Gum

Gum, gelatinous substance exuded by plants. Gums are composed of complex organic acids, called gum acids, or the salts of these acids. When hydrolysed, gum acids, such as arabin, yield sugars such as arabinose, galactose, and xylose, and simple acids. Gums have a consistency similar to glue when moist but are hard when dry. They are colourless and odourless and will not dissolve in organic solvents, although they are readily soluble in water. Gums are used as a base for mucilage, in cloth finishing and calico printing, and as emulsifying or soothing constituents of medicines.

Gum arabic, an exudate of several species of acacia, is typical of gums that contain arabin. Gum arabic of the finest quality is obtained from Acacia senegal and Acacia arabica, found in western and northern Africa. The gum forms a clear, thick solution in water. When ethyl alcohol is added to a solution of gum arabic and water, which has been slightly acidified with hydrochloric acid, arabin is produced. A similar gum, cherry tree gum, is exuded from the bark of several species of Prunus, such as common cherry and plum trees.

Gum tragacanth, which is obtained from various Turkish and Iranian species of Astragalus, particularly Astragalus gummifer, is typical of gums that contain bassorin. Gum tragacanth, like gelatin, absorbs water readily; it will take up as much as 50 times its weight in water, forming a thick mucilage.

Many gum resins and other plant exudates are commonly called gums. Gum resins are substances that contain both gum and resin, so that both water and alcohol are required to dissolve them. The principal gum resins are the so-called gums of ammoniac, asafetida, benzoin, galbanum, gamboge, myrrh, and sandarac. Latex, from which chicle, rubber, and gutta-percha are derived, is composed of gum resins, waxes, and fats. Chewing gum is usually made from chicle. See also Resins.