Acacia gum known as gum arabic, is one of the oldest food ingredient. It is used in texturing, film forming, emulsifying or stabilising properties. Acacia Gum has been known for many years by scientific community as a potential source of dietary fibre. More recently there has been an emergence of a new concept called functional food which is designed for promoting health and nutritional benefits. The fibre fortified foods have the same organoleptic qualities as conventional food in addition to a significant amount of dietary fibre. These foods may have effects like sandy texture, increase in viscosity or bad taste or colour which maybe limiting factors. On the other hand, Acacia gum can be added in very large amount without affecting the original taste and texture of the food in which it is incorporated. This is because of the highly complex polymeric structure.
Acacia Gum appears in many 'Pharmacopoeias'. The gummy exudate which flow naturally or obtained by the incision of the trunk and branches of Acacia Senegal and other species of African Origin is known as Acacia gum (gum arabic). these gummy exudates are purified by a simple physical process centrifugation and filtration without chemical or enzymatic modification. A simple sterilisation procedure ensures bacterial safety and finally high water soluble powders are obtained by spray drying and or granulation process.
Dissolution in water
Centrifugation
Filtration
Microfiltration
Sterilisation
Spray Drying Granulation
Fig1: Purification process of acacia gum
Fig2: schematic representation of Chemical Structure of Acacia Gum:
Polysaccharide
Protein Core (1-5%)
Reference http://www.lsbu.ac.uk/water/ref4.html
P. A. Williams and G. O. Phillips, Gum arabic, in G. O. Phillips and P. A. Williams, Handbook of hydrocolloids, CRC Press, Cambridge, England (2000) pp. 155-168
Structural unit
Gum arabic found in nature is a mixture of calcium, magnesium, potassium salt of a polysaccharide acid (arabic acid). The chemical composition of gum arabic is a complex and variable mixture of arabinogalactan oligosaccharides, polysaccharides and glycoprotein and depending on the source, the glycan components contain a greater proportion of L-arabinose relative to D-galactose (Acacia seyal) or D-galactose relative to L-arabinose (Acacia senegal). The gum from Acacia seyal also contains significantly more 4-O-methyl-D-glucuronic acid but less L-rhamnose and un-substituted D-glucuronic acid than that of Acacia senegal. The presence of any other carbohydrate material indicates that the gum is impure or contains impurities.
Physically gum arabic is a complex, highly branched globular molecule which is closely packed and thus has low viscosity. And shaped like a short stiff spiral or coil. The length of the main molecule chain ranges from 1050A to 2400A, depending on the amount of charge.
Molecular structure
Gum arabic consists of a combination of lower molecular weight polysaccharide (M.Wt. ~0.25x106; major component) and higher molecular weight hydroxyproline-rich glycoprotein (M.Wt. ~2.5x106 minor component) with wide variability between commercial samples. As the material is a combination or mixture the molecular structures have significant variations and exact structures are uncertain. The glycoprotein is a high molecular weight hydroxyproline rich arabinogalactan (~2% protein) containing a repetitive and almost symmetrical 19-residue consensus motif -ser-hypa-hypa-hypa-thr-leu-ser-hypb-ser-hypb-thr-hyp-thr-hypa-hypa-hypa-gly-pro-his with contiguous hydroxyproline (a) attached to oligo-α-1,3-L-arabinofurans and non-contiguous hydroxyproline (b) attached to galactose residues of oligo-arabinogalactans. combining a β-1, 3-D-galactopyran core with rhamno glucurono arabino galactose pentasaccharide side chains joined to the main chain via 1, 6-linkages.
Fig 3: schematic representation of molecular structure of Acacia gum.
GAL = Galactose ARA = Arabinose
GlcA = Glucuronic acid RHA = Rhamnose
4-MeGlcA = 4-O-methylglucuronic acid
http://eu.lib.kmutt.ac.th/elearning/Courseware/BCT611/Chap4/Chap4_intro.files/image026.jpg
fig 4: chemical composition representation
Gum Arabic Properties:
Solubility:
Gum arabic is unique due to its high solubility in water. Most common gums cannot be dissolved in water at higher concentration than about 5% because of their high viscosities. However a solution of gum arabic can be made of about 55% concentration which acts as a high viscous gel like mass. Due to its versatility gum arabic can be used in lower concentration in combination with other gums as thickeners and binders. Good grade gums like "KLTA" give solutions that are colourless, and have no taste (flavour). Poor grade gums like "GCA" have characteristics colour and flavour due to presence of tannins. Gum arabic is insoluble in oil and most inorganic solvents. It is soluble in aqueous ethanol up to l 60% ethanol as a higher limit.
Viscosity and pH relationship
Gum arabic is not very viscous at low concentration. High viscosities are obtained with high concentration of 40%-50%. This ability of forming high concentrated solution is responsible for the excellent stabilizing and emulsifying properties of gum arabic when mixed with large amount of insoluble materials. It has been concluded by Warner and Araujo that the first order rate for viscosity change may have a definite relationship to the rate of hydrolysis of gum arabic. Viscosity of gum arabic rises sharply with increasing PH to a maximum of about 6 and falls gradually at about 12. Solution of gum arabic is slightly acidic with a pH of about 4-6 Because of highly branched structure; acacia gun occupies a very low hydrodynamic volume and thus develops a very low viscosity. weakly acid character in water (pH 4.5), pH of the solution, affects the viscosity.
.
Fig5: Influence of concentration on viscosity of an acacia gum solution.
Fig6: Reheological characteristics of various concentration of gum arabic (Araujo 1966)
Note:
Rhehological gum arabic solution exhibit typical Newtonian behaviour at concentration up to 40% and above solution become pseudo plastic as shown by decrease in viscosity with increasing shearing stress.
• viscosity is shear rate independent
viscosity decreases in the presence of electrolytes (salts) shown in fig
Fig7: realtion between viscosity ratio and concentration of salts.
Comparison with other thickening agents shows lower viscosity of Gum Arabic solutions even at higher concentrations
Emulsifying properties
Gum arabic is an effective emulsifying agent because of its protective colloidal functionality. Thus can be used in oil in water food emulsion. It can produce stable emulsions in oil with a wide range of pH and in the presence of an electrolyte. Mechanism of emulsification is still not clear, but gum arabic forms a visible film at oil interface. As a film forming agent it prevents coalescence of the oil globules, thus permitting a high degree of dispersion by diminution of the diameter of the globule. With the dispersed phase (oil) the relative viscosity of emulsion changes also there is noticeable change in volume fraction which maybe because of the presence of a stabilizing agent or its thickness... emulsifier for essential oils and flavours, proteins adsorb on the surface of the oil droplets formed emulsions remain stable for a long period of time
Disadvantage:
Prolonged heating (> 70 °C) of Gum Arabic solutions causes precipitation of the proteinous components.
Explanation of gum arabic in beverage emulsion (gum and stabilisers for food industry 15)
Reference: Matthias Schiutz, industry requirement for hydrocolloids in beverage emulsion, Science and Technology, Flavour Delivery Science, Switzerland
.For beverage emulsion A.senegal derived gum is generally used. The protein backbone of that fraction is thought to constitute the hydrophobic part of the molecule with an affinity to the oil phase where as the hydrophilic polysaccharide side chain remain in the aqueous phase. Recently the existence of glycosyl-phosphatidylinositol lips attached to AGP fraction and their importance for the emulsifying efficiency of the gum has been reported. Attention has been paid on the interfacial rehology experiments suggesting there mechanical stability of the adsorption layer formed by the gum around an emulsion droplet plays a significant role reflected by substantial shear elasticity of the interface. The droplet size in a model emulsion found not to decrease any further when increasing gum concentration beyond 10%. Typical gum concentration in beverage emulsion with an oil phase content of up to 20wt%. Excess gum will be necessary to stabilize the emulsion i.e. to increase the viscosity of the emulsion to effectively prevent collisions and coalescence.
Functionality of Gum arabic.
Gum arabic is used as an ingredient of many food products; it is used to stabilize flavour oil emulsion in the soft drink industry. It has been already introduced some of the underlying mechanism involved in the adsorption of acacia type gums into the interface, and by far these have been most widely studied in gum arabic. Further studies have given consideration to the effect of dilution on the bulk continuous phase on the surface rheology of gum arabic-adsorbed o/w films and to the correlation between surface viscosity and emulsion stability. The studies show that dilution in bulk phase ensures a reduction on the surface viscosity of adsorbed gum arabic film (Dickinson et al 1989), however this effect proceeds slowly which suggests some small but finite loss of macromolecule material from interfacial region. It also presumes that structural deformation takes place. In conclusion this hypothesis postulates that small fraction of gum arabic macromolecule displays good film forming properties. It has been studied whether a relationship holds between N content of high molecular weight fraction which is responsible for stabilizing the gum against flocculation and the strength and thickness of the film formed at o/w interface stabilized by gum arabic sample of varying N content. (Dickenson and Stainsby 1988). The evidence showed that effective emulsifying behaviour is found for gum having high N content which when adsorbed gives excellent steric stabilization.
Interaction of polysaccharides with adsorbed proteins
Eric Dickinson, an introduction to food colloids, oxford university press, 1992
The ability of polysaccharide hydrocolloids to stabilize colloidal dispersion and emulsion is usually explained in terms of a modification of the structure and rheological properties of the aqueous continuous phase (Dickinsion and Stainsby). Due to their predominantly hydrophilic character most polysaccharide has low surface activity at air -water and oil-water interfaces. They are not expected to form adsorbed layers in food colloids which contain proteins and low molecular weight surfactants. Due to the low surface activity of polysaccharides they are rarely used ad emulsifying agents but are used as emulsion stabilizers. While considering the composition and structure of adsorbed film in food colloids, the nature and interaction between polysaccharide & adsorbed proteins and its influence on colloidal stability are of importance. Thus it can be said that an attractive protein-polysaccharide interaction can enhance stability by forming a thicker and stronger steric-stabilizing layer or it can destabilize an emulsion by polysaccharide bridging between protein-coated droplets. On the other hand a repulsive protein-polysaccharide interaction can stabilize colloids by immobilizing the dispersed particle in a weak or strong gel network, or it can destabilize effect by inducing phase separation or depletion flocculation. The ideal combination of protein polysaccharide would produce optimum result in the formulation of food colloids by bringing the emulsifying property of protein along with the stabilizing property of polysaccharide.
The strongest type of protein-polysaccharide interaction is where there is a covalent linkage of protein and carbohydrate moieties to form a single hybrid amphiphilic macromolecule. The main advantage of covalent bonding over noncovalent protein-polysaccharide interaction (e.g. electrostatic) is the maintenance of solubility and molecular integrity over a wide range of solvent condition (ph, ionic strength, temperature.).
A natural protein-polysaccharide hybrid is gum arabic (Acacia Senegal). Serine and hydroxyproline residues of the proteins fractions are involved in covalent linkages to carbohydrate, resulting in a mixture of arbinogalactan-protein complex, each containing from one to several branched polysaccharide units linked to a common proteins core the so called Wattle Bossoms model.
Gum arabic is a polysaccharide heterogeneous material. Its surface activity is low compared with food proteins, it is compensated for in practise by using high concentration of gums during emulsification, i.e. 1:1 gum to oil ratio, as compared with 1:10 protein-to-oil ratio for a protein-stabilized emulsion.
Shotton and Wibberley (1959) demonstrated the surface activity of arabic acid and its salt and their ability to form thick viscoelastic films at oil-water interfaces.
Nakamura et al (1983) have studied the dependence on molecular weight of the surface rheology of gums arabic film at the coconut oil-water interface and it is shown that there exist correlation between the surface viscosity and emulsion stability. The study proved that the rheological parameters are increasing functions of the weight average molecular weight, where as
interfacial tension under same experimental condition is essentially independent of molecular weight.
Anderson et al (1986) proved that gum arabic is similar to guar, Xanthan etc in its low level of nitrogen. Protein content bound to the periphery of the gum molecule has major impact on its surface activity.
Testing Emulsifier efficiency
when understanding the effectiveness of a food emulsifying agent, it is useful to distinguish between emulsifying capacity (ability to make an emulsion) and emulsion stability (ability to keep an emulsion in its freshly made state).
The proper way to compare the emulsifying properties of food proteins is to make emulsion under well controlled conditions, preferably using a high pressure homogenizer. then to compare droplet size distribution of this emulsion as a function of time. Taking creaming behaviour of the emulsion on storage and the state of aggregation of the droplet. Such testing helps lessen the sever emulsifying condition by using lower protein concentration. There are two main ways to determine droplet size distribution for O/W emulsion, i.e. Coulter counter and light scattering technique. In this study light scattering technique was used.
.In light scattering technique particle sizing below 0.5um is used. Average particle size of droplet is determined at fixed angle. Sometime biasing towards the small particle end leads to overestimation of total surface area and relative insensitivity to the growth of large droplets which is crucial in assessing emulsion stability. In Malven Matersizer total intensity of static light scattering over a wide range of scattering angles is used to give the complete droplet size distribution. this technique the only uncertainty is the values of optical input parameters
Malvern Matersizer.
The Malvern Mastersizer S is a single lens laser diffraction system. It uses a laser of 2 mill watts helium neon power to measure the size of particles. Laser scattering theory used are Mie theory, and Fraunhofer analysis. Knowledge of the scattering theory and particle properties is used to transform the scattered light data to a distribution of particle size information to the software running on PC. Particle dispersion size is in the range of 0.5-3500 micron depending on the texture and product. The samples are introduced into the laser by means of the appropriate accessories. The small volume sample dispersion unit stirs and pumps small volumes of sample through the test cell. It is activated by a control box, which displays and adjusts the speed. This is connected to the Mastersizer digital I/O so that the speed can be automatically recorded by the test software.
Please attach the photograph and sample result
UV-Spectrophotometer
A UV spectrophotometer is a device used to study the interaction between radiation and materials in the form of liquid concentration at a corresponding wavelength at 400-700 nanometre. It measures visible light and visible range of ultraviolet and infrared spectrum. In our experiment we have used wavelength of 495 (nm) for different concentration of gum. The absorbance values are plotted by taking the reference sample at 495(nm) containing the mix of the dye, ethanol and deionised water. The absorbance graph is plotted against the concentration with the available spectrophotometric reading and characteristic curve is plotted. This helped us to read the difference in behaviour of KLTA and GCA at different phi
Effect of high-hydrostatic pressure and Ph on gum arabic.
A.G.Panteloglou, A.E.Bell, F.Ma, Division of food science (Nottingham) & Department of food science (Reading), Elsevier, February 2010.
The potential of high pressure is been used in both food processing and preserving method in food industry. Gum arabic is a natural extract, particularly useful in acidic condition (Dickinson, 2003). Currently as per studies it is accepted that high pressure may bring about changes in hydrophobic association, hydrogen bonding and electrostatic interaction (Ledward, 1995). High pressure leaves covalent bonds intact and affects only the non covalent ones.
It has been demonstrated by Gross et al. (1994) that the covalent bonds and the primary structure of the proteins and polysaccharide are not ruptured by pressure due to the negligible compressibility of the covalent bonds. Ability to affect the secondary bonds leaving the covalent bonds largely intact, high pressure processing denatures high molecular weight molecules and cell structure and change their functionality and leaving low molecular weight chemically unaffected. Till date very less has been published on effect of high pressure on physical properties of gum arabic.
In this study two gum of different qualities where used they were pressure treated at different pH (2, 4, 8). Type1 gum was good gum called KLTA. this is a spray dried preparation of kordofan gums, described as light type due to its pale colour. Type 2 gum was poor gum called GCA .which is a spray dried gum, derived from a gum corbretum and usually has inferior rheological and emulsifying properties.
Effect of high pressure on viscoelastic behaviour of gum (native) and pressurised
Effect of ph on rheological behaviour of gum (native) and pressurised.
