The starch is now fit for boxing, by shovelling the cleaned deposit into wooden chests, about 4 feet long, 12 inches broad, and 6 inches deep, perforated throughout, and lined with thin canvas. When it is drained and dried into a compact mass, it is turned out by inverting the chests upon a clean table, where it is broken into pieces 4 or 5 inches square, by laying a ruler underneath the cake, and giving its surface a cut with a knife, after which the slightest pressure with the hand will make the fracture. These pieces are set upon half-burned bricks, which by their porous capillarity imbibe the moisture of the starch, so that its under surface may not become hard and horny. When sufficiently dried upon the bricks, it is put into a stove (which resembles that of a sugar refinery), and left there till tolerably dry. It is now removed to a table, when all the sides are carefully scraped with a knife; it is next packed up in the papers in which it is sold; these packages are returned into the stove, and subjected to a gentle heat during some days; a point which requires to be skilfully regulated.
Mr. Samuel Hall obtained a patent for bleaching starch by chloride of lime in 1821. Chlorine water would probably be preferable, and might prove useful in operating upon damaged wheat.
The sour water of the starch manufacture contains, according to Vauquelin, acetic acid, acetate of ammonia, alcohol, phosphate of lime, and gluten.
During the drying, starch splits into small prismatic columns, of considerable regularity. When kept dry, it remains unaltered for a very long period. When it is heated to a certain degree in water, the envelopes of its spheroidal particles burst, and the farina forms a mucilaginous emulsion, magma, or paste. When this apparent solution is evaporated to dryness, a brittle horny-looking substance is obtained, quite different in aspect from starch, but similar in chemical habitudes. When the moist paste is exposed for 2 or 3 months to the air in summer, the starch is converted into sugar to the amount of one-third or one-half of its weight, into gum, and gelatinous starch called amidine by De Saussure, with occasionally a resinous matter. This curious change goes on even in close vessels.
Starch from potatos.—From the following table of analyses, it appears that potatos contain from 24 to 30 per cent. of dry substance:—
| Starch. | Fibrous paren- chyma. | Veg. Albumen. | Gum, Sugar, and Salts. | Water. | |
|---|---|---|---|---|---|
| Red potato | 15·0 | 7·0 | 1·4 | 9·2 | 75·0 |
| Germinating potatos | 15·2 | 6·8 | 1·3 | 3·7 | 73·0 |
| Kidney potatos | 9·1 | 8·8 | 0·8 | — | 81·3 |
| Large red potatos | 12·9 | 6·0 | 0·7 | — | 78·0 |
| Sweet potatos | 15·1 | 8·2 | 0·8 | — | 74·3 |
| Peruvian potatos | 15·0 | 5·2 | 1·9 | 1·9 | 76·0 |
| English potatos | 12·9 | 6·8 | 1·1 | 1·7 | 77·5 |
| Parisian potatos | 13·3 | 6·8 | 0·9 | 4·8 | 73·1 |
Manufacture of potato starch.—The potatos are first washed in a cylindrical cage formed of wooden spars, made to revolve upon a horizontal axis, in a trough filled with water to the level of the axis. They are then reduced to a pulp by a rasping machine, similar to that represented in [figs. 1047], [1048.], where a is a wooden drum covered with sheet-iron, roughened outside with numerous prominences, made by punching out holes from the opposite side. It is turned by a winch fixed upon each end of the shaft. The drum is enclosed in a square wooden box, to prevent the potato-mash from being scattered about. The hopper b is attached to the upper frame, has its bottom concentric with the rasp-drum, and nearly in contact with it. The pulp chest c is made to slide out, so as when full to be readily replaced by another. The two slanting boards d, d, conduct the pulp into it. A moderate stream of water should be made to play into the hopper upon the potatos, to prevent the surface of the rasp from getting foul with fibrous matter. Two men, with one for a relay, will rasp, with such a machine, from 21⁄2 to 3 tons of potatos in 12 hours.
The potato pulp must be now elutriated upon a fine wire or hair sieve, which is set upon a frame in the mouth of a large vat, while water is made to flow upon it from a spout with many jets. The pulp meanwhile must be stirred and kneaded by the hand, or by a mechanical brush-agitator, till almost nothing but fibrous particles are left upon the sieve. These, however, generally retain about 5 per cent. of starch, which cannot be separated in this way. This parenchyma should therefore be subjected to a separate rasping upon another cylinder. The water turbid with starch is allowed to settle for some time in a back; the supernatant liquor is then run by a cock into a second back, and after some time into a third, whereby the whole starch will be precipitated. The finest powder collects in the last vessel. The starch thus obtained, containing 33 per cent. of water, may be used either in the moist state, under the name of green fecula, for various purposes, as for the preparation of dextrine, and starch syrup; or it may be preserved under a thin layer of water, which must be renewed from time to time, to prevent fermentation; or lastly, it may be taken out and dried.