Gums

GUMS. The generic name given to a class of uncrystallizable substances, composed of carbon, hydrogen and oxygen, which occur widely in plants, and to a limited extent in animals. Their char acteristic property is that they form viscous solutions or mucilages either by dissolving in water (soluble gums) or by absorbing many times their own volume of that solvent (insoluble gums). Com mercial gums contain more or less mineral matter, chiefly calcium, magnesium and potassium. A little nitrogen is also often present; but it is not an essential constituent. In this the gums differ from the gelatins, glues and proteins, which form solutions of a similar character; but which are definitely nitrogenous bodies.

The best known gum is gum arabic (gum acacia), which has been used in the arts from the earliest times. It is a product of the various varieties of acacia which grow widely throughout Africa and in India and Australia. The gum exudes from the tree, the process being sometimes assisted by making incisions in the bark. The world's supply of gum arabic is obtained chiefly from the Sudan (especially from the province of Kordofan) and from the French colony of Senegal, while smaller quantities come from Northern Nigeria, Morocco, India and Australia. In the Sudan (where the trees are often cultivated in groves) and in Senegal the grey-barked acacia, Acacia Senegal, is the chief gum-producing tree. The Sudanese name for this tree is hashab. The gum is col lected in the dry season and shipped to European ports where it is sorted into grades for the market.

The origin and function of the gums in plant life is not yet clearly understood. They are generally regarded, in the case of trees at least, as being due to the breaking down of cellular tissue; but while some authorities hold that they are normal products of plant metabolism, others believe that they are caused by an unor ganized ferment or enzyme produced by an invading fungus, and others again suggest that they are due to bacterial action. It is said that the more sickly the tree the better the yield of gum; but of course in such cases it may be that a body typical of healthy life in the plant is being overproduced owing to functional dis turbances, without the intervention of fungi or bacteria. It is cer tain that soil, climate and season affect the yield from a given variety of tree.

Gum arabic comes on the market in the form of rounded or vermiform nodules or "tears." The best qualities are Sudan or Kordofan gum (also known as hasliab gum and picked Turkey gum) and gomme blanche from Senegal. These are colourless or slightly yellow; whereas the lower grades have a distinctly yellow colour. The most remarkable property of gum is the viscous col loidal solution it forms with water (see COLLOIDS). It is insoluble in alcohol and the addition of the latter in sufficient quantity to an aqueous solution precipitates the gum.

Chemistry of Gum.

Up to the middle of the i 9th century when modern organic chemistry was still in its infancy, it was supposed that living matter was composed of a relatively small number of compounds which were called proximate principles, and we find the gums classed among the "proximate principles of the vegetals" (W. T. Brande, Manual of Chemistry, 1848) . In 18S4 Neubauer showed that the chief constituent of commercial gum arabic was an acid substance which he called arabin or arabic acid. This body was composed of carbon, hydrogen and oxygen, and analysis appeared to show that the two latter elements were in the proportion in which they are present in water. The gums were, therefore, placed among the carbohydrates (q.v.) . In 1868 Scheibler obtained, by the decomposition of arabin, a new sugar which he called arabinose. Notwithstanding this important dis covery, the chemistry of the gums remained obscure for many years, no doubt because their uncrystallizable character made puri fication difficult and uncertain. The key to the problem was fi nally found by C. O'Sullivan, in a series of researches, the results of which appeared in the journal of the Chemical Society be tween 1884 and 19o1. By introducing a novel method of treat ment, O'Sullivan was able to show that arabin consisted of an acid nucleus to which a number of molecules of the sugars galactose and arabinose were chemically united. To the acid he gave the formula C23H38022 and the name arabic acid, while to arabin he assigned the formula and named it di-arabinan-tetragalactan-arabic acid. The words arabinan and galactan refer respectively to two molecules of the sugars arabinose and galactose minus two molecules of water. It will be observed that the acid in the corn pound is minus It is interesting to note that the pectins which cause fruit juices to "gel" in the making of jam have recently been shown to consist of compounds of sugars with an acid.

Gum tragacantli is the chief insoluble gum. It is derived from various species of Astragalus occurring in south-west Europe, Greece and Turkey. It consists of a mixture of several gum acids combined with sugars (O'Sullivan).

Gums have a great variety of }uses, all of which depend on their viscoscity or their adhesiveness. The best qualities are used in clearing liqueurs, in sizing silk, textiles and paper, in confectionery, pharmacy and calico printing. Inferior kinds find application in the manufacture of stationery, matches and ink.

Gum resins are mixtures of gums and resins exuded by certain plants. The best known are ammoniacum, asafoetida, galbanum, gamboge, myrrh and olibanum or frankincense.

For the history of the gums see Fliickiger and Hanbury's Phar macographia. For the chemistry see H. H. Robinson, Report of the British Association, 1906, p. 227. For a general account, with statistics of production see Bulletin of the Imperial Institute, vol. 6, i9o8 ; also H. S. Blunt, Gum Arabic, with special reference to its Production in the Sudan (Oxford, 1926).

See also DEXTRIN. For Chewing-gum, see CHICLE. (T. D.)