Source, Etc

Tragacanth gum is a dried, gummy exudation from the stem of Astragalus gummifer, Labillardiere (N.O. Leguminosoe), and other species of Astragalus.

These plants are small, branching, thorny shrubs, about a metre in height, and are natives of southern and eastern Europe and especially of Asiatic Turkey and Persia, where they form one of the most characteristic forms of vegetation. When the stem is incised a gum exudes and dries, the form that it assumes being dependent on the form of the incision, vertical slits yielding flat, ribbon-shaped pieces and punctures vermiform tears. It is produced by the transformation of the cell-walls of the pith and medullary rays into gum, which easily absorbs water, and, swelling, exerts considerable pressure on the surrounding tissue. When, therefore, the stem is wounded the gum is forcibly pressed out, a piece about 2 cm. long being exuded in half an hour; it carries with it the starch grains present in the cells in a more or less unaltered condition, and these are therefore a natural constituent of the drug.

Of the influences that induce gummosis of the cell-wall, nothing definite is known.

In some districts accidental wounds by grazing cattle appear to suffice for the production of tragacanth. In Asiatic Turkey it is collected chiefly in Anatolia and shipped from Smyrna; in Persia the Baktiari mountains, south of Ispahan, and the neighbouring districts yield much tragacanth, which is conveyed from the Persian Gulf ports to Bombay, and thence to Europe. The former variety is known as Smyrna, the latter, which alone is official, as Syrian or Persian.

Description

Syrian or Persian tragacanth occurs in thin, flattened curved, ribbon-shaped flakes of a translucent, horny appearance and nearly colourless or faintly yellowish. The flakes are often 3 cm. long and 1 cm. wide, and are marked with numerous concentric ridges, conveying the impression that the gum has been exuded in successive portions. The flakes break with a short fracture, are odourless and almost tasteless. Soaked in cold water they swell considerably, forming a gelatinous mass, but only about 8 or 10 per cent, dissolves.

Smyrna tragacanth occurs in similar pieces in which, however, the ribbon-like character is less pronounced, and which are more opaque than the Persian, so that the two, when placed side by side, can easily be distinguished. Moreover, Smyrna tragacanth contains sufficient starch to give a decided reaction with the iodine test, whereas Persian responds only very faintly.

Constituents

That portion of tragacanth that is soluble in water consists chiefly of a polyarabinan-trigalactan-geddic acid and yields by hydrolysis arabinose, galactose, and geddic acid; the insoluble part of the gum is termed bassorin; it is converted by baryta water into isomeric a- and β-tragacanthan-xylan-bassoric acids which yield by hydrolysis tragacanthose, xylose, and bassoric acid. Tragacanth contains also water, traces of starch, cellulose, and nitrogenous substances, and yields about 3 per cent, of ash.

Uses

Tragacanth is chiefly employed medicinally as a means of temporarily suspending insoluble powders in mixtures, and to give the requisite firmness to pill-masses. Very large quantities are used as a thickening agent in calico printing.

Varieties, Etc

In addition to the tragacanth described (flake tragacanth), much is imported of a very inferior quality (' hog ' gum, Caramania gum), the botanical origin of which is doubtful; such gum is usually in tears or irregular vermiform pieces, and darker in colour. Sometimes it is whitened with lead carbonate before being used to adulterate the finer qualities.

Vermicelli tragacanth is in thin vermiform pieces often of good colour; tragacanth of this nature is collected in northern Morea from A. cylleneus, Boissier et Heldreich.

Indian Tragacanth (Sterculia gum, Karaya gum) is obtained from Sterculia urens, Roxburgh (N.O. Sterculiaceoe), and possibly other species. It occurs in irregular, striated, often vermiform, whitish, or pale brownish or pinkish brown pieces, here and there with fragments of bark attached. It has a distinctly acid odour, forms a transparent, colourless jelly with water and assumes at most a slight brownish colour when boiled with 5 per cent, solution of potash (tragacanth turns canary-yellow). The powder has been used to adulterate powdered tragacanth. Scoville gives the following test: - Shake 2 grammes, moistened with alcohol, with 50 c.c. of water till homogeneous; add 2 grammes of borax dissolved in 50 c.c. of water, shake vigorously and stand over night. If Indian tragacanth is present the mucilage will be stringy. The presence of scleren-chymatous cells (from adhering bark) in the powder points to the presence of Indian tragacanth. It is largely used in India as a substitute for tragacanth.

Powdered acacia may be detected in powdered tragacanth by making a mucilage 1.5 in 50, adding guaiacol 0 5 in 50 of water and 1 drop of hydrogen peroxide. Pure tragacanth remains colourless; if acacia is present a brownish colour is developed (Caesar and Loretz).