Thin layer drying of millet and effect of temperature on drying characteristics

Thin layer drying characteristics of two varieties of millet EX-BORNO and SOSAT C88 at air temperatures of 40, 50, 60 and 70oC was investigated. The drying data were applied to five moisture ratio models, namely, exponential, Henderson and Pabis, page, modified page and Logarithmic. The models performance was evaluated by comparing the coefficient of determination (r²) and root mean square error (RMSE) relating the experimental and predicted moisture ratios through nonlinear regression analysis. The main factor controlling the drying rate was temperature and falling rate period characterised the entire drying process. The r² and RMSE vary between 0.969 - 0.997, 0.961 - 0.993 and 0.021 - 0.064, 0.024 - 0.0535 for SOSAT C88 and EX-BORNO respectively for the five models. The modified Page and Page drying models satisfactorily describe the drying behaviour with highest R² and lowest RMSE and gave best fitting curves. The effective moisture diffusivity ranged between 2.86 x 10^-9 - 5.72 x 10^-9m2/s and 1.17 x 10^-9 - 2.98 x 10^-9 while the activation energy values were respectively 36.19 and 43.94 kJ/mol for SOSAT C88 and EX-BORNO varieties respectively showing that EX-BORNO requires more energy to trigger the diffusion process.