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United States Department of Agriculture

Agricultural Research Service

Title: Moisture Diffusion Coefficients of Single Wheat Kernels with Assumed Simplified Geometries: Analytical Approach

Authors
item Kang, Sukwon - NEW JERSEY STATE UNIV.
item Delwiche, Stephen

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 11, 2000
Publication Date: November 1, 2000

Interpretive Summary: Moisture tempering in wheat, that is the direct addition of water to the grain, is an essential step in maximizing flour extraction from wheat kernels. As the pericarp of a wheat kernel absorbs moisture, it toughens which causes fewer small pericarp particles to be released during milling. However, as moisture increases further, certain rheological properties change, which may result in a decrease of extracted flour. Therefore, the present study was undertaken for the purpose of gaining a better understanding of kernel moisture absorption during tempering. Nine common varieties, representing six U.S. wheat classes or subclasses, were analytically modeled and tested in soaking experiments. Absorption, as characterized by the diffusion coefficient (a universally defined index), occurred more rapidly in soft wheats than hard wheats. Especially true for hard wheats, the pericarp, with a lower diffusion coefficient than the endosperm, acted as a barrier toward moisture. Use of diffusion coefficients and the mathematical model which uses geometrical correction factors to account for non-spherical kernel shape is of direct benefit to the wheat processing industry. These values and procedures provide industry with a means to determine the effect of varying process conditions without the need for actual measurement.

Technical Abstract: Using a combination of soaking data and an analytical solution of the diffusion equation, moisture diffusion coefficients of single wheat kernels were determined for nine commercial varieties representing six market classes. Two geometric conditions were examined: the whole kernel as a prolate spheroid, and the endosperm (also modeled as prolate spheroidal) and pericarp as separate components. Values from the analytical solution for a sphere were adjusted by a geometrical correction factor to more closely represent the response of a prolate spheroid. The ranges in diffusion coefficients were 0.39 x 10E-10 to 1.04 x 10E-10 m**2/s for endosperm and 0.04 x 10E-10 to 0.29 x 10E-10 m**2/s for pericarp. Compared to the pericarp, moisture diffused more rapidly in the endosperm. Soft wheats tended to have a more permeable pericarp layer than hard wheats, which resulted in a greater overall rate of diffusion, despite the endosperm of these two groups being nearly equivalent in diffusion coefficient value.

Last Modified: 8/27/2014
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