Maize in addition to protein contains an oil-containing germ which it is necessary to remove. In this case the grain is first steeped for 2 to 4 days in warm water containing 1% of sulphur dioxide, ground in burr stone mills, and the resulting diffluent mass diluted with water is passed through a long V-shaped tank where the germ floats and is skimmed off.
Wheat contains gluten which forms an elastic paste with water and cannot be removed by levigation. The procedure carried out in this case is to reduce the grain to flour which is then made into a dough with water, divided into small pieces and placed in a semicircular silk or fine brass sieve where a travelling roller presses out the starch which is removed by a fine spray of water. The starch liquor goes through a very similar purification process to that employed for maize, of ten being treated with dilute caustic soda solution to swell the gluten and assist its removal between the sieving processes. It is necessary to carry out the final drying process at very low temperatures to prevent the gelatinization of the starch granules. Several methods, similar in principle for starches from all sources, are in use in the large factories, and consist of either bringing the starch lumps or blocks in contact with a continuous current of warm air, or evaporating off the excess of moisture in a vacuum chamber. For producing the starch in the form of powder or flour the moist lumps are either placed on an endless band running backwards and forwards in an enclosed chamber through which warm air is drawn, or they are placed on trays mounted on a trolley and passed through a tunnel in contact with warm air. In the tropics drying is usually carried
out in the open air.
Prism or crystal starch is prepared from maize or rice starch for laundry purposes, and is produced by forming the damp starch into blocks by draining off the water from the starch liquor in boxes with perforated bottoms covered with filtering cloths ; the blocks are cut into two or more sections which are partly dried in a warm chamber; the yellowish brown crust which forms on the outside is scraped off, and the blocks wrapped in paper, returned to the drying chamber and dried very slowly. Under this treat ment the blocks break up into irregularly shaped prisms techni cally termed "crystals." Pearl tapioca is made by heating the slightly moist starch from manioc in a steam heated copper pan with constant stirring, some of the cells becoming gelatinized and adhering together in small, irregular masses. Further drying follows and the crude lumps are passed through rollers and graded.
Granulated or pearl sago is prepared by drying the starch in such a way that it forms a plastic dough in which a portion of the granules have become gelatinized, forcing this through sieves, and drying the resulting granules in the air.