Description—Arrowroot is a brilliant white, insipid, inodorous, powder, more or less aggregated into lumps which seldom exceed a pea in size; when pressed it emits a slight crackling sound. It exhibits the general properties of starch, consisting entirely of granules which are subspherical, or broadly and irregularly egg-shaped; when seen in water they show a distinct stratification in the form of fine concentric rings around a small star-like hilum. They have a diameter of 5 to 7 mkm. when observed in the air or under benzol. If the water in which they lie be cautiously heated on the object-stage of the microscope, the tumefaction of the granules will be found to begin exactly at 70° C. Heated to 100° C. with 20 parts of distilled water, arrowroot yields a semi-transparent jelly of somewhat earthy taste and smell. By hydrochloric acid of sp. gr. 1·06, arrowroot is but imperfectly dissolved at 40° C.
The specific gravity of all varieties of starch is affected by the water which they retain at the ordinary temperature of the air. Arrowroot after prolonged exposure to an atmosphere of average moisture, and then kept at 100° C. till its weight was constant, was found to have lost 13·3 per cent. of water. On subsequent exposure to the air, it regained its former proportion of water.
Weighed in any liquid which is entirely devoid of action on starch, as petroleum or benzol, the sp. gr. of arrowroot was found by one of us to be 1·504; but 1·565 when the powder had been previously dried at 100° C.
Microscopic Structure of Arrowroot and of Starch in general—The granules are built up of layers,—a structure which may be rendered evident by the gradual action of chloride of calcium, chromic acid, or an ammoniacal solution of cupric oxide. When one of these liquids in a proper state of dilution is made to act upon starch, or when for that purpose a liquid is chosen which does not act upon it energetically, such as diastase, bile, pepsin, or saliva, it is easy to obtain a residue, which according to Nägeli, is no longer capable of swelling up in boiling water, nor is immediately turned blue by iodine, except on the addition of sulphuric acid; but which is dissolved by ammoniacal cupric oxide. These are the essential properties of cellulose; and this residue has been regarded as such by Nägeli, while the dissolved portion has been distinguished as Granulose (Maschke, 1852).
C. Nägeli in his important monograph on starch[2339] has described the action of saliva when digested with starch for a day, at a temperature of 40° to 47° C.; he says that the residue is a skeleton, corresponding in form to the original grain but somewhat smaller, light, and very mobile in water. He concludes that its interstitial spaces must have been previously filled with granulose.
This experiment, which has been repeated by one of us (F.), does not in our opinion warrant all the inferences that Nägeli has drawn from it: it is true that many separate parts of the grain are dissolved by the saliva, while others have disappeared down to a mere film, and others again have been attacked in a very irregular manner. But we cannot agree with the statement that anything comparable to a skeleton of the grain has been left. After longer action at a higher temperature, which however must not exceed 65° C., a more copious dissolution of the starch, either by saliva or by bile, takes place; but in no case is it complete.[2340]
Chemistry of Starch—Its composition answers to the formula (C₆H₁₀O₅)₂+3 OH₂, or when dried at 100° C., C₆H₁₀O₅. Musculus however showed, in 1861, that by the action of dilute acids or of Diastase, starch is resolved into Dextrin, C₁₂H₂₀O₁₀, and Dextrose, C₆H₁₂O₆, with which decomposition, the formula, C₁₈H₃₀O₁₅, would be more in accord. Sachsse (1877) on the other hand advocates the formula C₃₀H₆₂O₃₁ + 12 OH₂.
Cold water is not without action on starch; if the latter be continuously triturated with it, the filtrate, in which no particles can be detected by the microscope, will assume a blue colour on addition of iodine, without the formation of a precipitate. The proportion of starch thus brought into solution is infinitely small, and always at the expense of the integrity of the grains. It is even probable that the solution in this case is due to the minute amount of heat, which must of necessity be developed by the trituration.
Certain reagents capable of attacking starch act upon it in very different ways. The action in the cold of concentrated aqueous solutions of easily soluble neutral salts or of chloral hydrate is remarkable. Potassium bromide or iodide, or calcium chloride for instance, cause the grains to swell, and render them soluble in cold water. At a certain degree of dilution a perfectly clear liquid is formed, which at first contains neither dextrin nor sugar; it is coloured blue, but is not precipitated by iodine water; and starch can be thrown down from it by alcohol. This precipitate, though entirely devoid of the structural peculiarity of starch, still exhibits some of the leading properties of that substance; it is coloured in the same manner by iodine, does not dissolve even when fresh in ammoniacal cupric oxide, and after drying is insoluble in water, whether cold or boiling. The progress of the solvent is most easily traced when calcium chloride is used, as this salt acts more slowly than the others we have mentioned. It leaves scarcely any perceptible residue. This fact in our opinion militates against the notion that starch is composed of a peculiar amylaceous substance, deposited within a skeleton of cellulose.
The remarkable action of iodine upon starch was discovered in 1814 by Colin and Gaultier de Claubry. It is extremely different in degree, according to the peculiar kind of starch, the proportion of iodine, and the nature of the substance the grains are impregnated with, before or after their treatment with iodine. The action is even entirely arrested (no blue colour being produced) by the presence in certain proportion of quinine, tannin, Aqua Picis, and of other bodies.