The oil can be used repeatedly, requiring only occasional replenishing as it becomes oxidized. The roasting-cylinders are best made of wronght-iron, and about 10 ft. long and 1 ft. in diameter. They arc placed on a slight incline, and may be so adjusted as to work continuously, the finished product escaping at one end while the dry starch enters at the other. Several such cylinders may be placed over the same furnace, and rotated by cog-wheels. The darkest qualities of dextrine require a second roasting at a high temperature..
Payen's hot-blast furnace consists of an upper portion filled with brass trays, in which the dry starch is spread in a layer of 1-li in.; air, entering and circulating through passages, is heated by a furnace, and escapes into the upper chamber containing the starch ; parting with its heat to the latter, it descends again to be reheated. During the first part of the operation, an exit is provided for the moisture-laden air. The great faults of this system are the impossibility of regulating the temperature, and the inequality in the roasting by reason of the starch remaining stationary.
The mannfactnre of dextrine by the acid process depends on the fact that anhydrous starch, moistened with a dilute scarcely-volatile acid, and heated to 100°-125° (212°-257° F.), is trans formed into dextrine. Great care is necessary in arresting the process as soon as the dextrine is formed, to prevent its further conversion into sugar. The starch is mixed with a quantity (deter mined by experience) of dilute acid so as to form a damp powder ; this is exposed to a temperature of 100°-120° (212°-248° F.) until the transfbrmation is c,omplete. The acids chiefly used are nitric and hydrochloric ; it is essential that they contain no free chlorine, or it would pass over into the dex trine, and bleach the colours employed with the latter in printing paper, calico, &o. Sulphuric acid is seldom availed of, except for making liquid dextrine ; dextrine made with it never becomes really dry, and is generally of darker colour. For the production of liquid dextrine, both oxalic and lactic acids are likewise employed, their excess after completion of the transformation being neutralized by carbonate of lime.
By Anthon's method, the pure starch is replaced by potatoes themselves, previously freed of soluble ingredients by treatment with acidulated or alkaline water, then dried, and ground fine.
This material is acidulated with nitro-hydrochloric acid ut the rate of 0.05-0.1 per cent. on the weight of starch. Tho acidulated mass is spread on linen hnrdles in a drying-room at 38°-44° (100r-111}° F.) till it ceases to lose weight, when the temperature is raised to 70-75° (158' 167° F.) for a time, and is finally increased to 90° (194° F.), and thns maintained for hour, when the perfectly dry substanee, while still hot, is placed in tin-plate moulds at 100°-125° (212° 257° F.) for 1-2 hours, by which the formation of the dextrine is completed. The final heating in the moulds is conducted in the apparatus shown in Fig. 1172. This consists of a double-jacketed kettle, whose outer receptacle c serves as a salt water- nr oil-bath, being supplied through d. The outer shell is encased in felt and wood ; the false bottom e helps to facilitate the circulation of the con tents of the bath. The inner receptacle is divided into a series of flat cells f about 1 in. diam., containing the starch to be converted. The pipe g serves to admit cold air or liquid into the jacket, of which the pipe h is the outlet.
An approved recipe for making dextrine by oxalic.
acid is :-500 lb. potato-starch (nr an eqnivalent of grain-starch), 1500 lb. water, and 8 lb. oxalic acid, heated in a water-bath till the liquor ceases to give a blue colour with tincture of iodine, cooled off, neu tralized with chalk, left for several days, filtered, evaporated to a doughy consistence, and slowly and completely di ied.
The mannfacture nf dextrine by means of fermentative bodies, such as diastase, is now conducted only on a very small scale ; the product always contains an appreciable quantity of sugar, and is qualified by the term sucree among the French firms who make it. The process consists iu heating a mixture of starch, diastase, and water at 65°-75^ 49°-167° F.), and boiling the mass immedi ately the conversion is finished, so as to arrest as far as possible the further transformation into anat.. The syrupy nature of the article makes it difficult of transport, and it is very liable to ferment.