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Gum arabic
Sources and Extraction
Arabic gum consists of the dried exudates extracted from the trunk and the branches of Acacia senegal or related species of acacias (family Leguminosae) (Sudan, Senegal). The exudates secrete as resinlike mass occurring from natural or artificial injuries in the bark, and harden within a few weeks to become hyaline tubers or scales. After being gathered manually and cleaned from bark splits, they are devided into different quality grades, according to their colour. The annual production is estimated at approx. 60 000 tons. Exudates of other plants (e.g. Combretum types) are sometimes wrongly offered as Arabic gum.
Properties
Appearance
Unground Arabic gum consists of white or yellowish spheric, drop shaped structures of different sizes or of square fractions, occasionally containing darker fragments or pieces of bark splits. For use in foodstuffs, this raw material is cleaned by solution in water, cleaning of the solution and subsequent spray drying. Arabic gum is also traded in the form of white to yellowish flakes, grains or powder.
Composition
Arabic gum is a neutral to slightly acid, complex polysaccharide. The components contained in the macromolecule are D-galactose, L-arabinose, L-rhamnose and D-glucuronic acid in the molar proportion of approx. 3:3:1:1. The structure is widely ramified, with a main chain of β-(1-3)-linked galactose units, to which galactose units with a terminal glucuronic acid group are linked in α- and β-(1-6)-bond. The acid groups are partly neutralised through Ca-, Mg-, Na- and K-ions, and partly exist as they are. The side chains are ramified with rhamnose and arabinose units, respectively. The molecular weight of Arabic gum ranges within large limits, the medium weight is at approx. 350 000.
Solubility
Arabic gum is easily and quickly soluble in cold water. Highly concentrated solutions with relatively low viscosity can be produced. Arabic gum is also soluble in hot ethylene glycol and glycerol as well as in aqueous ethyl alcohol up to 60%.
At concentrations of less than approx. 40%, solutions react like authentic Newtonian fluids; at higher concentrations they show pseudo-plastic behaviour. In aqueous solutions, Arabic gum shows slightly acid character (pH 4-5). In the pH-range of 4 – 9, the viscosity changes only insignificantly; in a stronger acid or alkaline milieu rapid degradation occurs, causing a decrease of viscosity of aqueous solutions. Also precipitation through electrolytes reduces the viscosity (e.g. CaCl2). With gelatin, Coacervat forming (i.e. precipitation) occurs.
Application properties
The main issue for its use are the formation of protective colloids and thus the stabilising effect in emulsions and dispersions. Arabic gum forms clear, soluble but not hygroscopic coatings on dragées; it can also be applied as glue for stamps. The pseudo-plastic behaviour is used for gum sweets and the like.
Toxicologic Evaluation
Arabic gum is not being decomposed and absorbed in the stomache and the small intestine. The flora of the colon splits it up slowly; the absorption occurred by short-chain fatty acids produces up to 3,5 Kcal/g (80% of the starch). Arabic gum has been used for decades as food additive and in pharmaceutic preparations. Therefore, it did not appear necessary to limit its application as food additive by determining an ADI (acceptable daily intake) indicated as mg additive/kg body mass.