Numerical modeling of wheat fractionation role of starch volume fraction
Résumé
Wheat endosperm is a cemented granular material composed of a binary-sized mixture of
almost spherical starch granules bounded to one another with an amorphous protein
matrix. The milling properties of wheat grains depend on this typical microstructure, largely
controlled by the genetic background and the growing conditions. An original Atomic Force
Microscopy nano-scratch methodology has been developed [1] to assess the local
rheological properties of the protein matrix, the starch granules and their interface. The
determined relative stiffness and failure strength, together with information on the phase
distribution, were then used to construct 2D numerical samples of wheat endosperm. The
granular structure of the sample with different granular packing was computed using a
Molecular Dynamics approach. The protein matrix was added in the form of bridges
connecting neighboring particles. The samples were then meshed using a triangular lattice
of one-dimensional spring elements that were characterized by stiffness and yield force.
The rheological properties of each element were set according to the location of its two
nodes leading to five different elements: starch, matrix, starch-matrix, starch-starch and
voids. The samples were then subjected to an increasing uniaxial tensile stress until failure
using an iterative procedure based on conjugate gradient minimization. The Lattice
Element Method, developed by Topin [2,3], was used for the simulations and a parametric
study was performed where the protein content, the starch granular packing and the
starch-protein adhesion, suggested to be responsible of the wheat fragmentation, were
varied. The results showed that, depending on the sample porosity, the bulk elastic
properties do not follow the mixing law of diluted composites, highlighting the granular
backbone effect of percolating particles. A non-linear evolution of the bulk elastic modulus
as a function of the sample porosity was also noted, with little effect of the granule solid
fraction. Concerning the failure properties, the results showed that the particle volume
fraction has a greater influence on the yield stress at high starch-protein adhesion and that
for the same porosity, increasing particles volume fraction leads to higher yield stress.
Finally, we noted that the yield stress strongly depends on the sample porosity at high
starch-protein adhesion and is weakly affected by the porosity at low adhesion, whatever
the particle volume fractions.
Domaines
Ingénierie des aliments
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