The combination of iodine with starch does not take place in equivalent proportions, and is moreover easily overcome by heat. The iodine combined with starch amounts at the utmost to 7·5 per cent. The compound is most readily formed in the presence of water, and then produces a deep indigo-blue. Almost all other substances capable of penetrating starch grains, weaken the colour of the iodine compound to violet, reddish yellow, yellow, or greenish blue. These different shades, the production of which has been described by Nägeli with great diffuseness, are merely the colours which belong to iodine itself in the solid, liquid, or gaseous form. They must be referred to the fact that the particles of iodine diffuse themselves in a peculiar but hitherto unexplained manner within the grain or in the swollen and dissolved starch.
Commerce of Arrowroot—The chief kinds of arrowroot found in commerce are known as Bermuda, St. Vincent, and Natal; but that of Jamaica and other West India Islands, of Brazil, Sierra Leone, and the East Indies, are quoted in price-currents, at least occasionally. Of these the Bermuda enjoys the highest reputation and commands by far the highest price; but its good quality is shared by the arrowroot of other localities, from which, when equally pure, it can in nowise be distinguished. Greenish,[2341] however, points out that in Natal arrowroot the layers (or laminæ) are more obvious than in other varieties, although it appears that the former is also produced by Maranta.
The importations of arrowroot into the United Kingdom during the year 1870 amounted to 21,770 cwt., value £33,063. Of this quantity the island of St. Vincent in the West Indies furnished nearly 17,000 cwt., and the colony of Natal about 3000 cwt. The exports from St. Vincent in 1874 were 2,608,100 lb, those of the Bermudas in 1876 only 45,520 lb.[2342] The shipments from the colony of Natal during the years 1866 to 1876 varied from 1,076 cwt. in 1873 to 4,305 cwt. in 1867.[2343]
Uses—Arrowroot boiled with water or milk is a much-valued food in the sick-room. It is also an agreeable article of diet in the form of pudding or blancmange.
Adulteration—Other starches than that of Maranta are occasionally sold under the name of Arrowroot. Their recognition is only possible by the aid of the microscope.
Substitutes for Arrowroot.
Potato Starch—This substance, known in trade as Farina or Potato Flour, is made from the tubers of the potato (Solanum tuberosum L.) by a process analogous to that followed in the preparation of arrowroot. It has the following characters:—examined under the microscope, the granules are seen to be chiefly of two sorts, the first small and spherical, the second of much larger size, often 100 mkm. in length, having an irregularly circular, oval or egg-shaped outline, finely marked with concentric rings round a minute inconspicuous hilum. When heated in water, the grains swell considerably even at 60° C. Hydrochloric acid, sp. gr. 1·06, dissolves them at 40° quickly and almost completely, the granules being no longer deposited, as in the case of arrowroot similarly treated. The mixture of arrowroot and hydrochloric acid is inodorous, but that of potato starch has a peculiar though not powerful odour.
Canna Starch, Tous-les-Mois,[2344] Toulema, Tolomane—A species of Canna is cultivated in the West India Islands, especially St. Kitts, for the sake of a peculiar starch which, since about the year 1836, has been extracted from its rhizomes by a process similar to that adopted in making arrowroot. The specific name of the plant is still undetermined; it is said to agree with Canna edulis Ker (C. indica Ruiz et Pavon).[2345]
The starch, which bears the same name as the plant, is a dull white powder, having a peculiar satiny or lustrous aspect, by reason of the extraordinary magnitude of the starch granules of which it is composed. These granules examined under the microscope are seen to be flattened and of irregular form, as circular, oval, oblong, or oval-truncate. The centre of the numerous concentric rings with which each granule is marked, is usually at one end rather than in the centre of a granule. The hilum is inconspicuous. The granules though far larger than those of the potato, are of the same density as the smaller forms of that starch, and, like them, float perfectly on chloroform. When heated, they begin to burst at 72° C. Dilute hydrochloric acid acts upon them as it does on arrowroot.
Canna starch boiled with 20 times its weight of water affords a jelly less clear and more tenacious than that of arrowroot, yet applicable to exactly the same purposes. The starch is but little known and not much esteemed in Europe; it was exported in 1876 from St. Kitts to the amount of 51,873 lb, besides 5,300 lb arrowroot starch.[2346]