Influence of the drying conditions on the drying constants and moisture diffusivity during the thin-layer drying of figs

In order to establish the influence of the drying air characteristics on the drying performance of figs (ficus carica) several drying tests have been carried out in a laboratory scale tunnel-dryer. The dryer using ambient heated air and working in closed loop was equipped with a continuous monitoring system. The investigation of the drying characteristics has been conducted in the temperature range of 55-85 degreesC and the airflow in the range of 0.5-3 m/s. An Arrhenius-type equation was used to interpret the influence of the drying air parameters on the effective diffusivity, calculated with the method of slopes in terms of energy of activation, and this was found to be insensitive to air velocity values higher than 2 m/s. The effect of the air temperature and velocity on the drying constants was determined by fitting the experimental data using regression analysis techniques. The influence of the air temperature on the drying kinetics of figs has been shown to follow the Arrhenius relationship. The strong influence of air temperature and velocity at the early stages of drying was evident, as well as the relative insensitivity of the drying process at the later stages. A value beyond which the increase of the airflow velocity has no significant effect on the drying rate was encountered and was determined to be 2 m/s, indicating the predominance of the internal mass transfer resistance over the external one. The investigation revealed that the drying kinetics is most significantly affected by temperature, with the airflow velocity having a limited influence on the drying process. Correlations expressing the drying constants and effective moisture diffusivity dependence on the drying-air parameters are reported. (C) 2004 Elsevier Ltd. All rights reserved.