PHYSICOCHEMICAL AND RHEOLOGICAL MODELLING OF SYNERGISTIC INTERACTION OF SOME PLANT GUM EXUDATES
The visco-elastic properties of Khayasenegalensis (KS gum), Anacardium occidentale (AO gum) and Acacia Senegal (AS gum) blends in dilute solution were investigated. The gums were evaluated for intrinsic viscosity and the elastic component. Molecular conformation of the complex of AS-KS was assessed by the power law and the results showed that the b values for AS 40%―KS 60% and AS 20%― KS 80% given as 1.98 and 1.67 which were significantly larger than that of the pure gums and all other blend used within the course of this research. A 20% AS–80% KS blend exhibited the strongest attraction between Khaya gum and gum Arabic molecules since the blend had the highest value of intrinsic viscosity (69.1 dL/g), while 80% AS–20% AO blend had the least attraction as evidenced by the polymer miscibility coefficient (0.0009), the elastic component and a positive Huggins coefficient found to be less than one, indicating there is no aggregation in the blend. The power-law model was successfully applied to predict the molecular conformation of AS, AO and KS alone in dilute solutions and they exhibited random coil conformation except for AO gum. Their hydrodynamic interaction value indicates that the blends become more soluble in water when more concentration of gum Arabic is added to the blends (AS:AO, AS:KS). The power-law coefficient decreased with an increased addition of the KS fraction in the blends, suggesting a more flexible AS-KS, AS-AO complex dependent on KS and AO respectively. FTIR and GCMS analyses were carried out for proper characterization of the components of the gum blends, other analysis included rheological study on the gum with corresponding effects from increased temperature and effects of added salts. Physicochemical properties of the gum blends as well as the pure gums were determined, these included salinity, pH, turbidity, total dissolve solid, conductivity, density, colour and solubility.
1.1 Background of the study
Gums are considered to be pathological products formed following injury to the plant or owing to unfavorable conditions, such as drought, by a breakdown of cell) (Jani et al.,2009). They are complex carbohydrate derivatives of a polysaccharide nature and are either soluble in water as in the case of gum arabic or form mucilages by the absorption of large amounts of water (gum tragacanth). Their principal use is in foodstuffs owing to their ability to impart desired qualities to foods by influencing their viscosity, body and texture; most frequently in confectionery food, flavouring and soft drinks. They also have pharmaceutical and industrial applications as demulcents, adhesives in pill manufacture, lithography, paints, inks, corrosion inhibitors and as emulsifying agents. The use of natural gums taken from the exudates and extracts of plants have been given a strong attention due to the many and lucrative possibilities for industrialization and to the excellent international market, example being gum arabic which in current production potential is around 30,000 to 40,000 tonnes per annum, of which bulk (80%) originates in Sudan; Nigeria being the second largest producer (Da silva et al., 1992). Virtually all gum arabic in the Sahelian zone is exported, either immediately or after a period of storage or stockpiling. Sudan dominates the world exports, accounting for 70% to 80%, the balance being accounted for by the Sahelian countries of West Africa (Nigeria, Mali, Niger, Burkina Faso, Chad, Tanzania and Kenya). One billion pounds are consumed in the United States each year where the growth in demand exceeds 8% per year (JECFA/FAO, 1988). Another example is the cashew gum, which by natural exudation or by means of incisions, produces a gum or resin of a yellowish color, soluble in water, and which presents a great potential for industrialization, appears on the trunk and branches of the cashew tree. It is similar to gum arabic and may be used asa substitute for liquid glue in book binding, in the pharmaceutical cosmetic industry as an agglutinant for capsules and pills, and in the food industry as a stabilizer of juices, beer and ice cream, as well as for clarification of juices, and can also be utilized in the making of cashew wine (JECEFA/FAO, 1986).
Besides proving to be strong wood glue when mixed with water, it presents a fungicidal and insecticidal action, and because of this is much used in book binding. Research already exists on its utilization in the making of inks and varnishes (JECFA/FAO, 1988).
Cashew gum extraction represents one more source of revenue for the producer, in addition to the cashew nut and the peduncle, as well as an alternative for the utilization of unproductive cashew trees, in phase of decline or senescence (Jani et al., 2007).
From a wider point of view, cashew gum not only can end the importation of gum arabic, which costs Brazil US$ 1,900,000/year, but can also become an export item (Jani et al., 2007).
The Embrapa Tropical Agro industry, which develops the technology, invites potential industrial partners for the processing and industrialization phases of the product and, later, for marketing. This is a unique opportunity to gain a potentially significant market for importation substitution and for participating in the external market.
These gums found wider application because of their physical, rheological and chemical properties (such properties include solubility, water sorption, swelling capacity, pH, effect of temperature, and viscosity among others) (De Paul et al., 2001).
There is increasing demands for gums globally because of its vast application, which causes the increase in prices of the existing gums in the local and international market. For the price to be stable and less expensive there is need to look for alternative or discoveries of more suitable natural gums which will be of the same quality or even surpass the existing ones.
Studies on the physicochemical and rheological properties of some gum exudates have been carried out. Gum exudates from Khaya senegalensis plants grown in northern Nigeria were investigated for its physicochemical properties such as pH, water sorption, swelling capacity and viscosities at different temperatures using standard methods as reported in literature. It was found that Khaya gum appeared to be colourless to reddish brown translucent tears. 5 % w/v mucilage has pH of 4.2 at 28 °C. The gum is slightly soluble in water and practically insoluble in organic solvents. Water sorption studies revealed that it absorbs water readily and is easily dehydrated in the presence of desiccants. A 5 %w/v mucilage concentration gave a viscosity value which was unaffected at temperature ranges (28 – 40°C). The results indicated that the swelling ability of Khaya senegalensis gum may provide potentials for its use as a disintegrant in tablet formulation, as a hydro gel in modified release dosage forms and the rheological flow properties may also provide potentials for its use as suspending and emulsifying agents owing to its pseudo plastic and thixotropic flow patterns.
Mhinzi (2002) analysed gum samples from three selected Albizia species from Tanzania and determined their commercial potential by comparing their properties with those of Albizia zygia and Acacia gums. The properties of the gum exudates from Albizia amara, Albizia pertesiana and Albizia harveyi were found to be similar to those of A. zygia gum except that their aqueous solutions possess slightly lower viscosity and higher levels of tannin. The Albizia gums were much less soluble in water than acacia gums; However their methoxyl contents and acid equivalent weights (AEW) were similar to those of some Acacia gums.
Rheological properties of Xanthan and locust beans gums have been studied by Higiro et al. (2006) and the gums were found to obey Huggins and power law models and from the rheological modeling of the gums, the conformation of the gums were established. Xanthan gum interacts with galactomannans to form mixed gels with high viscosity at low-total-polysaccharide concentrations (Takoet al., 1984), and this interaction is more pronounced with locust bean gum (LBG) than with any other polysaccharide or galactomannan (Deaet al., 1977). The interaction between xanthan and LBG is largely exploited in food applications in which thickening or gelling is desired. Considerable work has been published to elucidate the mechanisms behind xanthan and LBG interaction (Cairnset al., 1986; Cairns et al., 1987; Tako et al., 1984; Wanget al., 2002; Williamset al., 1991). Physical and physicochemical techniques have been used to study xanthan–galactomannans interaction, and results demonstrated the existence of an ‗‗order (helix)–disorder (coil)‘‘ transition for xanthan; this transition responds to changes in ionic strength and temperature, and may play a major role in such interactions (Morris, 1995a). High temperatures favor the ‗‗disordered‘‘ transition, whereas high ionic strength favors the ‗‗ordered‘‘ transition (Morris, 1995a).
Lopeset al.,(1992) studied the interaction of xanthan and guar gum at low temperature in water and 2×10-2M NaCl by using viscosity methods. The authors noticed a small synergistic effect between the two gums in 2×10-2M NaCl; the effect became more pronounced in water. They concluded that xanthan adopted a disordered conformation in water, whereas the conformation was in an ordered form in 2×10-2M NaCl. These findings were supported by Dalbe (1992); he used small deformation oscillation methods to study xanthan–glucomannan mixture and reported that addition of 8.56×10-2M NaCl or 6.71×10-2M KCl to gum mixtures led to a dramatic reduction in gel strength, which was not altered by further addition of electrolytes. Similar results were reported by Wang et al. (2002). The authors used rheological methods to study the conformational role of xanthan in interaction with LBG, and reported a decrease in viscosityand texture strength for xanthan–LBG solutions and gels, respectively, associated with the addition of 4×10-2M NaCl. In the absence of electrolytes, the xanthan molecules may self associate to reduce the interfacial energy.
Early work suggested that a specific interaction occurred between the ordered xanthan molecule and the galactomannan chain (Dea and Morrison, 1975; Dea et al., 1977; Morris et al., 1977; McCleary 1979). This interaction depended on the mannose/galactose ratio, as well as on the fine structure of the galactomannan. Tako et al.,(1984), Tako and Nakamura (1985) and Tako (1991) reported that the intermolecular interaction between xanthan and galactomannans occurred between the side chains of xanthan and the backbone of galactomannans, as in a lock-and-key model. Cairns et al., (1986, 1987), by means of X-ray fiber diffraction, suggested that gelation occurred only if xanthan was first denatured by heating above the order-disorder transition temperature and the interaction occurred between the xanthan backbone in an extended 2- fold cellulose-like conformation and LBG in a similar 2-fold conformation. Recent evidence strongly suggested that destabilization of the xanthan helix facilitated xanthan and galactomannan binding (Cheetham and Mashimba, 1988; Zhan et al., 1993; Foster and Morris 1994; Goycoolea et al., 1994).
It has been shown that gelation could occur when xanthan and LBG were mixed at temperatures below the xanthan helix-coil transition (Williams et al., 1991; Mannion et al., 1992; Foster and Morris, 1994; Goycoolea et al., 1994); the melting temperatures of the mixed gels were independent of ionic strength (Zhan et al., 1993) and xanthan conformation (Goycoolea et al., 1994); the modulus of the mixed gels increased with increasing disorder of the xanthan helix (Zhan et al., 1993). Hence, it has been suggested that galactomannan acts like a denaturant to disturb the helix-coil equilibrium of xanthan and to displace the ordered conformation of xanthan to the conformation required for efficient binding to it within the heterotypic junctions (Zhan et al., 1993; Goycoolea et al., 1994; Morris et al., 1994; Morris, 1996; Morris, 1996).
To date, much work has been accomplished on the gelling properties of gums and their inherent visco-elastic properties, as evidenced by many published papers; but the evaluation of gums interaction in solution has been studied to a lesser extent (Casas et al., 1999; Cuvelier and Launay, 1986), and most of the research was performed by using the Ubbelohde or rotational viscometer.
1.1 Aim of the study
The aim of the present study is to investigate and model the functional properties of the blends prepared from Anacardium occidentale, Khaya senegalenses and Acacia senegal gum exudates using recommended methods of analyses.
1.2 Objectives of the study
The objectives of the study were as follows:
i. To collect, identify and purify gums from Anacardium occidentale L, Khaya senegalensis,Acacia Senegal;
ii. To investigate physicochemical (colour, pH, absorbance, solubility in various solvents, turbidity, total dissolved solid content, conductivity and salinity) and rheological properties of the gum exudates as well as their blends using using recommended methods and appropriate instruments;
iii. To investigate the structure and surface morphology of the natural polymers (gum exudates) blends in aqueous media;
iv. To adequately model Rheological properties of the gum blends using Huggins, Kraemer, Arrhenius, Tanglertpaibul and Rao models;
v. To determine the active functional groups in each of the gums using FTIR spectrophotometer;
vi. To carry out rheological studies on the gums using different types of viscometer and a rheometer. From rheological data, effect of viscosity on temperature, concentration, pH, ionic strength of various electrolyte (K+, Al3+, Br- and Cl-) and conformations of the gums (coil-coil overlap) shall be modeled using appropriate equations; and
vii. To determine and model kinetic and thermodynamic parameters of flow for the gums and these shall include activation energy of flow, activation entropy and enthalpy of flow and activation free energy of flow.
1.3 Justification for the study
This research entails the effect of ionic environment on the interaction of plant gums in dilute solution as well as other conditions: temperature, salt and acid etc. there by improving on the search for alternative gums with better quality through polymer blending and hence reduce the increasing demands for gums globally as well as their prices in the local and international market.