Source, Etc

The official varieties of starch (Amylum, B.P.) are those obtained from wheat, Triticum sativum, Lamarck, from maize, Zea Mays, Linne, and from rice, Oryza sativa, Linne (N.O. Gramineoe). Wheat is largely cultivated in temperate climates, whilst maize and rice are grown chiefly in warmer or subtropical countries.

In preparing the starch from the fruits, it is essential in the first place that the cells containing it should be ruptured in order to liberate the starch grains, and in the second place that the starch grains thus liberated should be separated from other matters, both soluble and insoluble, that accompany them, especially from the nitrogenous gluten which often clings to them pertinaciously. The cells are ruptured by grinding the softened grain to a pulp, and the gluten is removed by one of the following processes, viz.:

(a) A mixture of crushed grain and water is allowed to undergo putrefactive decomposition; the gluten is decomposed, lactic, acetic, and other acids being produced and removed by washing.

(b) The crushed grain is mixed with a dilute solution of caustic soda by which the gluten is dissolved.

(c) The grain is crushed and mixed with water to a dough from which the starch grains are washed by kneading it in a stream of water, leaving a mass of gluten behind.

The starch is purified by washing, straining and allowing the milky liquid containing starch grains in suspension to clear, by which a more or less complete separation into pure starch and starch mixed with varying amounts of cell-debris is effected. It is finally dried, during which the moist mass gradually splits up into angular fragments. These are then ground to form starch powder.

Description

From whatever source starch is obtained it forms either a fine white powder, or angular or columnar masses easily reducible to powder. It is inodorous and quite insoluble in water, to which it should impart not more than a faintly acid (wheat and potato starches) or faintly alkaline (maize and rice starches) reaction. It leaves, when incinerated, only traces of ash. Boiled with water and cooled it gives a cloudy, more or less gelatinous mixture, which is coloured deep blue by solution of iodine. Air-dry starch usually retains from 12 to 16 per cent. of moisture.

Fig. 219.   Wheat Starch. Magnified 300 diam. (Tschirch.)

Fig. 219. - Wheat Starch. Magnified 300 diam. (Tschirch).

Fig. 220.   Maize Starch. Magnified 300 diam. (Tschirch.)

Fig. 220. - Maize Starch. Magnified 300 diam. (Tschirch).

Although the different varieties of starch show certain differences in the temperature at which they gelatinise, the chief means of distinguishing them and ascertaining their freedom from admixture lies in the examination with the microscope.

Wheat starch consists of large and small grains mixed together, with few intermediate in size; the former are lenticular in shape and sometimes marked with faint concentric rings. The hilum is central but not conspicuous. The larger of the large grains average about 30 to

38 microns 1 (μ) in diameter, the smaller of the large grains 15 to 25, whilst the small grains average 6 to 7.

Maize Starch consists of grains that are nearly uniform in size and rather smaller than the large grains of wheat starch; (mostly 12μ to 18μ; they are polygonal with blunt angles, or more or less rounded. In the centre there is often a small cleft, or two or three radiating from a centre (the hilum).

Rice Starch consists of extremely minute grains, averaging about 6μ in diameter. They are polygonal, with sharp angles, and without evident concentric striae, hilum, or cleft.

Varieties

The following varieties of starch, although not official are sufficiently important to call for remark: -

Potato Starch, obtained from the tubers of Solarium tuberosum, Linne (N.O. Solanaceoe), is extensively employed for various technical purposes and has been frequently used to adulterate maranta starch and certain drugs.

Fig. 221.   Rice Starch. Magnified 300 diam. (Tschirch.)

Fig. 221. - Rice Starch. Magnified 300 diam. (Tschirch).

The grains are much larger than those of any of the official starches, varying mostly from 45μ to 75μ in length and from 45μ to 60μ in breadth; larger grains may occasionally be found and smaller ones are by no means uncommon. They have usually a flattened ovoid shape and possess a small hilum that is often situated near the narrow extremity of the grain, whilst the broad end frequently exhibits a sinuate outline; the concentric striae are well marked.

Maranta Starch, obtained from the rhizomes of Maranta arundinacea, Linne (N.O. Marantaceoe), is known in commerce as St. Vincent, Bermuda, and Natal arrowroot, or simply as arrowroot, but as the latter term is applied to a number of other starches, it is desirable that this, the arrowroot of English commerce, should be specified as maranta starch. Thus the starch of Curcuma angustifolia, Roxburgh, and C. leucorhiza, Roxburgh (N.O. Scitamineae), is known as East Indian arrowroot; that of Manihot utilissima, Pohl (N.O. Euphorbiaceoe), and that of Ipomoea Batatas, Choisy (N.O. Con-volvulaceoe), as Brazilian arrowroot; that of Carina edulis, Edwards, and other species of Carina (N.O. Scitamineoe) as Queensland arrowroot, etc.

1 A micron is the one-thousandth part of a millimetre.

Fig. 222. Potato Starch. Magnified 300 diam. (Tschirch.)

Fig. 222. Potato Starch. Magnified 300 diam. (Tschirch).

The grains of maranta starch resemble in general appearance those of potato starch, but they are as a rule decidedly smaller, averaging only 30μ to 45μ in length (exceptionally 65μ, to 75μ) and 24μ to 30μ in breadth. They are rather irregularly ovoid in shape and exhibit distinct concentric striae. The position of the hilum is usually indicated by a cleft or by three or four radiating fissures, and in the majority of grains is near the broad end. The size of the grain, the character and position of the hilum sufficiently distinguish this starch from potato starch, which it otherwise resembles.

Fig. 223.   Maranta Starch. Magnified 300 diam. (Tschirch.)

Fig. 223. - Maranta Starch. Magnified 300 diam. (Tschirch).

Curcuma starch, from the rhizomes of G. angustifolia, Roxburgh, and C. leucorhiza, Roxburgh (N.O. Scitamineae), is known as East Indian arrowroot and is largely used in India, although it does not find its way, to any great extent, to this country.

The grains average about 36μ to 60μ in length, although both larger and smaller ones may be found. They vary considerably in shape, but typical ones are of an elongated ovate outline. The hilum is situated in a little projection from the narrow end of the grain, and is therefore remarkably eccentric; it is often indistinct, but its position can be found by following the concentric striae which are usually discernible. The grains are nearly flat, appearing as rods when viewed from the edge.

Fig. 224.   Curcuma Starch. Magnified 300 diam. (Tschirch.)

Fig. 224. - Curcuma Starch. Magnified 300 diam. (Tschirch).

Starch is a polysaccharose of the formula (C6H10O5)n. Boiled with water the grains swell, burst and partially dissolve, forming an opalescent solution which, sufficiently concentrated, gelatinises on cooling (starch paste). Infusion of malt or a solution of diastase acting upon starch paste converts the starch into dextrin and maltose. Saliva and pancreatic juice have a similar effect. Boiling with dilute sulphuric acid converts it almost completely into dextrose.

Dextrin is commonly prepared by heating potato starch by means of superheated steam to 180° to 200° {yellow dextrin), or by mixing the starch with 2 to 9 per cent. of nitric acid mixed with a little water and heating in thin layers to 110°(white dextrin). Both varieties are soluble in water. In yellow dextrin the form of the starch grain is but little altered.

Tapioca consists of the partially gelatinised starch from the rhizomes of Manihot utilissima, Pohl (N.O. Euphorbiaceoe); the starch is either raked on heated plates (flake tapioca) or pressed through a sieve and heated (seed, pearl, bullet pearl tapioca).

Sago is similarly prepared from the starch from the stem of Metroxylon Rumphii, Martius, or M. Sagu, Rottboell, (N.O. Palmoe). Substitutes may be prepared from other starches, e.g. potato, etc.