Effect of Soaking Temperature on the Physical Properties and Water Diffusion Coefficients of Selected Cowpea Varieties
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Date
November, 2016
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Abstract
The purpose of this study was to investigate the effect of soaking temperature on the physical properties and water diffusion coefficients of three cowpea varieties (Asontem, Hewale and Asomdwee) grown in Ghana. 10 g of all three cowpea varieties were subjected to four soaking temperatures (30, 40, 50 and 60 ℃) for 10 h during which measurements were taken at 30 min interval. The initial values for moisture content, length, breadth, thickness, equivalent radius, and thousand seed weight of Asontem were 16.53% (d.b), 7.19 mm, 6.01 mm, 4.78 mm, 3.46 mm and 146.9 g, whereas the geometric mean diameter, surface area, seed volume and sphericity were 5.90 mm, 111.00 mm2, 341.06 mm3, and 0.83, respectively. The initial values for moisture content, length, breadth, thickness, equivalent radius, geometric mean diameter, surface area, seed volume, thousand seed weight and sphericity of Hewale were 12.40% (d.b), 6.88 mm, 5.54 mm, 4.45 mm, 3.26 mm, 5.53 mm, 96.44 mm2, 281.75 mm3, 122.27 g and 0.81, respectively. The initial average values for moisture content, length, breadth, thickness, equivalent radius, geometric mean diameter, surface area, seed volume, thousand seed weight and sphericity of Asomdwee were 13.63% (d.b), 7.20 mm, 5.44 mm, 4.63 mm, 3.08 mm, 5.66 mm, 100.81mm2, 299.99 mm3, 125.36 g and 0.79, respectively. As soaking temperature increased from 30 ℃ to 60 ℃, the values of the physical properties of the three cowpea varieties decreased linearly. The length, breadth, thickness, geometric mean diameter, surface area, seed volume and sphericity of Asontem decreased linearly from 9.60 mm, 7.13 mm, 5.98 mm, 7.42 mm, 173.5 mm2, 678.2 mm3, 0.77 to 8.85 mm, 6.61 mm, 5.56 mm, 6.91 mm, 149.9 mm2, 542.7 mm3 and 0.77, respectively. The length, breadth, thickness, geometric mean diameter, surface area, seed volume and sphericity of Hewale also decreased
linearly from 9.93 mm, 6.98 mm, 5.89 mm, 7.42 mm, 173.1 mm2, 673.6 mm3, 0.75 to 9.64 mm, 6.59 mm, 5.50 mm, 7.06 mm, 156.1 mm2, 576.6 mm3 and 0.73, respectively. The length, breadth, thickness, geometric mean diameter, surface area, seed volume and sphericity of Asomdwee decreased linearly from 9.69 mm, 6.59 mm, 5.52 mm, 7.07 mm, 156.9 mm2, 581.4 mm3, 0.73 to 9.42 mm, 6.34 mm, 5.20 mm, 6.77 mm, 144.0 mm2, 511.3 mm3 and 0.72, respectively. The water absorption kinetics of Asontem, Hewale and Asomdwee cowpea varieties followed the Fick’s law of diffusion during the first hours of soaking. The values of water diffusion coefficients determined for Asontem, Hewale and Asomdwee cowpea varieties, within the temperature variation from 30 ºC to 60 ºC during soaking ranged from 5.12 x 10-10 m2/s to 6.64 x 10-10 m2/s, 3.96 x 10-10 m2/s to 5.12 x 10-10 m2/s and 4.93 x 10-10 m2/s to 6.08 x 10-10 m2/s, respectively. The influence of temperature on the water diffusion coefficient was adequately described by an Arrhenius-type equation giving activation energy values for Asontem, Hewale and Asomdwee cowpea varieties as 7.27 kJ/mol, 7.26 kJ/mol and 6.26 kJ/mol respectively.
Description
A Thesis submitted to the Department of Agricultural Engineering, College of
Engineering in partial fulfilment of the requirements for the degree of Master of Philosophy in Food and Post Harvest Engineering,
Keywords
Cowpea Varieties, Physical Properties, Modelling, Water Absorption, Arrhenius – type equation, Diffusion coefficient, Activation Energy