PURPOSE: To permit an easy powdering of a hygroscopic substance by adding starch to an aqueous solution containing the hygroscopic substance, heating the same until the double refraction of the powder particles is lost and thereafter drying the resulting slurry while retaining a thin film outer structure of the particles.

CONSTITUTION: The starch produced from barley, corn, potatoes, etc. is added to an aqueous solution or dispersion containing a hygroscopic substance such as various amino acids, straight chain oligosaccharide and organic salts, which is then hated until most of the starch particles have lost the double refraction. The resulting slurry is thereafter dried while the thin film outer structure of the starch particles are substantially retained. In this way the hygroscopic substance can be adsorbed and retained in a large amount and easily powdered while controlling the blocking and the bridge formation taking place between the particles originated by the hygroscopicity.