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Chapter 13

Preparation and Use of Gum Arabic and Its Products

Joseph K. Mwangi and Francis M. Mwaura

Gum arabic is a natural gum that is obtained from Acacia senegal and A. seyal, two leguminous trees originating from Africa. Acacias are one of the most common trees in Africa’s semi-arid savannas and A. senegal occurs throughout the Sahel and Sudanese woodlands, in the Great Rift Valley and into Southern Africa (White, 1983).  These trees have compound leaves with small leaflets, small white to cream colored flowers and form thin woody pods.  Sudan is the dominant producer of gum arabic, accounting for over 80% of world production, with Chad the world’s second largest producer.  

Gum arabic is collected from exudates on tree branches that are produced through a process called gummosis, one that serves to seal tree wounds and protect against extreme heat and desiccation (Figure 1).  In Kenya, similar material is also harvested from Acacia meansii and Acacia abysinica.  Processed gum arabic has well-established characteristics in terms of solubility, color, emulsification and film formation, and these properties are very important within many commercial and industrial processes.

The quality of the gum is determined by the maturity of the tree with older trees producing better quality gum than younger ones.  The colour of the harvested gum is an important consideration in quality assessment.  A dry, colourless-to honey-brown tree secrection produces higher quality gum. 

The trunk and branches may be cut at several positions to induce greater exudation. The exudates dry within a few days after secretion and are then ready for collection. If the exudates are too wet, it is recommended to air dry them  immediately. It is also recommended to harvest the exudates during the dry season as they are better than those processed from exudates harvested in the rainy season. After harvesting, the gum exudates are sorted depending on the intended use of the final processed gum. On average, each tree produces approximately 20 kg of gum per month in Kenya. Other trees such as wattle (A. mearnsii), also produce useful gum exudates, but these trees are less productive, yielding only 2 to 3 kg of secretions per month.

Processing Gum Arabic

1. Crush the dried clumps of exudates into small pieces of ≤ 2 cm³.

2. Put the crushed exudates in a container and add a small amount of ‘Guar’ gum to make the crystals dissolve more evenly. Guar gum can be bought from food processors. The ratio of guar gum to the tree gum exudates depends on the quality of the gum to be made. Higher quality gum arabic requires the addition of less guar.

3. Add water at a ratio of 1 part gum to 10 parts water.  For example, 1 kg of crushed gum is added to 10 litres of water. When the gum is completely dissolved, check for expected viscosity to further adjust the ratio of water to gum.

4. Filter the mixture using a clean cloth (Figure 2).

5. If necessary, add a coloring agent.  For example, food coloring may be added to give the gum a prefered colour.

6. Package the gum in plastic containers of sizes that meet different customers’ requirements in the market.

Uses of Gum Arabic

Gum arabic is an important material to many industries including confections and baked goods, beverages, encapsulated flavors, pharmaceuticals, cosmetics, printing, textiles and other applications, as well as the preferred “sticker” for seed inoculants in agriculture (Box 1).

Confectionery and baked goods.  Gum arabic retards sugar crystallization in gum drops and emulsifies and distributes fat particles in caramel and toffee.  In panned sugar confections, it serves as a coating agent and film-former. Gum arabic is a lubricant and binder in extruded snack cereals, at levels from 2-5%. At 15-40% levels, gum arabic provides adhesion of dry flavors in peanuts and similar products. In low-fat cake and muffin mixes, gum arabic functions as a partial oil replacer as well as a moisture binder.  Gum arabic’s water solubility, low viscosity, and adhesiveness gives it value as a glaze for buns to provide gloss and flexibility and also makes it useful as a component in toppings and icing bases, and in applications where its emulsifying power is important. It can be used to encapsulate baking flavors, such as cinnamon oil, for dispersal in vegetable fats and for use where flavor release is wanted at specific melting temperatures.

Beverages.  Gum arabic is a useful and inexpensive hydrocolloid emulsifier, texturizer and film-former that is widely used in the soft drink industry to stabilize flavors and essential oils. The simultaneous presence of hydrophilic carbohydrate and hydrophobic protein in the gum arabic enhances its emulsification and stabilization properties. Emulsification is particularly improved due to molecular flexibility that allows greater surface interaction with oil droplets. Gum arabic is used in confectionery and pastries and as a foam stabilizer in marshmallows. The gum arabic glycoprotein possesses a flexible but compact conformation. It is readily soluble to give relatively low viscous Newtonian solutions even at high concentrations of about 20-30 % (Fennema, 1996). The effectiveness of gum arabic as an emulsifier has given it broad application in foods. It has an especially strong position in the soft drink industry as a stabilizer of citrus oil emulsion concentrates. It fills an important application in beverages as a cloud-producing agent and dry mixture where a spray-dried emulsion of gum arabic and hydrogenated vegetable oil produces a stable, free-flowing powder that, on dispersal in water, provides a cloudiness or turbidity typical of citrus or other juices. A modified form of this procedure is used in formulation of several dry beverage mixes. The foam-stabilizing ability of gum arabic is used in beer and certain soft drinks to stabilize the foam "lace" on the side of the glass.

Encapsulated flavors.  Gum arabic is an ideal carrier in flavor encapsulation because of its natural emulsifying and surface-active properties, good retention of volatile flavor components, high solubility in water (up to 50%) and pH stability. Additional advantages include its neutral flavor, low hygroscopicity and ability to protect flavors from oxidation. For example, acacia gums protect orange oil against oxidation more effectively than does modified starch.

Pharmaceuticals.  Supplementation with gum arabic increases feacal nitrogen excretion and lowers serum urea nitrogen concentration in chronic renal failure patients consuming a low-protein diet.  A daily dose of 25 g would approximately double the amount of energy available to the colonies of bacteria that inhabit the human digestive tract. These bacteria grow by fermenting dietary fiber, and as they grow they absorb nitrogen. They are also capable of degrading urea to ammonia. They themselves are then excreted in feaces, taking some of the body's nitrogen waste with them. Only small quantities of gum arabic are used in pharmaceuticals. These uses depend upon its emulsifying, suspending, demulcent or coating characteristics. The gum maintains suspensions when used in rather high concentrations. Owing to its mild ability to complex heavy metal ions, it brings about better suspension of these salts when needed, for example, in the suspension of calamine lotion.  It also functions as a binder in cough drops and lozenges.

Cosmetics.  Gum arabic stabilizes lotions and protective creams. It increases the viscosity, assists in imparting spreading, adds a smooth feel to the skin, and forms a protective coating. It is also a binding agent in the formulation of compact cakes and rouges, and acts as an adhesive in the preparation of facial masks. A typical compact cake is composed chiefly of a pigment, a mineral oil, and an aqueous solution of gum arabic. The gum is also used as a foam stabilizer in liquid soap. Gum arabic has been recommended for use as a fixative and binder in hair creams and as a stabilizer and film former in protective creams.

Printing and textiles.  Since the nineteenth century, gum arabic has been combined with a sensitizer and a soluble pigment, applied to paper, and exposed through a negative under a powerful light source. This can produce beautiful prints only surpassed by adding further layers of gum pigments in registration. It is also possible to print color separated black-white negatives to produce true color prints, but precise registration is required.

In the past, gum arabic was extensively used in the aluminium plate printing process. Normally after printing, the plate would be cleaned with chemicals and covered by a gum arabic solution before re-using the plate.  The process works by sensitizing a part of the printing plate so that it will accept grease, oil and printing ink. An image is drawn or transferred on to an aluminum or zinc plate and ‘etched’ with a solution of gum arabic, water, and nitric acid.  Offset lithography continues to be an important printing process because of its low costs for film preparation and press operation, although the advent of digital processing has reduced its importance somewhat.

Gum arabic is used to make the yarn stronger and increase its tensile strength. Many textile manufacturers use modified starch mixed with gum arabic. Gum arabic can also be added to the painting formula to fix the pigment in the fabric, thus saving fabric printing costs.

Other industrial and household applications.  The essential ingredients in watercolors are pigments, a binding agent (usually gum arabic), and water. When combined these three components create transparent watercolor.  Pigments are ground and a liquid gum arabic solution is added to produce paint that is more opaque and which imparts a dusty quality to the surface. Gum arabic is resoluble even after drying, therefore it can be stored as dehydrated cakes. Occasionally oxgall (a wetting agent) is added to water color to aid dispersion of the pigment.

Industrial moulds must be covered with lubricant prior to injection to make the moulded material easier to extract, but paraffin wax alone is too difficult to remove. Alternatively, manufacturers may mix gum arabic with the paraffin and then place the mold into hot water, allowing the gum arabic to dissolve and remove the accompanying wax.

Ink may be prepared using gum arabic as follows. Collect “lamp black” the soot resulting from burning and slowly add water to produce an inky black solution.   Then, add a small amount of gum arabic solution to thicken the ink for writing.  For a longer shelf life, store the ink in an air-tight container.  A basic permanent ink can be prepared by mixing 1 egg yolk, 1 teaspoon full gum arabic and 1 cup honey in a small bowl. Add 1 teaspoon lamp black to make a thick paste and store in jar.

Powdered gum arabic is a simple adhesive for paper products and may be used directly after dissolution in two or three times its weight in water. A 40% aqueous solution has been made as mucilage for general office purposes. Gum arabic solutions are also excellent adhesives for “sticking” inoculants of beneficial bacteria onto legume seeds (Lowther et al., 1989).

Conclusion

The price of gum arabic varies with its grade and intended use, with the purest gum arabic used in pharmaceuticals (KSh 400 per kg) and food processing (KSh 200 per kg). Industrial grade gum arabic markets for KSh. 80 to 100 per kg. Gum arabic has “untapped” potential to stimulate income generation and industrial activities, however, the supply of the raw material must be sustained through protection of existing stands and additional tree planting of selected Acacia spp.  Training of traders and collectors in gum collection and storage will improve the quality of the final products. Acacia trees and gum arabic are important organic resources in Africa and should be considered by planning and development agencies as an important component for rural transformation in semi-arid areas.

References

Fennema, O.R. 1996. Food Chemistry. Marcel Dekker, New York

Lowther, W.L., Hoglund, J.H. and Mac Farlane, M.J. 1989. Aspects that limit the survival of legume seedlings. In: Marten, G.C. et al. (Eds.) Persistence of Forage Legumes. American Society of Agronomy, Madison, USA. pp 265-275

White, F. 1983. The Vegetation of Africa. UNESCO. Paris. 356 pp

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