Interactions of sorbed water with starch studied using proton nuclear magnetic resonance spectroscopy

Proton NMR measurements, made on hydrated starch, indicate that when small amounts of water (ca. 10% by weight) are sorbed by the polysaccharide, the water is highly mobile and reorients anisotropically. This may be compared with the dynamics of water in small crystalline organic hydrates, such as α-cyclodextrin hexahydrate, where the water motion is generally more constrained. Analysis of the wideline proton spectra indicates that an increase in sample crystallinity on addition of water is accompanied by an increase in the starch chain mobility. Cross-relaxation is shown to occur between the starch and the water protons via the secular ‘flip-flop’ process, rather than by proton chemical exchange. This process influences the proton spin–lattice relaxation and results in relatively fast communication between the different proton populations.