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Methods for improvement of the dehulling efficiency of sorghum and millet were investigated.
These included: studying the fundamental physical and mechanical properties of the grain and
how these properties are influenced by different process variables, the use of different pre
treatment methods to loosen the grain seed coat and hence facilitate its removal during the
dehulling process, the use of alkali dehulling as an alternative method to conventional abrasive
and traditional dehulling methods, development of a numerical model which could be used as
a tool for further study of the abrasive dehulling process and finally development of a prototype
dehuller combining dehulling principles from both traditional and mechanical dehulling systems
which could dehull tempered grain without problems experienced in current mechanical
dehullers.
The effects of grain and process variables, such as moisture content, temperature and loading
conditions on mechanical properties of grain were studied under uniaxial compression of
individual whole grain kernels. Five temperature and moisture dependent parameters, which
included modulus of deformability, ultimate strain, ultimate stress, energy to break point, force
to break point and modulus of toughness were evaluated from force-deformation relations
obtained. Each of these parameters except ultimate strain decreased with increase in kernel
moisture content and temperature. Grain strength properties also depended heavily on the
orientation of the grain during loading (flat loading vs. side loading), indicating that sorghum
kernel was anisotropic with respect to mechanical properties.
Effect of different pre-treatments on dehulling efficiency was investigated using hydrothermal
pre-treatments. These included, treating the grain with steam or tempering with distilled water
for different durations followed by drying using either unheated ambient air at 20°C or heated
air at 60°C to their initial moisture content (12%db). A tangential abrasive dehuller was used to
dehull the pre-treated grain and the effect of different pre-treatments on dehulling efficiency of
the pre-treated grain was evaluated based on the extent of seed coat removal and crude fibre
reduction in the dehulled grain. Results indicated that hydrothermal pre-treatments improved
the dehulling efficiency of sorghum and millet by an average of 8.5% and 20% respectively in
terms of seed coat removal and by 16% and 13.7% respectively in terms of crude fibre
reduction in the dehulled grain compared to untreated grain.
Tempering the grain for a short duration (<15 minutes) reduced the seed coat adhesion in
sorghum and millet substantially without affecting the grain strength properties significantly.
Seed coat adhesion strength at different tempering durations was quantified by measuring the
tangential force required to remove a unit area of seed coat using a friction apparatus.
Tempering for 15 minutes reduced the seed coat adhesion strength per unit area by 91.1% and 95.7% compared to untreated grain for sorghum and millet respectively. The effect of seed
coat adhesion reduction due to tempering, however, was reversible as the grain kernel was
dried.
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Alkali dehulling of sorghum and millet was investigated using an aqueous solution of sodium
hydroxide as a dehulling agent. Results indicated that alkali concentration, soaking time and
temperature were significant factors during the dehulling process. Soaking the grain in 10%
aqueous solution of sodium hydroxide for 10 minutes at 60°C was found to be the optimum
alkali dehulling conditions for sorghum and millet. Under these conditions 90% yield of
unbroken endosperm at 89% dehulling efficiency was achieved from a soft red sorghum
variety, which is higher than can be achieved using any of the conventional dehulling systems.
Moisture absorption characteristics of sorghum and millet during tempering were studied at
different temperatures, initial grain moisture contents, and endosperm textures. Temperature
had a significant effect on moisture absorption characteristics of both sorghum and millet,
moisture absorption increased with increase in temperature for both sorghum and millet. The
moisture absorption characteristic was modelled using Fickian diffusion model and Peleg's
model. Peleg’s model was able to simulate the moisture absorption characteristics of both
sorghum and millet at initial soaking stages more accurately than the Fickian diffusion model.
A general model, based on Peleg’s model and Arrhenius function, was developed for
prediction of tempering duration at different temperatures within 20 - 50°C temperature range.
The abrasive dehuliing process was successfully modelled using a three-dimensional discrete
element model. The computer code was validated by experiment, and good correlation was
obtained between simulation and experimental results. Several numerical experiments were
also carried out using the developed computer code to investigate the influence of mill and
grain parameters on dehulling of sorghum and millet. This model thus provides a tool which
can be used to study the dehulling process in more detail than is currently possible using other
methods.
Based on experimental results from the grain physical and mechanical property and pre
treatment tests, a prototype dehuller incorporating a dehuliing surface, which enabled it to
dehull moist grain without clogging, was designed and constructed. Grain-grain and grain-
dehuller surface friction was the main dehuliing principle. The dehuller was able to achieve
higher yield, dehulling efficiency and colour reduction
tangential abrasive dehuliing device (TADD). |
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