Acacia. U. S. (Br.). Acacia.
Acac. [Gum Arabic]
"The dried gummy exudation of Acacia Senegal Willdenow, and of other African species of Acacia (Fam. Leguminosae)." U. S. "A. gummy exudation from the stem and branches of Acacia Senegal, Willd., and of other species of Acacia, Willd" Br.
Acaciae Gummi, Br.; Gummi Africanum; Gummi Mimosa; Gum Acacia; Galam Gum; Gomme Arabique Vraie, Fr.; Gummi Arabicum, P. G.; Arabisches Gummi, G.; Gomma Arabica, Gomma del Cordofan. It.; Goma Arabiga, Sp.: Samagh Arabee, Arab.
The name Acacia was employed by the ancient Greeks to designate the gum tree of Egypt, and has been appropriately applied to the genus in which that plant is included.
The most important of the gum-yielding Acacias is the official A. Senegal Willd. This is a small tree rarely exceeding a height of 6 m., with a grayish bark, the inner layers of which are strongly fibrous, bipinnate leaves, dense spikes of small yellow flowers longer than the leaves, and broad pods containing 5 or 6 seeds. It forms large forests in Western Africa, north of the river Senegal, and is abundant in Eastern Africa, Kordofan, and Southern Nubia. It is known by the natives of Senegambia as Verek and of Kordofan as Hashab.
Nearly all species of Acacia growing in Africa yield a gum. The commercial Somali gum, which is usually of fair quality, is yielded by A. glaucophylla Steud. and A. abyssinica Hochst., shrubs growing in Abyssinia and the Somali country. The following species yield a gum having a brownish or reddish color (called Amrad Gums), and hence are less valuable, viz., A. arabica Willd., A. stenocarpa Hochst., A. Seyal Del. and A. Ehrenbergiana Hayne. Inferior gums are also yielded by the following: A. horrida Willd., which furnishes the so-called Cape gum and is distinguished by being very brittle and yielding a less adhesive mucilage. Talca or Sennarr gum is derived from A. Fistula Schweinf. This gum has a greenish tinge and yields a ropy mucilage. Australian gum has a reddish color, said to be due to the presence of tannin. This gum is also spoken of as Wattle gum or Australian gum, and is derived from the Golden Wattle (A. pycnantha Benth.., a shrub growing in New South Wales, Victoria and Southern Australia. For commercial history of the several varieties of gum arabic, see U. S. D., 19th edition, p. 2. For further information in regard to gum bearing trees of Northern Africa, see P. J., Aug., 1873; C. R. A. S., t. lxxix, p. 1175; Toxicologie Africaine, vol. ii.
The bark and unripe fruit of the acacia contain both tannic and gallic acids. The dried juice of the pod was used by the ancient Greeks; and an extract is still sold in the bazaars of India under the name of Akakia. This extract is heavy, hard, of an agreeable odor, varying in color from greenish to dark-reddish, or, when seen in bulk, blackish. It has a sweet, astringent taste, and yields a mucilaginous infusion. A similar preparation, acacia nostras, has been prepared in Europe by expression and inspissation from the unripe fruit of Prunus spinosa, or wild plum tree.
The gum of the acacias exudes spontaneously from the bark, and hardens on exposure; but incisions are sometimes made in order to facilitate the exudation. The gum is said also to be found immediately under the bark, where it is sometimes collected in regular cavities. It is formed within the plant by metamorphosis of the cells of the inner bark. The tissues involved are chiefly those of the sieve and the cambiform cells. While to some extent it is a natural change, yet it is usually looked upon as being in part a pathological production, as gummosus develops more largely upon the wounding of the trees. The attack of the Acacia plants by various parasites is held by some to account for the enormous production of gums in these plants. The investigations of Smith tends to show that all vegetable gums are of bacterial origin and that the differences in the several gums are due to the differences in the nature of the bacteria producing them. (Proc. Linn. Soc. N. S. Wales, 1904, p. 217;) For further discussion on the origin of acacia gum see Tschirch, "Handbuch der Pharmakognosie."
The trees are not tapped for gum until they are about six years old. The annual yield varies very greatly, being from 188 to 2856 Gm. in young trees and from 379 to 6754 Gm. in large trees. The average annual yield of gum from young trees is about 900 Gm. and from old trees over 2 kilos. (Edie, 4th Report of the Wellcome Tropical Research Laboratory.)
It is stated by Jackson that, in Morocco, the greatest product is obtained in the driest and hottest weather, and from the most sickly trees. An elevated temperature appears to be essential, for in cooler climates, though the tree may flourish, it yields no gum. It is probable that some species of acacia yield finer gum than others, but it is also certain that the same tree will often yield some gum of the finest quality in regular tears or globular masses, and some irregular shaped, dark colored fragments of inferior value. Thus, from the same tree it will exude frothy or thick, and clear or dark colored, and will assume, upon hardening, different shapes and sizes; so that the pieces, when collected, require to be assorted before being delivered into commerce. This sorting is usually done on its arrival in European ports and only the selected or picked gum corresponding to the U. S. P. description should be used.
Commercial History and Varieties.—There are two principal commercial varieties of gum arabic: 1. The Khordofan, Turkey or Arabian Gum and 2. The Senegal or West African Gum, both of these being derived from A. Senegal. The former of these has the finer commercial qualities, being nearly white or faint yellowish-white and yielding a more or less transparent viscid mucilage.
KHORDOFAN, TURKEY, OR ARABIAN GUM.—This variety was formerly the only kind designated as gum arabic and entered commerce almost exclusively through Egypt, being collected in Upper Egypt, Nubia, Khordofan, Darfur, and other regions of the Upper Nile, and carried to Alexandria, from whence it passed directly into the world's commerce or entered the latter through Smyrna, Trieste, or some other Mediterranean entrepot. At one time the more or less colored varieties were known as gum gedda, while the white and fine drug was known as gum turic, names derived from Jiddah and Tor, Red Sea ports, through which the varieties were supposed to be respectively exported. Three chief commercial varieties are recognized. (a) Hashabi or Khordofan gum, the finest of these varieties, was collected in the country westward of the White Nile; formerly it constituted the bulk of the superior gum arabic of commerce. (b) Sennari gum was an inferior variety, yielding a mucilage which turned sour more quickly than that produced by Khordofan gum. It was collected in a country eastward of the White Nile, and in the region of the Blue Nile. (c) Ghezzirah or Gezireh gum is obtained from Ghezzirah and neighboring countries and sent to Trieste. It has much of the same appearance as the official Khordofan gum.
Formerly, Khordofan gum was collected in all parts of the Soudan and Upper Egypt, and forwarded on the backs of camels to Assouan, at the head of the Nile navigation, often being many months en route. The closure of the Soudan by the Mahdi caused Egyptian gum almost to disappear from commerce, but the reopening of the region by the British government has resulted in large quantities of the gum being brought by rail to Assouan, whence after being sorted into three varieties it is put in palm-leaf sacks and sent down the Nile. The finest Egyptian gum consists of large roundish or smaller more or less irregular fragments, transparent but usually rendered opaque upon the surface by innumerable minute fissures. The inferior gum varies from yellow to dark-reddish as the quality deteriorates, and often contains impurities. H. C. Wood found that the gum arabic comes into Assouan in sacks or mats which are simply piled in rectangular roofless spaces surrounded by walls about ten feet high, made of dry mud. He was informed by the traders that the gum is gathered during the months of January, February and March by collectors having vested rights in a certain portion of the forests. Long incisions are made vertically through the bark and the exuding gum allowed to harden, the trees not being injured by the process, the collections going on from year to year. Gedda gum, sometimes spoken of as an Egyptian gum, enters commerce through Geddah, or Jiddah, on the Arabian side of the Red Sea. It seems to be the same as Mecca or El Wisch or Aden gums, which are sometimes spoken of as Egyptian gums, but are probably produced in the triangular peninsula which forms the eastern extremity of Africa.
Suakin gum, Talca or Talha gum, from A. stenocarpa and A. Seyal, is exceedingly brittle, and usually semi-pulverulent. It is a mixture of nearly colorless and brownish gums, is exported at Alexandria, and is sometimes termed gum savakin.
SENEGAL OR WEST AFRICAN GUM.—This variety is sometimes incorrectly referred to as synonymous with the official gum. It. is derived from A. Senegal and other species of Acacia growing in the Soudan and Senegal. It yields a good adhesive mucilage and is valuable for technical purposes. Some of the best qualities of Senegal gum are also adapted for certain pharmaceutical uses. There are a number of different commercial varieties named from their geographical sources. It was introduced into Europe by the Dutch. The French afterwards planted a colony on the western coast of Africa, and took possession of the trade. St. Louis, at the mouth of the Senegal, and Portendic, considerably farther north, are the ports in which the commerce in gum chiefly centers. Immense forests exist in the interior, containing many species of the genus Acacia, all of which are said to yield gum, as is affirmed do also various trees belonging to other genera. The chief harvest begins in October and ends in December, although gum is also collected in March. The dry winds, which prevail after the rainy season, cause the bark to crack; the juice flows out and hardens in masses, which are often as large as a pigeon's egg, and sometimes as that of an ostrich. It is affirmed that the exudation is also largely caused by a parasitic plant, Loranthus Acaciae Zucc., the gummy exudation freely oozing out at the point where the parasite penetrates the bark. (Ph. Centralh., Aug., 1895.) Senegal gum is usually in roundish or oval unbroken pieces, or in straight or curled cylindrical pieces of various sizes, in the finest grades whitish or colorless, but generally yellowish, reddish, or brownish red. The pieces are larger than those of Turkey gum, less brittle and pulverizable, and breaking with a more conchoidal fracture. In recent years the commercial supply of gum Senegal has been a variable one. In 1911 the gum was extremely scarce, there was practically none on the market, owing to a failure of the crop.
INDIA GUM.—Persian gum, which is said to be sent from Persia to Assouan to be packed as genuine gum arabic, can be distinguished from the latter, which it closely resembles, by its not dissolving in water. Sickenberger thinks that it is the product of Prunus bokharensis or of P. puddum. India gum is not produced in India, but reaches Bombay from Aden and the Red Sea ports, and is almost certainly the product of the Soudan, being therefore identical with Egyptian gum. It occurs in India in two forms, "Maklai," in large round tears or vermicular pieces, white, yellow or reddish, and much fissured; and "Maswai," in fragments and vermicular pieces similar in color and fissuring to the other variety. It is often crudely adulterated before reaching Europe, especially with Bassora gum, distinguished by its insolubility in water. Of recent years various India gums have come into commerce in increasing quantities as substitutes for true gum arabic. India gum is brought to this country partly from Calcutta or Bombay, and partly by way of England. It usually comes in large cases. We have seen a parcel said to have come directly from the Red Sea, enclosed in large sacks made of a kind of matting, and bearing a close resemblance to the gum from Calcutta, except that it was more impure, and contained numerous large, irregular, very brittle masses, not much less than the fist in size. The term India gum should not be confused with India gum Tragacanth, which is called simply by the name India gum in the confectionery and ice cream trades where it is used as a filler and to give bulk and firmness; nor with Indian gum. (See Gummi Indicum.)
SUBSTITUTES FOB GUM ARABIC.—According to A. Mander, the East India gums appearing in the London market are:
Amrad, Amra or Oomra Gum.—A name applied to various East Indian gums having a reddish color and being in more or less irregular stalactitic pieces. (See U. S. D., 19th ed., p. 4.)
Ghatti Gum.—A pale gum consisting of rounded or vermiform pieces varying in size, clear internally, but dull and roughened on the surface, apparently caused by shrinkage in drying; from brownish-yellow to perfectly colorless and transparent. According to the Pharmacographia Indica, this gum is readily distinguished from all others by its dull white, roughened surface and glassy fracture free from cracks; it possesses about double the viscosity of gum arabic.
Ghatti gum is said to be obtained from Anogeissus latifolia Wallich in enormous quantities, to be much used in India, and to be exported from Bombay in the pure state.
Small quantities of gum are produced in India by a large number of trees often having no botanical relations with the Acacias. For an account of these gums see Pharmacographia Indica, vol. i.
Under the names of Brazilian gum, Para gum, and gum angico, large quantities of a gum occurring in large dark-amber or dark-brown glossy drops, soluble in water, are occasionally found in commerce. It is said to be the product of Acacia angico. Its mucilage is very adhesive, but usually too dark in color for pharmaceutical purposes. It must be distinguished from the gum resin often known as Brazilian gum, which is said to be obtained from Hymenaea courbaril, and is used in making varnishes.
Chagual or Maguey gum of Chili occurs in hollow cylindrical pieces from 0.2 to 1.5 cm. in thickness, occasionally having the form of stalactites or irregular tubers. On their inner surface they are longitudinally streaked, while their outer surface is usually numerously fissured, the fissures penetrating deeply toward the interior. In the absence of these, the pieces are of glassy brightness, transparent, and of very dense structure internally. The color varies from colorless, through yellowish and brownish to a tolerably deep brown, isolated pieces being almost black. According to the experiments made by Guehm, the commercial drug is scarcely fitted for technical use as a gum, but the clear pieces when made into a concentrated mucilage by prolonged heating answer the purposes of the calico printer well. Puya chilensis, P. lanuginosa, and P. lanata are commonly said to be the sources of the gum, though the researches of Hartwich make this uncertain. The exudation is asserted to be the result of the bite of a caterpillar, Kastina elegans. (Zeit. Oest. Apoth. Ver., Aug. 1, 1896.)
Specimens of gum arabic are sometimes found in commerce, which are soluble in water with difficulty. According to Kochlin, if ten parts of such gum, fifty parts of water, and three parts of a 12 per cent. solution of hydrogen dioxide be heated together for two or three hours, the gum is rendered easily soluble. (Nat. Drug., 1894, 176.) Related to the acacia gums are wood gum, from the wood of foliage trees, yielding xylose on hydrolysis; cherry gum, the gum of cherry and almond trees, yielding ß-arabinose on hydrolysis; peach gum, from the peach tree, yielding arabinose and galactose on hydrolysis; barley gum, obtained in the nitrogen-free extractive material of cereals, yielding galactose and xylose. Martina examined twenty-seven varieties of gum, and the composition of some of the principal ones is given in the table below.
|Origin.||Source.||Ash.||Lime.||Mucic Acid.||Galactose.||Furfurol.||Pentaglucose.||Total Glucose.|
|Acacia dealbata||Van Diemen||0.65||....||39.09||52.12||8.89||17.68||73.93|
|Gum of Apricot||4.20||1.85||9.16||12.21||17.27||34.52||43.48|
|Gum of Plum||2.15||1.07||5.19||6.92||31.03||62.06||66.47|
|Gum of Cherry||2.50||1.00||6.13||8.17||23.07||46.14||56.38|
Thos. Maben gives the following method of testing mucilage obtained from various gums sold for gum arabic as the best that he has been able to devise after much experimentation. Two or three drops of the mucilage prepared from the gum are placed on a glass or porcelain slab, and one or two drops of the following reagents added; these are then stirred together with a glass rod and the results compared. In the case of borax, acacia mucilage at once agglutinates or hardens into a gummy mass, similarly with basic lead acetate and ferric chloride, while it gelatinizes or forms a softer mass with potassium silicate. Similar reactions are given by the Senegal gums, the Indian Amrad gums, white Barbary, white and brown Cape, and Geddah gums. Barbary brown and Amrad give a jelly only with borax, otherwise they react as acacia. Australian gum agglutinates with borax, but only gelatinizes with basic lead acetate, and has no reaction with ferric chloride and potassium silicate. Brazilian gum has no reaction with potassium silicate, but gelatinizes with borax and ferric chloride and slightly with basic lead acetate. Ghatti gum gelatinizes with all four reagents, but in a slight degree only with potassium silicate. Oomra gum reacts similarly to acacia, except that it is entirely unaffected by basic lead acetate, and forms a softer jelly with ferric chloride. There are, of course, shades of difference in the various reactions which cannot be indicated by these terms, but, generally speaking, a fair idea is given of the nature of the gum. (P. J., March 1, 1890, 717.)
General Properties.—Gum arabic is "In ovoid, more or less spheroidal tears, or in broken, angular fragments from 2 to 30 mm. in diameter, varying from white or yellowish-white to light amber-colored; translucent; very brittle; fractured surface glass-like, sometimes iridescent; nearly inodorous; taste insipid, mucilaginous. Insoluble in alcohol; slowly and almost completely soluble in twice its weight of water, forming a mucilaginous liquid, which has a slight, characteristic odor and is acid to litmus. When either 0.1 mil of basic lead acetate T.S., or 0.1 mil of a concentrated solution of sodium borate, or 0.1 mil of ferric chloride T.S. is added to 10 mils of a 10 per cent. aqueous solution of Acacia, a gelatinous precipitate is produced. The precipitate produced with ferric chloride T.S. is neither black nor brownish-black (tannin). The powder is whitish, with few or no starch grains or fragments of vegetable tissues. Not more than 1 per cent. of powdered Acacia is insoluble in water (plant tissues, sand, or dirt). Acacia yields not more than 4 per cent. of ash, and the powder contains not more than 15 per cent. of moisture." U. S.
"When dissolved in an equal weight of water, the solution is not glairy, and after admixture with more water, yields no gummy deposit on standing. An aqueous solution (1 in 10) exhibits slight laevorotation (absence of dextrin, certain sugars, etc.). 10 millilitres of the same solution are not precipitated by solution of lead acetate; are not, after previous boiling and cooling, colored blue or brown by 0.1 millilitre of N/10 solution of iodine (absence of starch and dextrin), or bluish-black by T. Sol. of ferric chloride (absence of tannin). Ash not more than 4 per cent." Br.
The commercial gum arabic contains 17 per cent. of water and 3 per cent. of ash, consisting almost entirely of calcium, potassium and magnesium carbonates.
The gum dissolves slowly, at ordinary temperature, in twice its weight of water, forming a thick glutinous liquid of distinctly acid reaction. It is insoluble in alcohol, ether, and the oils. One hundred parts of 22 per cent. alcohol, by volume, dissolve 57 parts of gum, diluted alcohol containing 40 per cent. alcohol takes up 10 parts, and 50 per cent. alcohol only 4 parts (Flückiger). On adding hydrochloric acid to the aqueous solution and precipitating with alcohol, a colorless amorphous substance is obtained. This is arabic acid. On hydrolysis, it yields galactose, arabinose, and a pentabiose named arabinon. The arabin (or arabic acid) may also be prepared by placing a solution of gum, acidulated with hydrochloric acid, on a dialyzer, when calcium chloride will diffuse out, leaving behind solution of arabin.
Arabic acid dried at 100° C. (212° F.) has the composition 2C5H10O5 + H2O; the water separates when it unites with bases. It has a decided tendency to form acid salts. Concentrated nitric acid forms with it nitro-compounds; diluted nitric acid, on the other hand, gives rise to mucic and saccharic acids, together with oxalic and a little tartaric acid. Diluted sulphuric acid on prolonged boiling gives rise to arabinose, or arabin sugar (pectinose, or pectin sugar), C2H10O5, which reduces alkaline copper solution and turns the plane of polarization 121° to the right. Kiliani (Ber. d. Chem. Ges., 1887, 339) first established the formula as given above, and it is now recognized as belonging to the class of pentoses. They are not fermentable, and, on prolonged boiling with diluted hydrochloric acid, lose the elements of water and yield furfurol, C5H4O2. Neutral lead acetate does not precipitate an aqueous solution of gum arabic, but the basic acetate forms, even in a very diluted solution, a precipitate.
Prolonged heating of the dry gum causes it to change readily into metarabic (metagummic) acid, which is identical with the cerasin found in the beet and in cherry gum. Sulphuric acid will also change arabic into metarabic acid. 25 Gm. pure gum arabic are covered with 50 mils strong alcohol, 10 mils water, and 5 mils sulphuric acid, and allowed to stand twenty-four hours. On pouring off the fluid, and washing the residue with alcohol and with water, metarabic acid remains behind as a voluminous mass, which dries to a white, tasteless, and odorless powder of acid reaction. (Graeger, Jahresbericht der Chem., 1872, 781.) The metapectic acid which is prepared by Scheibler from the sugar beet is identical with metarabic acid.
The principle separated by cold water from the soluble arabin proves to be the same as metarabic (metagummic) acid prepared direct from the pure gum arabic by heating, or by the action of sulphuric acid. It is also identical with gum extracted from the sugar beet by Scheibler. In the normal and sound beet this gum is insoluble in water, and merely swells up like metarabic acid, while in altered beets there is found a portion (arabin) soluble in water. (Scheibler, Ber. d. Chem. Ges., 1873, p. 612.)
The similarity of the reactions and composition of arabinose and galactose (from sugar of milk by inversion) led Kiliani to assert the identity of these two varieties of sugar, but later studies by himself, Claesson, and Scheibler have shown that they are distinct. Thus, galactose is fermentable, while arabinose is not; galactose yields mucic acid when oxidized with nitric acid, and dulcite when reduced with sodium amalgam, while arabinose does not yield either; the fusing point of the crystallized galactose is given at 142° to 144° C. (287.6°-291.2° F.), while that of arabinose is 160° C. (320° F.); galactose yields with phenylhydrazin a light yellow compound, fusing at 170° to 171° C. (338°-339.8° F.), while arabinose forms a brownish-yellow compound, fusing at 157° to 158° C. (314.6°-316.4° F.). (Scheibler, Ber. d. Chem. Ges., 17, 1731.) Arabinose is said to be obtainable only from those varieties of gum arabic that yield no mucic acid when treated with nitric acid. (Claesson, Ber. d. Chem. Ges., 14, 1271.)
Gum arabic undergoes no change by age, when kept in a dry place. Its concentrated aqueous solution remains for a considerable time unaltered, but ultimately becomes sour, from the production of acetic acid. The disposition to sour is increased by employing hot water in making the solution. The tendency of a weak solution to become moldy is said to be obviated by adding a few drops of sulphuric acid, and decanting from the calcium sulphate deposited. (A. J. P., 1872, 353.) Solution of gum arabic does not ferment upon the addition of yeast, saliva, or gastric juice; the addition of chalk and cheese, however, starts a fermentation which gives rise to lactic acid and alcohol, but not to mannite or glycerin. The addition of a solution of gum to an acidified albumin solution causes a precipitate, which disappears on further addition of gum, but the solution will then curdle and become flocculent on application of heat. Gum may be distinguished from dextrin by the following tests: 1. Gum contains no dextroglucose, which, however, is present in dextrin, and may be recognized by the copper test. (See Alkaline Cupric Tartrate Volumetric Solution, Part III.) 2. Gum contains a lime compound; hence its solution is rendered milky by oxalic acid, while a solution of dextrin remains almost clear. 3. Gum gives a slimy, yellow deposit when its solution is mixed with a neutral ferric salt.
J. Henry Schroeder examined twelve specimens of powdered acacia, and states that dextrin is not frequently used as an adulterant, and that if in using the alkaline cupric tartrate test the heat be prolonged during twenty minutes, a well denned reduction was produced even when pure Senegal gum was used. (A. J. P., 1897, 195.) R. G. Shoults (A. J. P., 1900, 267) supplemented the conclusions of Schroeder and proposed to use the difference in rotatory power of acacia and dextrin as a test. He found a solution of pure acacia to have a laevorotatory power of -18°, while one of dextrin was dextrorotatory varying to the extent of from +120° to +138°. This method of testing the presence of dextrin may be replaced by the simpler one with the microscope. (See also Kebler's paper, Am. Drug., 1901, 343.) The following test is given in the Ph. Post, 1894, 563. Add 3 mils of a solution consisting of 15 drops of solution of ferric chloride, 15 drops of a saturated solution of potassium ferrocyanide, 5 drops of HCl (1-125), and 60 mils of water to a 20 per cent. solution of the gum. If the gum arabic is pure, it will remain a clear yellow for from eight to ten hours. If there is dextrin present, the color changes to blue.
The properties above enumerated belong to gum arabic generally. There are, however, varieties with differences which deserve notice.
1. Gum that is transparent and readily soluble. This constitutes by far the greater portion of the commercial varieties distinguished by the names of Turkey and Senegal gum. It is characterized by its transparency, ready solubility, and the comparatively slight degree of thickness and viscidity of its solution. Under this head may be included the gomme blanche fendillee of Guibourt. It is distinguished by the whiteness and deficient transparency of the pieces, attributable to the minute cracks or fissures with which they abound, and which render them very brittle and easily pulverizable. This peculiar structure is generally ascribed to the influence of solar heat and light, but is conjectured by Hayne to arise from the exudation of the juice in the frothy state noticed by Ehrenberg. Though the unbroken pieces are somewhat opaque, each minute fragment is perfectly transparent and homogeneous. This variety, in consequence of its prompt and entire solubility, is usually preferred for medicinal use and for most purposes in pharmacy.
2. Gum less transparent and less soluble. Guibourt has proposed for portions of this gum the name of gomme pelliculee, from the circumstance that the masses are always apparently covered, on some part of their surface, by a yellowish opaque pellicle. Other portions of it have a mammillary appearance on the surface. It is less transparent than the former variety, is less freely and completely dissolved by water, and forms a more viscid solution. It dissolves with difficulty in the mouth, and adheres tenaciously to the teeth. It is found in all the commercial varieties of gum, but least in that from Egypt. Its peculiarities have been ascribed to variable proportions of bassorin or cerasin associated with the soluble arabin. Between these two varieties there are insensible gradations, so that it is not always easy to classify specimens.
E. Bourquelot (J. P. C., 1904 , 19, 473, 474) states that acacia contains an active oxidizing ferment which renders it unsuitable for use in many pharmaceutical preparations, and that, notwithstanding the wide use of acacia as an excipient, it is possible that certain active ingredients may become altered in its presence. Nor is acacia the only gum which contains a ferment; myrrh, frankincense and bdellium also contain an oxydase. Among the substances stated as being incompatible with acacia are pyrogallol, morphine, vanillin, ordinary phenol, cresylol, ortho- and metaxylol, thymol, carvol, a- and b-naphthol, pyrocatechol, guaiacol, phenols, lysol, cresols, acetylguaiacol, veratrol, creosol, eugenol, acetyleugenol, methylaniline, ethylaniline, paratoluidine, crude aniline, xylidine, a-naphthylamine, pyramidon apomorphine, eserine, adrenaline, isobarbaloin, caffeotannic acid, gallic acid and tannin. Paraxylenol, hydroquinone, resorcinol, anisol and phenetol do not appear to be affected. Nor is the action confined to chemical substances; acacia is incompatible with the opium and Calabar bean products, suprarenal extract, aloetic preparations, and all substances flavored with vanillin, tannin-containing extracts, such as those of rhatany, catechu, and rhubarb; fluidextract of viburnum prunifolium, and kola preparations. Obviously, in cases where mucilage of acacia is concerned, this oxidizing action may be eliminated by heating the liquid to 100° C. (212° F.), and thus destroying the ferment.
Impurities and Adulterations.—As gum arabic is usually collected in huge piles at Khartoum before being shipped to Port Soudan the sand and impurities are likely to sift to the bottom. As a consequence the first orders are likely to be filled with the cleaner article while the latter containing the siftings may run as high as 4 per cent. of ash. The inferior grades are often mixed with, or substituted for, the better kinds, especially in powder. Very occasionally flour or foreign starch is fraudulently added to the powder, but may be easily detected by the use of the microscope and the blue color which it produces with tincture of iodine. In consequence of the impurities and difference in quality, gum arabic should generally be garbled. In fact, it is surprising what a good effect the removal of 1 or 2 per cent. of objectionable matter has upon the remainder of the drug. It has been proposed to change the arabinic acid of the sugar beet, by the method of Scheibler, into metarabinic acid, as the foundation of a true artificial gum arabic, but the artificial gums of the market have no such close chemical relation with the natural gum; many of them are mixtures of various substances, others are produced from starch by the action of sulphuric acid or by other means. Universal gum, a patented product obtained from potato starch, has been highly commended for the permanency and adhesiveness of its mucilage, but is said not to act well as an emulsifier. A substitute has also been made from Irish moss. (See Chondrus.) Gelatin is sometimes added to acacia as an adulterant; formaldehyde has been recommended by Trillet (Ph. Post, 1899, 629) as an agent for its detection. (See also Proc. A. Ph. A., 1900, 643.)
Uses.—Acacia is used in medicine chiefly aa a demulcent. Hence it is advantageously employed in catarrhal affections and irritation of the fauces, by being held in the mouth and allowed to dissolve slowly. It has been used as a food, but has very little if any nutritive value. In pharmacy, gum arabic is extensively used for the suspension of insoluble substances in water, and for the formation of pills and troches. Two kinds of powdered gum arabic are used, one a coarse powder called granulated, the other finely dusted. The granulated dissolves more readily in water, according to Hager, because it has lost during desiccation only two per cent. of moisture, while in preparing the "finely dusted" powder the high heat necessarily used to thoroughly dry it drives off ten per cent. of water. Its easy solubility and absence of tendency to form " lumps" cause the coarse powder to be preferred for solutions, emulsions, etc.
Acacia is incompatible with strongly alcoholic solutions, solutions of ferric chloride and lead subacetate, and strong solutions of sodium berate.