Thin Layer Drying Kinetics and Modelling of Okra (Abelmoschus Esculentus (L.) Moench) Slices under Natural and Forced Convective Air Drying

Abstract

The effect of sample thickness (10 and 20 mm), method of drying (open sun, solar and hot air drying) and drying air temperature (50, 60 and 70 ^oC) on the drying characteristics and kinetics of okra slices were investigated. The results showed that sample thickness, method of drying and drying air temperature significantly (P = 0.05) affected the drying rate and thus the drying time. It was observed that okra slices would dry perfectly within 216 – 240 h, 192 -216 h, and 12 – 19 h under open sun, solar and hot air drying, respectively. Irrespective of the drying method, all the samples dried in the falling rate period with no constant rate period. Four thin-layer semi-empirical mathematical drying models (Newton, Page, Henderson and Pabis, and Logarithmic models) were fitted to the experimental drying curves. The models were compared using the coefficient of determination ( ) and the root mean square error (RMSE). The logarithmic model has shown a better fit to the experimental data obtained from the open sun, solar and hot air drying respectively as relatively compared to other tested models. Correlation between the model parameters and the drying air temperature (under hot air drying) to calculate moisture ratio in relation to the drying time were also determined. The transport of water during drying was described by application of Fick’s diffusion model and the effective moisture diffusivity was estimated. The value ranges from 0.253 to 0.901 × 10^-10 m²/s for open sun, 0.31 to 1.01 × 10^-10 m²/s for solar drying and 3.29 to 14.7 × 10^-10 m²/s for hot air drying, respectively. The Arrhenius-type relationship describes the temperature dependence of effective moisture diffusivity and was determined to be 16.74 kJ/mol and 10.39 kJ/mol for 10 and 20 mm sample sizes, respectively.

Keywords: Okra; Open sun drying; Solar drying; Hot air drying; Mathematical modelling; Effective moisture diffusivity.