Deuterium Nuclear Magnetic Resonance Studies of Potato Starch Hydration

Abstract

Deuterium Nuclear Magnetic Resonance (NMR) measurements at 4.7 Tesla were used to study the hydration properties of potato starch suspensions as a function of the starch-to-water ratio. The deuterium NMR spectrum of potato starch suspensions consisted of a relatively tall, single Lorentzian (D₂O) peak and a resolved doublet (‘powder’ pattern) of about 1 kHz quadrupole splitting and low amplitude. The deuterium NMR transverse relaxation rate (R*₂) was measured for the single Lorentzian, deuterium oxide peak; this rate increased with increasing starch-to-water ratio. Deviations of such R*₂ dependences from linearity were observed only at high ratios of starch-to-water, above ~ 40% solids. In addition to the ‘free’ or bulk population (which has a very fast rotational correlation time of 5 ps), a second population of water weakly sorbed on the starch granule surface was monitored and found to be in fast exchange with the bulk water; this second water population has a rotational correlation time of 17 ps at 298 K, only about three times slower than that of bulk water. Additionally, a third population of slowly exchangeable water is present, which is “anisotropically bound” and has a highly restricted motion within the potato starch granule structure; well-defined quadrupole splittings are observed from this latter population of water in potato starch. Potato starch exhibits unique hydration properties that have not been found in cereal starches.