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Acacia senegal (L.) Willd.

Syn.: Acacia verek Guill. et Perr.
Mimosaceae
Gum Arabic, Senegal Gum, Sudan Gum Arabic, Kher, Kumta

Source: James A. Duke. 1983. Handbook of Energy Crops. unpublished.


  1. Uses
  2. Folk Medicine
  3. Chemistry
  4. Toxicity
  5. Description
  6. Germplasm
  7. Distribution
  8. Ecology
  9. Cultivation
  10. Harvesting
  11. Yields and Economics
  12. Energy
  13. Biotic Factors
  14. References

Uses

Tree yields commercial gum arabic, used extensively in pharmaceutical preparations, inks, pottery pigments, water-colors, wax polishes, and liquid gum; for dressing fabrics, giving lustre to silk and crepe; for thickening colors and mordants in calico-printing; in confections and sweetmeats. Causing partial destruction of many alkaloids including atropine, hyoscyamine, scopolamine, homatropine, morphine, apomorphine, cocaine, and physostigmine, gum arabic might be viewed as a possible antidote. Pharmaceutically used mainly in the manufacture of emulsions and in making pills and troches (as an excipient); as demulcent for inflammations of the throat or stomach and as masking agent for acrid tasting substances such as capsicum; also as a film-forming agent in peel-off masks. Its major use is in foods, for example, as suspending or emulsifying agent, stabilizer, adhesive, flavor fixative, and to prevent crystallization of sugar, etc. Used in practically all categories of processed foods (candy, snack foods, alcoholic and nonalcoholic beverages, baked goods, frozen dairy desserts, gelatins, and puddings, imitation dairy products, breakfast cereals, and fats and oils). Use levels range from less than 0.004% (40 ppm) in soups and milk products, 0.7 to 2.9% in nonalcoholic beverages, imitation dairy, and snack foods, to as high as 45% in candy products (Leung, 1980). Strong rope made from bark fibers. White wood used for tool handles, black heartwood for weaver's shuttles. The long flexible strands of surface roots provide one of the strongest of local fibers, used for cordage, well-ropes, fishing nets, horsegirdles, footropes, etc. Seeds are dried and preserved for human consumption (NAS, 1980). Young foliage makes good forage. Plants useful for afforestation of arid tracts, soil reclamation, and windbreaks (Duke, 1981a). In modern pharmacy, it is commonly employed as a demulcent in preparations designed to treat diarrhea, dysentery, coughs, throat irritation, and fevers. It serves as an emulsifying agent and gives viscosity to powdered drug materials; is used as a binding agent in making pills and tablets and particularly cough drops and lozenges. Because of its enzyme, the gum is not suitable for use in products having readily oxidizable ingredients. For example, it reduces the vitamin A content of cod liver oil by 54% within three weeks. It is incompatible with aminopyrine, morphine, vanillin, phenol, thymol, a- and b-naphthol, guiacol, cresols, creosol, eugenol, apomorphine, eserine, epinephrine, isobarbaloin, gallic acid, and tannin; also with strongly alcoholic liquids, solutions of ferric chloride and lead subacetate and strong solutions of sodium borate. It was formerly given intravenously to counteract low blood pressure after hemorrhages and surgery and to treat edema associated with nephrosis, but such practices caused kidney and liver damage and allergic reactions and have been abandoned (Morton, 1977).

Folk Medicine

The demulcent, emollient gum is used internally in inflammation of intestinal mucosa, and externally to cover inflamed surfaces, as burns, sore nipples and nodular leprosy. Also said to be used for antitussive, astringent, catarrh, colds, coughs, diarrhea, dysentery, expectorant, gonorrhea, hemorrhage, sore throat, typhoid, urinary tract (Duke and Wain, 1981).

Chemistry

Gum acacia contains neutral sugars (rhamnose, arabinose, and galactose), acids (glucuronic acid and 4-methoxyglucuronic acid), calcium, magnesium, potassium, and sodium. Its complex structure is still not completely known. Its backbone chain consists of D-galactose units, and its side chains are composed of D-glucuronic acid units with l-rhamnose or l-arabinose as end units. The molecular weight has been reported to be between 200,000 to 300,000 and as high as 600,000 (Leung, 1980).

Toxicity

Ingested orally, acacia is nontoxic. However, some people are allergic to its dust and develop skin lesions and severe asthmatic attacks when in contact with it. Acacia can be digested by rats to an extent of 71%; guinea pigs and rabbits also seem to utilize it for energy, as does man to a certain extent. Gum arabic may actually elevate serum or tissue cholesterol levels in rats (Leung, 1980).

Description

Savanna shrub or tree, up to 20 m tall, over 1.3 m in girth, spiny; bark gray to brown or blackish, scaly, rough; young branchlets densely to sparsely pubescent, soon glabrescent, crown dense; stipules not spinescent; prickles just below the nodes, either in threes up to 7 mm long, with the middle one hooked downwards and the lateral ones curved upwards, or solitary with the laterals absent; leaves bioinnate, up to 2.5 cm long; leaf-axis finely downy with 2 glands; pinnae 6–20 pairs; leaflets small, 7–25 pairs, rigid, leathery, glabrous, linear to elliptic-oblong, ciliate on margins, pale glaucous-green, apex obtuse to subacute; flowers in spikes 5–10 cm long, not very dense, on peduncles 0.7–2 cm long, normally produced with the leaves; calyx bell-shaped, glabrous, deeply toothed; corolla white to yellowish, fragrant, sessile; pod straight or slightly curved, retrap-shaped, 7.5–18 cm long, 2.5 cm wide, thin, light brown or gray, papery or woody, firm, indehiscent, glabrous, 5–6-(-15) seeded; seeds greenish-brown. Fl. Jan.–Mar.; fr. Jan.–Apr., July, Aug. or Oct. (Duke, 1981a).

Germplasm

Tree with a single central stem and a dense flat-topped crown, bark without any papery peel, rough, gray or brown, with pubescent, rarely glabrous inflorescence, and pods variable in size, rounded to somewhat pointed but not rostrate or acuminate at apex. Variety rostrata Brenan is a shrub, branching at or close to base, or a small tree, with a single stem, 1–6 m tall, with dense flattened crown, bark normally with a flaking papery peel, creamy-yellow to yellow-green or gray-brown, inflorescence axis always pubescent and pods 2–3.5 times as long as wide, rostrate or acuminate at apex. Variety leiorhachis Brenan, is always a tree with central stem, and rounded or irregular with straggling branches; bark with conspicuous yellow papery peel, and inflorescence axis always glabrous. Variety pseudoglaucophylla occurs on fixed sand duned in Africa. Assigned to the African Center of Diversity, gum arabic is reported to exhibit tolerance to alkali, drought, fire, high pH, poor soil, sand, slope, and wind. (2n=26) (Duke, 1981a)

Distribution

Widespread in tropical Africa from Mozambique, Zambia to Somalia, Sudan, Ethiopia, Kenya, and Tanzania. Cultivated in India, Nigeria, and Pakistan.

Ecology

Thrives on dry rocky hills, in low-lying dry savannas, and areas where annual rainfall is 25–36 cm. This hardy species survives many adverse conditions, and seems to be favored by low rainfall and absence of frost. Ranging from Warm Temperate Thorn through Tropical Thorn to Tropical Dry Forest Life Zones, gum arabic is reported to tolerate annual precipitation of 3.8–22.8 dm (mean of 9 cases = 12.4 dm), annual mean temperature of 16.2–27.8°C (mean of 9 cases 23.8°C), and pH of 5.0–7.7 (mean of 7 cases = 6.4), but Cheema and Qadir (1973) report 7.4–8.2.

Cultivation

In Sudan, trees are cultivated over a very large area. Best propagated from seeds which are produced once every few years, grown in Sudan, in special "gum gardens." Elsewhere, it is collected from wild trees. In Pakistan, the best period for afforestation is the early monsoon season (Apr.–Jun.). Surface sowing is recommended in mildly alkaline sandy soils. Plants can also be reproduced by shoot cuttings. Trees coppice well (NAS, 1980).

Harvesting

Gum exudes froin cracks in bark of wild trees, mostly in the dry season, with little or none in the rainy season when flowers are out. In some areas, a long strip of bark is torn off and the gum allowed to exude. In Africa, it is regularly tapped from trees which are about 6 years old by making narrow transverse incisions in bark in February and March. In about a month, tears of gum form on surface and are gathered. Trees begin to bear between 4–18 years of age and are said to yield only when they are in unhealthy state due to poor soil, lack of moisture or damaged. Attempts to improve conditions tend to reduce yield. Gum from wild trees is variable and somewhat darker colored than that from cultivated plants. Collected gum is carefully freed of extraneous matter, sorted and sometimes ripened in sun before export. Gum arabic is oderless with a bland taste, yellowish and some tears are vermiform in shape. Ripened or bleached gum occurs in rounded or ovoid tears over 2.5 cm in diameter, and in broken fragments. Tears are nearly white or pale yellow and break readily with a glassy fracture. Gum is almost completely soluble in an equal volume of water and gives a translucent, viscous, slightly acid solution, but is insoluble in 90% alcohol. Kordofan (Sudan) Gum is yellow or pinkish, has fewer cracks and is more transparent (Duke, 1981a).

Yields and Economics

Annual yields from young trees may range from 188 to 2856 g (avg. 900 g), from older trees, 379 to 6754 g (avg. 2,000 g). Gum arabic is important export product for some areas in tropical Africa and Mauritania. From Africa some genuine gum is shipped to India then to Europe and America. Between 1940 and 1950, United States imports range from 3,179–8,989 MT (Duke, 1981a) Morton (1977) reports >11,000 MT more recently.

Energy

Considered the best firewood in Mauritius and Senegal, this is not a big yielder, annual running 0.5–5 m3/ha wood, with an energy value of ca 3,500 kcal/kg. A nitrogen,fixing species, it can be used to reestablish vegetation cover in degraded areas, as well as for sand-dune fixation and wind erosion control (NAS, 1980a).

Biotic Factors

Fungi reported on this crop are Cladosporium herbarum, Fusarum sp., Ravenelia acaciae-senegalae and R. acaciocola. Many insect visitors mimic the plant, the buffalo treehopper, Stictocephala bisonia, being a good example. Spiders (Cyclops sp.) may completely cover the young growing apex. Seedlings are often grazed by gazelles, goats, and pigs (Morton, 1977).

References

Complete list of references for Duke, Handbook of Energy Crops
Last update December 19, 1997