Characterization of galactomannans extracted from seeds of Gleditsia triacanthos and Sophora japonica through shear and extensional rheology: Comparison with guar gum and locust bean gum

Abstract

The rheological behaviour, under steady and dynamic shear and extensional conditions, of two non-conventional galactomannans isolated from Gleditsia triacanthos and Sophora japonica is determined and compared to those of locust bean gum and guar gum.

The studied galactomannans exhibit shear-thinning behaviour in the range of concentrations and shear rates evaluated. For similar concentrations and shear rates, the guar gum exhibits the highest viscosities. Experimental data in steady shear was correlated with the Cross model, which provided a good description of viscosity shear rate data. Under oscillatory shear, all gum solutions exhibited a behaviour typical of random-coil polysaccharide solutions.

The extensional rheology experiments showed that by increasing the polymer concentration and decreasing the temperature, the relaxation times, elastic moduli and rupture times increase.

These findings lead to the conclusion that G. triacanthos and S. japonica galactomannans can be used as efficient thickening hydrocolloids as alternative to conventional galactomannans.

Introduction

Galactomannans, storage polysaccharides isolated from the seed endosperm of some Leguminosae, are composed of a linear mannan main chain with side chains of a single galactose (Dea & Morrison, 1975).

The two galactomannans of major commercial importance are Guar Gum (GG) from Cyamopsis tetragonolobus and Locust Bean Gum (LBG), from Ceratonia siliqua, which have mannose/galactose ratios (M/G) of 1.8:1 and 3.5:1 (Dakia, Blecker, Roberta, Watheleta, & Paquota, 2008) and molecular weight in the range of 0.697 × 10⁶–0.94010⁶ Da (Parvathy, Nuggehalli, & Tharanathana, 2007) and 0.856 × 10⁶–1.20 × 10⁶ Da (Dakia et al., 2008), respectively. They are extensively applied in the food industry as thickening and stabilizing agents, due to their low cost and wide range of functional properties (Cheng, Brown, & Prud'homme, 2002).

Galactomannans have the ability to form very viscous solutions at low concentrations, which are lightly affected by pH, addition of electrolytes and heat processing (Sittikijyothin, Torres, & Gonçalves, 2005). These solutions usually exhibit a non-Newtonian behaviour, in which the viscosity decreases with the increase of shear rate (Garti, Madar, Aserin, & Sternheim, 1997). The Cross (1965) model has been successfully used to describe the shear-thinning behaviour of most galactomannans (da Silva et al., 1992, Sittikijyothin et al., 2005). When associated with other polysaccharides, such as xanthan gum and kappa-carrageenan, galactomannans can form gels with new properties (Andrade et al., 2000, Azero and Andrade, 2006, Bresolin et al., 1999, Casas et al., 2000, Fernandes et al., 1991 and Hernandez, Dolz, Dolz, Delegido, & Pellicer, 2001).

Their functional properties are connected to their molar mass and structural features: the solutions' viscosity depends mainly on the molar mass while the synergistic interactions seem to be determined by the mannose/galactose ratio (M/G) and fine structure of galactomannan chain (Dea et al., 1986, Fernandes et al., 1991, Schorsch et al., 1997).

Since the hydrocolloids are used to modify textural attributes, the study of their rheological behaviour is essential as it is recognized that rheological properties play an important role in process design, evaluation and modelling. Furthermore, rheological data is required for calculation in any process involving fluid flow (e.g. pump sizing, extraction, filtration, extrusion, purification) and play an important role in the analyses of flow conditions in food processes such as pasteurization, evaporation, drying and aseptic processing (Marcotte, Taherian, Trigui, & Ramaswamy, 2001).

While considerable work has been published on the rheological characterization of LBG and GG under shear flow (Choi and Yoo, 2008, Doublier and Launay, 1981, Patel et al., 1987, Sittikijyothin et al., 2005), the work done with non-traditional sources of galactomannans is scarce.

Mazzini and Cerezo (1979) first reported the presence of galactomannans in the seeds of Gleditsia triacanthos (a Leguminosae). This species can tolerate a wide range of climatic and soil conditions (Blair, 1990) and is spread in America, Middle Europe and Mediterranean countries (Üner & Altınkurt, 2004). Its seeds are composed of testa (27%), embryo (29%) and endosperm (34%) and galactomannans are the main polysaccharide constituents of the endosperm (Manzi, Mazzini, & Cerezo, 1984). The galactomannan of G. triacanthos used in this study was previously characterized in terms of M/G ratio, intrinsic viscosity and viscosity average molar mass and the obtained values were 2.82:1, 1042 ml/g and 1.62 × 10⁶ Da, respectively (Cerqueira et al., 2009). Sophora japonica (also a Leguminosae) is native from China, where it is widely cultivated and used as a hemostatic agent in traditional medicine and is currently spread all over the word (Ishida et al., 1989, Tang et al., 2001). The galactomannan extracted from S. japonica used in this study has been previously characterized and exhibits an unusually high M/G ratio (5.75:1), an intrinsic viscosity of 1001 ml/g and a viscosity average molar mass of 1.34 × 10⁶ Da (Cerqueira et al., 2009).

Extensional rheology can be used to complement data obtained from conventional shear rheometers and to fully characterize fluid properties (Odell & Carrington, 2006). It is much less studied, partly due to difficulties in creating a pure extensional flow. However, the extensional component is an important part of many food-processing operations, and it may be also very useful for consumer perception studies and quality evaluation of food products. Operations such as mixing, homogenization, sheeting, extrusion, extrudate expansion, baking expansion, flow in and out of constrictions and coating are all examples of processes with significant extensional component. From the consumer's standpoint, spreadability, pourability, mouthfeel, stinginess and stretchability are important attributes where extensional rheology dominates the textural perception (Padmanabham, 2003). An extensional deformation differs significantly from a shear deformation. During a shear deformation, the particles within the flowing body move in the same direction and slide over each other, while during an extensional deformation, as the material is either stretched or compressed, the particles within the fluid will either move away or towards each other (Chan et al., 2007). Contrary to shear rheology, in extensional measurements the cross-sectional area of the sample undergoes a modification as a consequence of the exponentional change in length. Therefore, the extensional rheology allows the evaluation of interfacial multicomponent phenomena (Steffe, 1996).

The extensional properties of a fluid can be quantified by various means, including a Capillary Break-up Elongation Rheometer (CaBER), as in the present work. The rheological analysis with a CaBER is based on the formation of an unstable fluid filament by imposing a rapid axial step-strain of prescribed magnitude. The filament is then allowed to relax until breakup under the action of its own dynamics. The relaxation and decay of the necked sample is governed by different forces, e.g. viscous, elastic, gravitational and capillary forces (Sujatha, Matallah, Banaai, & Webster, 2008). This technique allows the creation of a pure extensional flow and can be applicable to fluids over a wide range of viscosities.

Little is known about the extensional behaviour of galactomannans. In the present work, two non-conventional galactomannans were isolated from G. triacanthos and S. japonica seeds, and their rheological behaviour, in steady and dynamic shear and in extensional conditions was determined and compared with those of guar gum and locust bean gum, in order to investigate their effectiveness as alternative thickening hydrocolloids. The flow curves of each gum have been correlated with the Cross model.

The results of this study will contribute to the research of novel renewable sources of hydrocolloids, as alternatives to the traditional ones, and to the development of novel foods, addressing the claims of the modern consumer.