A NMR self-diffusion study of the porous structure of starch granules

The equilibrium self-diffusion of water contained within the porous structure of three different types of starch, viz. corn, wheat and potato starch was investigated. The samples were exposed to humid air until they reached a certain water content, the value of which was determined from the 1H NMR free induction decays. The water self-diffusion coefficient was determined by means of a pulsed-field-gradient stimulated-echo experiment. This experiment is susceptible to cross-relaxation effects, i.e. the transfer of longitudinal magnetization during the experiment; therefore, the cross-relaxation process was first quantified by means of Goldman-Shen-type experiments. The water self-diffusion could then be rationalized in terms of a distribution of diffusion coefficients, with an average value much reduced compared to bulk water. No dependence on the diffusion time in the range from 70 to 340 ms was observed, indicating that there are no impermeable barriers for the diffusion of water on a length scale of micron