On the Diffusion Constant of Water in Wheat

Abstract

Diffusion-weighted magnetic resonance imaging (MRI) was used to obtain diffusion constants for water in the embryo and endosperm of wheat. Our experiments showed a significant difference between the diffusion constant for the two components. It was also shown that water diffusion in both the endosperm and embryo deviates from the typically observed Gaussian behavior in bulk fluids, showing a time-dependent diffusion constant. Diffusion constants for the embryo and endosperm were shown to differ by an order of magnitude. Using a model for restricted diffusion, information on the endosperm pore size and the embryo cell dimensions could be obtained.

Measurement of Water Diffusivities in Barley Components Using Diffusion Weighted Imaging and Validation with a Drying Model

“…The pulse sequence used in this study is based on the spin-echo sequence described in detail by Gruwel et al [31] in which the PFGs were symmetrically positioned around the refocusing pulse and were applied in the read-out direction. The excitation and refocusing pulse (Hermite shaped) durations were 1000 and 633 ms, respectively.…”

“…The spin-echo PFGNMR sequence diagram, which is used in this study, is given elsewhere. [31] Cooper et al [40] mentioned the following advantages of a spin-echo PFGNMR technique: (1) interval between two successive gradient pulses (D) is a definite value during which the diffusion is measured, (2) D can be varied independent of time (s) to separate the RF pulses which avoids uncertainty caused by any change in s, and (3) g can be made larger because the duration of d can be reduced to a very small value at the time the RF pulses are applied, which permits the use of shorter diffusion times to observe restricted water diffusion. Due to diffusion, the spins precess at different frequencies as they move along the magnetic field gradient but the spins do not totally refocus at the time of the echo.…”

“…3. For D o , we used a value of 220 Â 10 À5 mm 2 =s for the diffusion coefficient of bulk water at 20.5 AE 0.5 C. [31] The Marquardt-Levenberg algorithm was employed in OriginPro to minimize the v 2 of the fit. While performing the three-parameter fit to Eq.…”

“…[24] The DWI is an imaging modification of the pulsed field gradient nuclear magnetic resonance (PFGNMR) technique. [26] The PFGNMR method has been successfully used by several researchers to measure D in cereal grains (wheat, [27][28][29][30][31] corn [32] ). However, these studies reported either a bulk D value of the whole grain or a single D value of only the endosperm or its parts.…”

“…We were not aware of any prior studies that reported D values of the embryo for any grain kernel. Gruwel et al [31] first took an initiative of using the DWI to measure the D values of individual wheat kernel components (i.e., endosperm and embryo) and to describe the pore geometry of the wheat kernels. Mitra et al [33] indicated that the PFGNMR can be used to study a range of pore geometries (0.10-10 mm), which are not accessible to standard techniques such as X-ray or small-angle neutron scattering.…”