Mobility of “Unfreezable” and “Freezable” Water in Waxy Corn Starch by ²H and ¹H 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.

Evolution of interactions between water and native corn starch as a function of moisture content

“…In fact, free water molecules, known to be very mobile, can create bonds with other bound water molecules, increasing their mobility. Besides, this interpretation is in agreement with the works of Li et al 12 and Tanner et al, 11 who observed the enhancement of water molecule mobility and a decrease in the residence time for water molecules, respectively, with NMR spectroscopy, independently of the specific limit at 30%. However, because the percentage of bound water does not change above 30% moisture, there is a balance between water molecules creating bonds with starch and other molecules destroying such links.…”

“…The appearance of free water from 30% moisture shows that the polymer may have created all the possible interactions with water, and the water, which is likely to be bound, cannot exceed 33% on a dry basis. This specific limit at MC ϭ 30% was also evoked by Li et al 12 in a study on a waxy corn starch, although they found a slight increase in the bound water content beyond 30% moisture. By carrying out TSC experiments on keratin, which is a protein and not a polysaccharide like starch, Leveque et al 4 noticed a boundary at a water content of 25%, beyond which the intensity of the thermocurrent did not change, whereas the amplitude dropped below 25%.…”

“…where ⌬H o is the crystallization enthalpy of permutated water and ⌬H is the crystallization enthalpy of the sample Other authors such as Li et al 12 used another available mode that consisted of cooling a sample at a definite rate from room temperature to about Ϫ40°C. Then, with heating, a fusion endotherm of water at about 0°C is observed and enables the calculation of W f and W u .…”

“…9 -11 Li et al 12 proposed simplifying the hydration problem by distinguishing water molecules, which create strong interactions with starch, from others. The first category corresponds to the bound water, and the second corresponds to the free water.…”

NMR and DSC Water Study During Osmotic Dehydration of Actinidia deliciosa and Actinidia chinensis Kiwifruit

“…10 Recently, the approach of combining different analytical techniques has been adopted to better investigate the status of water in several systems. [11][12][13] Differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR) can offer a different but complementary point of view in studying the water mobility. 11,13,14 The amount of unfreezable water within a sample after being cooled below zero is determined by DSC.…”