This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Acacia gum is a dried exudation from the stem and branches of various species of Acacia (N.O. Leguminosoe), especially of Acacia Senegal, Willdenow, a small tree attaining a height of 5 or 6 metres, and growing freely both in Western Africa (Sene-gambia) and in Eastern Africa (the upper Nile districts), possibly also in Central Africa, forming forests of considerable extent. The first formation of gum appears to take place in the cambium or the young bast tissue produced from the cambium and to be induced by a wound inflicted on the stem. Such wounds may be produced by animals (ants, beetles, &c), or by the cracking of the bark during the process of growth, or intentionally with the object of producing gum. Bacteria, moulds and other organisms may thus gain admittance to the gum-forming tissues, but whether these take an initial part in the formation of the gum is more than doubtful.
The best gum is produced near Kordofan from trees specially cultivated and worked for gum. Strips of bark 0.5 to 1 metre long and from to 2 to 8 cm. wide are removed from the trunks and large branches, care being taken not to injure the cambium. About two or three weeks afterwards the gum is collected, and the orchard is then picked over about every four days until the rain sets in, when the exudation of gum ceases. Gum is also spontaneously exuded from wild trees, but this is usually rather darker in colour and not so valuable. The trees begin to yield gum when they are about three years old and continue to yield for about fifteen years.
The gum as collected is in translucent tears which, ' ripened ' by exposure to the sun, develop cracks and become friable. Native girls clean it by picking out pieces of bark and sifting out the sand. It is then packed into sacks and sent from the nearest rail-head (El Obeid) to Port Sudan for shipment. Large quantities are sorted and packed in Omdurman.
In some districts another acacia yielding smaller quantities of gum (A. Seyal, Delile) occurs. The gum, which is called talk or talka gum, is collected by the natives with that of A. Senegal (hachab gum). As talka gum is of inferior quality it has to be separated by picking.
Kordofan (hachab) gum, which is the best variety, occurs in rounded or ovoid tears, varying in size from a pea to a hazel nut or even larger. They are often quite white, but sometimes show a yellowish tinge, and are opaque from the presence in the outer part of the tears of small fissures. In consequence of these they easily break up into a number of small, transparent, angular fragments with glistening, vitreous surfaces. The drug is practically inodorous, and has a bland, mucilaginous taste. Whilst the finest qualities are white, or have at most only a yellowish tinge, inferior grades have a decided yellow or reddish or brownish red colour and then contain traces of tannin.
Acacia gum is insoluble in alcohol, but dissolves freely in water, forming a translucent, viscid, but not glairy or ropy liquid, that feebly reddens litmus paper. A 10 per cent, aqueous solution of good qualities is slightly laevorotatory, and when boiled with an equal volume of Fehling's solution throws down a slight but distinct deposit of cuprous oxide. Solution of lead acetate produces no precipitate, but subacetate produces a copious white one; whilst a saturated solution of borax forms with a strong solution of gum a clear, translucent jelly. Inferior (brown) gum usually contains tannin which may be detected by solution of ferric chloride.
Acacia gum consists almost entirely of a gluco-sidal acid of high molecular weight, which has been termed arabic acid, combined with potassium, magnesium, and calcium; by hydrolysis each molecule yields two molecules of the sugar arabinose and four of galactose together with an organic acid to which the name of arabic acid has also been given, but which is better termed iso-geddic acid, as it is isomeric with the corresponding acid, geddic acid, obtained from geddah gum. The glucosidal acid of acacia, is therefore, a diarabinan-tetragalactan-isogeddic acid, the termination ' an ' indicating the anhydride of the corresponding sugar. This acid can be obtained from the gum by acidifying an aqueous solution with a mineral acid, dialysing it until the mineral constituents are removed, and fractionally precipitating with alcohol. Whilst moist it dissolves in water, but the dried acid only swells in water, dissolving on the addition of an alkali.
Gum acacia also contains an oxydase enzyme, and hence readily turns powdered guaiacum resin, or the tincture diluted with water, blue. It loses about 14 per cent, of moisture when dried at 100° and yields from 2.7 to 4.0 per cent, of ash. It contains further a small percentage of nitrogen, but this does not enter into the composition of the gum itself (distinction from gelatin, etc.); it is probably due to the enzyme, from which the gum cannot be entirely freed.
Acacia gum is used medicinally as a demulcent and as a means of suspending oils, resin, etc, in aqueous fluids.
Gum-yielding acacias are widely distributed over tropical and subtropical countries, and furnish large quantities of gums which, though in most cases unsuitable for medicinal use, are extensively employed for certain technical purposes.
Sennaar gum, Gedaref gum, Ghezireh gum, Talka gum, Somali gum, Aden gum, etc, are varieties of East African gums and are considered inferior to Kordofan gum; the latter gives a lsevo-rotatory solution, whereas many of the other East African gums give a dextrorotatory solution.
Senegal gum is exported from the Senegal river to Bordeaux. It is derived from A. Senegal, but may be distinguished from Kordofan gum by being (usually) slightly more coloured, less fissured and by containing vermiform tears.
Mogadore gum (A. gummifera, Willdenow); is mostly dark in colour and but little fissured; occasional white fissured tears are probably Sudan gums.
Indian (Acacia) gums include Amrad gum (A. arabica), Amritsar gum (A. modesta), etc. The flora of the deserts of Sind resembles that of the African. The gums are often in large tears, varying in colour from yellow to dark brown, and are used for calico printing, etc. (For ghatti gum see below).
Cape gum (A. horrida, Willdenow) and Australian gum (A. deal-bata, Link; A. pycnantha, Bentham) find application in various industries.
Many of these gums form glairy, ropy solutions with water, and when diluted throw down gelatinous deposits of gum that has swelled but not dissolved. An acacia gum suitable for pharmaceutical use should be free from both these characters, and should further give no reaction for starch (which might be present as an adulterant of powdered gum) or for tannin, which is present in certain inferior varieties of gum (such as Australian); a 10 per cent, aqueous solution should be slightly laevorotatory (absence of dextrin, from which an artificial gum has been prepared, certain sugars, &c).