Improvement of the dehulling efficiency of sorghum and millet

Abstract

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. I 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).