GB998775A - Process for preparing high viscosity phosphate-modified starches - Google Patents

Abstract

A process for treating alkali-metal phosphate-modified starch and separating therefrom an alkali-metal phosphate-modified starch fraction of increased viscosity when dispersed in water, said modified starch having been obtained by heating starch with an alkali-metal orthophosphate at a temperature between 120 DEG and 175 DEG C., comprises dispersing said alkali-metal phosphate-modified starch in a proportion between 10 and 20% by weight or higher up to saturation level, in an aqueous solution containing between 30 and 50 volume per cent of an inert oxygen-containing water-miscible organic liquid, at 30-70 DEG C., but below the gelatinization temperature; adding to the dispersion a sufficient quantity of the same organic liquid to precipitate an alkali-metal phosphate-modified starch fraction of substantially higher viscosity on aqueous dispersion than the original material while retaining unbound inorganic salts and lower viscosity material in the dispersed state; cooling the mixture, while agitating it, to a temperature of -25 DEG to 25 DEG C.; separating the precipitate from the cooled slurry; washing the precipitate at -20 DEG to 25 DEG c. with aqueous solutions containing higher volume per cent concentrations of the above organic liquid, and finally washing the precipitate with the above organic liquid in dry form at room temperature. The organic liquid may be one of those disclosed in Specification 998,775. In a preferred embodiment, the phosphate-modified starch has been obtained by soaking starch in an aqueous solution of alkali-metal orthophosphate, removing the liquid phase, drying the starch to a water content of below 15%, and heating the starch to 130-170 DEG C. for 1-15 hours, as disclosed in Specifications 812,363 and 812,339, the organic liquid in which the starch is dispersed is a monohydric aliphatic alcohol, e.g. methanol or ethanol, the quantity of alcohol added to the dispersion to precipitate the starch raises the concentration of alcohol in the dispersing liquid by about 10%, and the precipitated starch is washed with aqueous alcohol containing 50-80 volume per cent alcohol, then with 60-80 volume per cent alcohol, and finally with dry alcohol. During the cooling step, chlorine gas may be bubbled through the dispersion to bleach the modified starch. The alkali-metal phosphate modified starch fraction obtained may have a viscosity of 50,000-100,000 centipoises measured in 5% aqueous solution at 25 DEG C. Specifications 857,868 and 998,327 also are referred to.