| Abstract
| - The self-diffusion coefficient of water in soybean protein dispersion and tofu was measured by pulsedfield gradient (PFG) NMR. A soy protein isolate (SPI) dispersion (6 and 12%, w/w) in water, calciumcross-linked precipitate, and tofu were used for comparison. The self-diffusion coefficient of water(D) in the SPI dispersion, 2.23 × 10-9 m2/s, was estimated lower than that of free water, 2.6 × 10-9m2/s at 25 °C, and decreased as the SPI concentration increased. It further decreased by the additionof calcium chloride, reflecting the obstruction effect induced by the precipitates in addition to thehydration and hydrodynamic interaction in the protein dispersion. The two water regions in tofu wereinterpreted by the two-site Kärger model: D1 and D2 of soft tofu were 2.26 (±0.11) × 10-9 and 6.84(±0.34) × 10-11 m2/s, respectively. The relative amount of proton (water) was p1 = 0.98 and p2 =0.02 at 100 ms of diffusion time. The self-diffusion coefficients of water decreased in pressed tofu,and their relative amounts of water changed to p1 = 0.93 and p2 = 0.07. It was suggested that D1corresponded to obstructed water in the network structure and D2 corresponded to hydrated wateron the surface layer of pores formed in the protein network of tofu. The pore sizes estimated fromthe diffusion length of obstructed water were 21.3 μm in soft tofu and 20.8 μm in pressed tofu. Theremoval of fat from pressed tofu led to a decrease in D2 from 6.26 (±0.31) × 10-11 to 3.53 (±0.18)× 10-11 m2/s, and the relative amount of hydrated water increased from 0.07 to 0.14, which indicatedhydrophobic hydration. Keywords: Tofu; soybean curd; soy protein isolate (SPI); water self-diffusion coefficient; PFG-NMR
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