Mobility of "Unfreezable" and "Freezable" Water in Waxy Corn Starch by (2)H and (1)H NMR.

Abstract

"Freezable" and "unfreezable" water in waxy corn starch were characterized by thermal analysis, and the mobility in those states was characterized by solid state (2)H and (1)H NMR. Water was found to be isotropically mobile for samples over a range of water contents (6.3-47% by total weight) at room temperature. Mobility increased with increasing water content and temperature. According to (1)H and (2)H NMR data, a large fraction of "unfreezable" (DSC) was relatively mobile comparable to a liquid state even down to -32 degrees C. Some anisotropically immobile D(2)O at low temperatures exhibited a solid state Pake pattern in (2)H NMR spectra, which was similar to that of frozen D(2)O (ice) with a 144 kHz splitting. The decreasing fraction of mobile water with decreasing temperature suggested that only some of the so-called "unfreezable" water could be progressively immobilized as temperature decreased. However, much of the water (>50% of water present) remained very high in mobility, regardless of the relatively rigid starch molecules in the glassy solid state.