ABSTRACT

The effect of sample thickness (5, 10 and 15 mm) and method of drying; hot air drying (50, 60 and 70 ⁰C), microwave drying (200, 300 and 400W), open sun and solar drying on the drying characteristics and kinetics of cooking banana slices were investigated. Results showed that cooking banana drying is a diffusion-controlled process. The rate of drying increased with increase in drying temperature, microwave power rating and solar intensity. The values of drying rate constant (k) and effective moisture diffusivity (Deff) increased with the increase in microwave power and drying temperature levels. The dependence on effective diffusivity coefficient were expressed by an Arrhenius type relationship. The results also showed that sample thickness, method of drying, drying air temperature and microwave power levels affected the drying rate and thus the drying time. It was observed that cooking banana slices dried completely within 3hrs – 11hrs:30 mins, 15 - 45 mins, 10 – 30hrs and 18 – 42 hrs under hot air drying, microwave drying, solar and open sun respectively. Irrespective of the drying methods, all the samples exhibited constant rate and falling rate period.  Fifteen thin-layer mathematical drying models were fitted to the experimental data and compared with three statistical parameters, the coefficient of determination (R²), chi-square (X²) and root mean square error (RMSE). The Midilli and KucuK model (2002) was shown to have a better fit to the experimental data obtained from oven drying, microwave drying, solar and open sun drying respectively when compared to other tested models. Effective moisture diffusion coefficients (Deff) were determined utilizing Fick’s second law equation and the values of Deff found to be in range of 1.393 - 8.89 x 10^-8 m²/s and 1.074 x 10^-7 – 8.89x10^-8 m²/s for oven and microwave drying respectively. The Arrhenius-type relationship that describes the temperature dependence of effective moisture diffusivity for oven drying was determined to be 23.599kJ/mol, 24.809 kJ/mol and 24.223kJ/mol for 5, 10 and 15 mm sample thicknesses respectively. Modified Arrhenius equation was applied to identify the activation energy of microwave drying and its value ranged from 18.619 to 11.940kJ/mol for 5, 10 and 15 mm sample thicknesses. Respective empirical equations were also generated from Midilli-Kucuk (2002) thin layer drying model that can predict the drying curves of cooking banana under the listed drying methods