The starch-laden liquid from the sieves is allowed to settle in vats, and the crude starch thus secured is put through several washing, purifying and decolorizing pro cesses, going then to the dry-houses, where it is spread on steam-heated frames. This preliminary drying is succeeded by other processes of water-extraction by means of cloths, etc., or by air or vacuum pumps. Finally come refining, separating, bleaching, etc.
In some factories, the potatoes are sliced, steeped in water and allowed to ferment to facilitate the extraction of the starch. The starch-milk is also frequently run along sloping gutters, on which it deposits the starch.
Starch from Wheat.
There are several processes used for the extraction of starch from wheat. By the older methods, the grain is first steeped in water until sufficiently swollen, then, either still whole or bruised by passing through rollers, is placed in fermenting vats.
When the fermentation has been completed, the mass is, in large factories, placed in washing drums from which the starch-milk runs into tanks. When the crude starch has settled, the water is run off and the starch is mixed with clear water and passed through various sieves and then washed, refined, etc.
The fermentation greatly reduces the value of the residue, as the sour gluten is fit only for hog feed, but the process is still largely employed because it is the easiest method of loosening the especially sticky gluten of wheat.
Among the numerous methods of wheat-starch manufacture without fermentation, are several similar to that just described, except that the separation is made while the steeped grain is fresh, the sweet gluten residue having considerable commercial value.
By the most modern methods, the wheat flour is first formed into a dough or paste. If the former, it is then separated into small pieces and worked backwards and for wards by machinery over a fine sieve and under a stream of water, the starch being carried off in milk form and a glutenous mass being left. If the paste form is em ployed, the starch is extracted by washing without kneading. The most promising of all fresh-grain methods is by means of centrifugal machines, the action giving almost pure starch by separation from thin flour paste.
Starch from Rice.
Rice is of all grains the richest in starch, but it is in such form that the fresh grain processes will not extract it. Instead, the rice is first steeped in an alkaline solution, then washed and ground fine and again passed into the alkaline solution, where it remains with frequent stirring for twenty-four hours. This is followed by a rest of seventy hours, during which a partial separation is effected, the gluten having risen to the surface of the liquid and the fibrous portion of the grain and the starch having fallen to the bottom. The gluten and water are drawn off and the fibre and starch deposit stirred, then mixed and washed with an abundance of water and again allowed to stand. This second settling leaves the fibrous portion at the bottom of the tanks with a second layer of crude starch. The removal of the starch is followed by various purifying, washing, decolorizing and drying processes as for other starches.