Study of rice-starch structure by dynamic light scattering in aqueous solution

The expansion of various rice starches in solution was studied using dynamic light scattering (DLS) in order to provide information on their microstructure. Hydrodynamic diameters of starch molecules were found to change with solvent conditions. The component starches in the rice varieties studied had similar sizes in pure water (between 125 and 235 nm), but showed different expansion behaviour with changes in salt concentration, with addition of urea (which disrupts hydrogen bonding) and with the addition of 1-butanol (which reduces solvent polarity). Some starches showed a gradually increasing size with increasing [NaCl], while others showed an initial steep increase followed by more gradual behaviour. The size distributions from DLS indicated two (and possibly three) components at about 100 and 1000 nm. The larger component was largely responsible for the expansion with added salt, an effect which is ascribed to the greater ability of larger chains to expand. Chain length distributions (CLD) of debranched starch from the same samples were examined by capillary electrophoresis (CE) for correlation with the expansion behaviour obtained by DLS. The ratio of shorter to longer chains partly correlated with the classes of expansion behaviour. The data suggest that the expansion behaviour is sensitive to the branching structure (connectivity) as well as to the distribution of the lengths of the branches.