DIFFUSION AND REACTION IN WHOLE WHEAT GRAINS DURING BOILING

The boiling of whole wheat grains was studied using magnetic resonance imaging (MRI) and pulsed-gradient-spin-echo nuclear magnetic resonanc e (NMR) (PGSE NMR). MRI was used to image moisture distributions acros s a central section of grains boiled for different times at either 100 or 120 degrees C. A moisture front gradually moved into the grain, al though some moisture did penetrate ahead of the front. PGSE NMR, which measures self-diffusion indicated that two pools of water with differ ing diffusion characteristics may exist within the grain. Diffusion mo dels suggested from the PGSE results were incorporated into finite-ele ment simulations along with the Thomas-Windle model for diffusion/rela xation in polymers. Simulation results were compared to the MRI data, and the best fit was achieved using the Thomas-Windle model with a ste p-change in diffusivity above a critical moisture content. This behavi or is similar to that of diffusion in polymers undergoing a glass tran sition, a phenomenon widely accepted to occur when starch gelatinizes.