe & Razavi, 2017): where k is a constant (Pa.s n ), n is the slope of G 0 versus ω (À), and ω is the oscillatory frequency (Hz).

| Chemical composition of the protein
As it can be found, the protein content was 65 ± 1.5%, which is in line with the works of Kahyaoglu and Kaya (2006) and G omez-Arellano et al. (2017). However, the fat and the fiber were not detected in the samples; the moisture and carbohydrate were 7.5 ± 1.7% and 22.0 ± 0.7%, respectively. The mineral content of the sesame protein was 5.4 ± 0.5%.

| Strain sweep experiments
Dynamic rheological properties of sesame protein dispersions at different concentrations of 5.0%, 7.5%, and 10% were determined over strain 0.1% to 100% in linear and nonlinear regions at frequency of 1.0 Hz and 20 C. The changes of elastic (G 0 ) and viscous moduli (G 00 ) as a function of strain were provided in Figure 1. The G 0 value was greater than G 00 over the strain limit, and no crossover occurred, which explains the interconnection network structure of the protein dispersions. Therefore, the more pseudo-solid state of the proteins indicates the elasticity and energy storage capability of the protein dispersion (Rao, 1999). By increasing sesame protein, both G 0 and G 00 were increased, which has been also observed in the shear-thickening effect of sesame proteins at higher concentrations of 5% (G omez-Arellano et al., 2017). However, the LVE region of sesame proteins is to some extent large and approaches to about 5%; it was lower than many animal proteins such as whey (Rafe et al., 2016) and β-lactoglobulin  and some hydrocolloids such as alginate/guar gum (Mousavi et al., 2020). In contrast, it was higher than plant proteins including rice bran protein (Rafe et al., 2014) and basil seed gum . Thus, the G 0 , G 00 , and tan δ in the LVE region of sesame protein dispersions were measured and provided in Table 1. As can be seen, all of these parameters were also increased by concentration, and tan δ as an indicator of the physical state of the system was in the range of 0.35-0.38, which exhibited the nature of the viscoelasticity of the proteins.
Although strain 0.5 can be considered an LVE region in which the viscoelastic parameters are independent of the applied strain, the nonlinearity of the protein dispersions provides invaluable information concerning the yield behavior (Knudsen et al., 2006). Therefore, the yield or critical strain (γ) of the protein dispersions was measured by drawing the asymptotes, the low and large strain values of the elastic modulus, and the intersection point was considered as the yield point protein utilization in varying products such as high-protein-enriched beverages without the adverse effect of the high viscosity (Sze-Tao & Sathe, 2000). Furthermore, the fracture/yield strain or stress can also give more knowledge of the sesame proteins and depicted the elastic stress versus strain . As a result, the elastic stress versus strain at different concentrations of sesame protein is shown in Figure 2, and the values of the fracture stress/strain were calculated and given in Table 1. The protein dispersions showed a narrow range of fracture stress from 4.35 to 5.05 kPa, which is lower than many foods such as marshmallow (9 kPa), Monterey cheese (28 kPa), Cheddar cheese (43 kPa), and jelly candy (190 kPa) (Mousavi et al., 2020). Among the samples, the highest fracture strain of $3.0% and the corresponding fracture stress of 4.35 kPa were obtained for the protein dispersion of 10.0%. Furthermore, the maximum elastic stress or fracture stress was achieved for the same protein dispersion of 10%, revealing the highest intermolecular interactions between protein molecules.

| Mechanical properties
The elastic modulus and viscous moduli show the amount of energy stored and lost in the material during the deformation cycle, respectively. Frequency sweep of G 0 , G 00 , and dynamic viscosity (η*) for the sesame protein dispersions at varying levels are illustrated in Figure 3.
Both moduli were increased by increasing frequency, and the G 0 was greater than G 00 over the entire frequency range. Consequently, because of the G 0 > G 00 and lack of crossover point in the frequency range, the protein dispersions displayed a highly interconnected gellike structure (Zhang et al., 2016).
Furthermore, the results confirmed a typical weak gel behavior in which G 0 and G 00 dependent on frequency over four decades and the loss tangent (tan δ) approaching a value of $0.38. Tan δ reveals the differences in the temporary network structures of a gel (Shim & Mulvaney, 2001 it can be used to discover the type of gel developed. The n value is F I G U R E 2 Effect of increasing strain on elastic stress (G 0 γ) of sesame protein dispersion at different protein concentrations. Dashed lines indicate the maximum elastic stress, which was obtained in the strain range studied.
F I G U R E 3 Mechanical spectrum of sesame protein dispersions at different concentrations (γ = 0.5%, temperature = 20 C).
considered an indication of the viscoelastic nature of the gels and depends on the strength and gel structure. It is zero for purely elastic or covalent gels and becomes more with increasing relative contribution from the viscous component (less elastic), suggesting a physical gel (Foegeding et al., 2006). The n values for the sesame protein dispersions at different concentrations of 5.0%, 7.5%, and 1.0% were 0.13-0.16 revealing a relatively elastic gel structure.

| CONCLUSIONS
The rheological behavior of the sesame protein dispersions was investigated as a function of concentration. Sesame proteins displayed a high amount of protein that clearly exhibits a sustainable plant protein that can be used in many food products. The LVE region of sesame proteins is to some extent large and approaches to about 5% but is lower than the rice bran protein. The sesame protein showed low values of yield point that explained their feasibility utilization in varying products such as high-protein-enriched beverages without the adverse effect of the high viscosity. The protein dispersions showed a narrow range of fracture stress from 4.35 to 5.05 kPa, which is lower than many foods. The G 0 was greater than G 00 and lacked a crossover point in the frequency range, displaying a highly interconnected gellike structure in the protein dispersions. Complex viscosity (η*) was decreased linearly with frequency demonstrating the shear thinning phenomenon. Consequently, it can be inferred that the elastic behavior of the sesame protein dispersions comes from the electrostatic interactions between the protein molecule, and the sesame protein characteristics would permit the application of the sesame protein in varying food products without affecting the texture, whereas including the desirable sensorial and nutritional properties in preparation the tailor-made protein dispersions.

DATA AVAILABILITY STATEMENT
None.