Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 1999
Publication Date: N/A
Interpretive Summary: Milling is an essential step in wheat processing that converts the raw commodity into a form that is usable for human consumption. Traditionally, a moistening stage known as tempering occurs immediately prior to initial break. During this period, water is allowed to penetrate the kernel for the purposes of toughening the bran and softening the endosperm, both of which lead to an enhancement in the efficiency of flour extraction. Knowledge of the material properties of the wheat kernel, specifically the inherent rate of moisture uptake, known as the diffusion coefficient, is of great potential use to millers and processors who must estimate the optimal tempering time. A study was undertaken to determine the diffusion coefficients for various commercial varieties of wheat, ranging from the very soft (club wheats) to the very hard (durum wheats). These coefficients were determined by immersion experiments of intact kernels and kernels devoid of bran. Diffusion coefficients of wheat endosperm were directly determined from the bran-free immersions, while those of wheat bran were determined by a mathematical subtraction procedure that involved intact and bran-free immersion data. Coefficients were then used to mathematically model (by finite element analysis) the uptake of moisture in kernels whose shapes were assumed to be either spherical or ellipsoidal. Agreement between mathematical model and actual measurement was very good. Use of mathematical models is of great potential benefit to the wheat processing industry because the model provides information on the dynamics of water uptake without the need for actual measurement.
A finite element moisture diffusion model was refined for the purpose of examining the distribution and migration of water within single kernels of wheat. Among the material properties (endosperm, pericarp, and germ), moisture diffusion coefficients of the endosperm and pericarp were determined from immersion experiments and checked with a finite element model. Nine commercial varieties of wheat, representing six market classes or subclasses, were examined. Through separate immersion experiments of pearled wheat, representing the endosperm alone, and intact wheat, representing the pericarp by mathematical subtraction (neglecting the contribution from the germ), the ranges in diffusion coefficients across the nine varieties were as follows: 0.46 to 1.4 x 10E-10 m**2/s for endosperm, 0.042 to 0.42 x 10E-10 m**2/s for pericarp. During finite element modeling, spherical and ellipsoidal geometries were assumed for pearled and intact kernels, respectively. With the kernel moisture ratio defined as the overall moisture content normalized with respect to the level of moisture at equilibrium, the soft varieties (e.g., Rely, Penawawa, Vanna) demonstrated a higher ratio than hard varieties (e.g., TAM107). Agreement between modeled and measured values for overall moisture content was very good, as seen by the ranges in the standard deviation of differences being 0.014 to 0.041 kg/kg and 0.008 to 0.029 kg/kg for pearled and intact kernels, respectively.