Water Binding of Macromolecules Determined by Pulsed NMR

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Pulsed NMR was applied to measure the spin‐lattice (T 1 ) and the spin‐spin (T 1 ) relaxation times of the water adsorbed on sodium alginate, pectin, corn starch, casein and cellulose. T 1 was determined by means of repeated 90°‐90° pulse sequences and T 2 by the spin‐echo method. T 1 relaxation time curves for all the samples studied showed simple exponential, i.e., single phase, behavior. Plots of T 1 as a function of moisture content showed minima at 0.15–0.258 water/g dry matter; plots of T 1 vs water activity (Aw) yielded minima at Aw of 0.65. T 2 relaxation time curves for corn starch containing more than 0.56g water/g DM exhibited two‐phase behavior, indicating the existence of two water fractions of different mobility. The amount of water in the bound fraction showed a remarkable consistency (0.194 ± 0.011g water/g DM) among six samples of high moisture content. T 2 was found to increase with moisture content for all the macromolecules. Cellulose showed exceptionally long T 2 values compared to the other materials at the same moisture content, whereas pectin and sodium alginate showed short T 2 values. These results indicated that T 2 value is a measure of strength of water binding. Break points were observed in the T 2 ‐moisture curve. The moisture contents at these points corresponded to the bound water content determined by freezing and to the equilibrium moisture content at Aw of 0.9. T 2 increased exponentially with Aw.