Abstract

This study examined the drying behaviour, optimum drying conditions and proximate analysis of Moringa oleifera seeds in a tray dryer. Thirteen (13) experimental data sets were generated using Box-Behnken design of design expert at varying temperature, contact time and fan speed on response surface methodology. Data generated from the experiment were fitted into existing models to determine the best fit using Excel Solver. Experimental data was used to simulate mathematical model developed using Box-Behnken design with the objective of minimizing the moisture content of the moringa seed within the lower and upper bound of 25-50°C (drying temperature), 30-100 mins (contact time) and 0.5-2.5 m/secs (fan speed). It was shown that the moisture ratio decreases with increasing drying time in all the plots with equilibrium moisture content being achieved within the range of 60-90 minutes at different operating conditions. However, the experiments with temperature (50°C) and fan speed (2.5 m/s) exhibited lowest moisture ratio with varying time. Also, the drying rate decreased continuously throughout the drying period in all the graphs with no constant rate period at any of the operating conditions. Wang and Singh model best described the drying behaviour of the moringa oleifera seeds in a tray dryer with R2 and χ2 values of 0.9991 and 0.00017 respectively at drying temperature of 50°C, fan speed of 0.5 m/s and contact time of 65 min. The moringa seeds that were dried at temperature 37.5°C, contact time 65 mins and fan speed of 1.5 m/s have moisture content of 17.71% with highest crude carbohydrate of 75.49%. A minimum moisture content of 16.7390% was obtained at drying temperature of 25°C, contact time of 75 mins and fan speed of 2.5 m/s using the developed optimization model that gives an excellent prediction with R2 value of 0.9918.

Keywords: Optimization; Kinetics;  Drying; Proximate analysis; Tray dryer; Moringa oleifera