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The molecular mobility of water in corn starch powders (71 to 96% solids) was determined by deuterium ( 2 H) and oxygen‐17 ( 17 O) high‐field Nuclear Magnetic Resonance (NMR) spectroscopy, in deuterium (D 2 O) and oxygen‐17 enriched water (H 2 17 O), respectively. The NMR transverse relaxation rate (R 2 ) was related to both the concentration and activity of water (a w ). The concentration plot showed two regions of water mobility; trapped and monolayer water. The water activity plot showed two linear regions of water mobility; Region A, from 0.99 to 0.23 a w , and Region B, from 0.23 to 0.11 a w . The linear relationship in Region A between water mobility, as measured by R 2 and water activity is hypothesized to be due to the hydrophobic nature of the starch granules which results in diffusion limited water motion. Relaxation rates for 2 H and 17 O at high a w were compared with one another and literature data.
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Richardson et al. (1987) studied this question.
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