Dextrine (Lat. dexter, right; called also British gum, Alsace gum, starch gum, and torrefied starch), an isomeric condition of starch, having the composition C6H10O5 or C12H10O10. It is also isomeric with gum arabic, which it much resembles in appearance and in many properties, but differs in the remarkable one, from which it derives its name, of turning the plane of polarization to the right when polarized light is passed through a solution of it, instead of to the left, which is the case when a solution of gum arabic is used. Starch will also turn the plane of polarization to the right, but in a much less marked degree than dextrine; the latter having an optical rotary power of 138.68°. Dextrine is an intermediate stage between starch and grape sugar, passing into the latter by combining with H20, but differing from starch in physical qualities only. Another intermediate modification between starch and dextrine exists, according to Maschke and others, called soluble starch, which possesses a higher degree of right-hand polarizing power than dextrine and is turned blue by iodine. (See Starch.) Dextrine may be produced by several processes. 1. By carefully roasting starch in shallow pans or revolving cylinders, heated between 300° and 310° F. When the starch presents a light brown color, and emits the odor of strongly baked bread, the transformation is effected. 2. By subjecting starch to the action of nitric acid.
Payen's method was to mix 1,000 parts of dry starch with 2 parts of nitric acid of 36° Baume, diluted with 300 parts of water, and place the mixture in layers about an inch thick on brass drawers in an oven heated to about 240° F. The transformation is effected in an hour and a half or two hours. 3. By boiling starch with dilute sulphuric acid, about 11 parts of water, 4 of starch, and 1 of sulphuric acid being used. The starch is stirred in part of the water, and the acid diluted with the remainder. Both portions are then raised to about 194° F. and gradually mingled; the temperature being maintained until as great a quantity as possible of dextrine is obtained, when the liquid is boiled to arrest the production of grape sugar, which is always formed in the process. The dextrine can only be obtained pure by repeatedly dissolving it in water and precipitating with alcohol. 4. By the action of diastase on starch. If 8 or 10 parts of malt are stirred in about 400 parts of water at 80° F., and the mixture is raised to about 140°, and then 100 parts of starch are stirred in, the temperature being again raised to 158°-167°, and there maintained for about half an hour, the starch will be converted into dextrine; but unless the temperature is changed the latter will pass into grape sugar.
By raising the mixture to the boiling point, however, the transformation is arrested. - Dextrine, when pure, is solid, translucent, and uncrystallizable. It is ordinarily a brownish-colored powder, nearly tasteless, soluble in hot or cold water and in dilute alcohol, but insoluble in absolute alcohol. It is not colored blue, but red, by the action of iodine. When boiled with dilute acids and caustic alkalies, it is converted into glucose or grape sugar. If a small quantity of caustic potash is mixed with a solution of dextrine, and a dilute solution of sulphate of copper added drop by drop, the liquid will acquire a deep blue color, and will not yield a deposit while cold; but if heated to 185° F., it will be decomposed, with precipitation of oxide of copper. This test (Trom-mer's) distinguishes it from gum arabic. It has been found that the presence of dextrine prevents the blueing of starch by iodine, and will even discolor iodide of starch. Dextrine is used for stiffening cotton goods to prepare them for printing, and for sizing paper. It is a superior substitute for gum arabic, to give an adhesive layer to" postage stamps and the edges of envelopes; also to labels, particularly those for glass bottles, causing them to adhere more permanently than any other suitable substance.
Confectioners employ it in the manufacture of lozenges. It is often employed with great advantage in the preparation of bandages to keep broken bones in perfect relative position. For this purpose it is generally used as crudely prepared by the action of sulphuric acid. Potato starch is generally used in the manufacture of dextrine, on account of its cheapness and greater purity.