Peculiarities of aqueous amaranth starch suspensions

Among the starches the granules from amaranth starch (a. hypochondriacus, amylopectin type) are singular because of their extremely small size of 1-3 mum and high uniformity. However, large spherical particles of 30-80 mum in diameter were observed from spray-dried amaranth starch by environmental scanning electron microscopy (ESEM) which exhibited a characteristic fine structure. After the mixture was stirred in cold water, the large particles disintegrate into small ones of 1-3 mum diameter. The particles from the stirred suspension were characterized by static and dynamic light scattering and viscometry. Almost the same diameters were found by the three techniques which were 1.76 times larger than those for the dry starch particles investigated by ESEM. The difference in size is explained by reversible uptake of water. A molar mass of M-w,M-granule = 177 x 10(9) g/mol was measured for the granular particle. After dissolution in 0.5 N sodium hydroxide a molar mass M-w,M-amylopectin = 104 x 10(6) g/mol of the amylopectin was obtained that agreed satisfactorily with that of waxy maize. Thus the granule consists on average of 1700 amylopectin molecules. Furthermore, rheological measurements were carried out with aqueous suspensions at room temperature. A shear gradient dependence was found for concentrations higher than 6% (w/v) of granules. At c > 19% reversible gel formation was observed with G'(omega) > G(omega) and a plateau over 2 decades. The zero shear viscosity as a function of c[eta] shows behavior similar to glycogen and to latex particles. The granules, however, differ from common latex particles because of their capability to gel formation.