An assessment of the mechanisms for diffusion in the drying of bananas

This work has assessed the dominant type of diffusion in drying bananas, together with performing an analysis of the effect of temperature and moisture content on the diffusivity, investigating the variations in the diffusivities within and between the bananas, and studying the effect of the degree of ripeness on the diffusivity. Drying experiments were carried out at dry-bulb temperatures of 60 degrees C and 80 degrees C and wet-bulb temperatures of 30 degrees C and 40 degrees C. Samples prepared from different parts of the bananas and bananas at different degrees of ripeness were used in the experiments. The pore size distribution of the bananas was found to be from 1 nm to 18 nm, and the Knudsen numbers were estimated to be from 3 to 55, suggesting that Knudsen diffusion may be a significant type of diffusion occurring during the drying of bananas. However, the diffusivities, from 4.1 x 10(-7) to 7.94 x 10(-6) m(2)/s, for Knudsen diffusion are larger than those estimated experimentally. Surface diffusion may be the limiting transport mechanism for diffusion in dried bananas, which describes the diffusivity in the same way as Fickian diffusion. The diffusivity for bananas was best described as a function of temperature and moisture content with a standard error of 0.06 kg/kg. The diffusivities within and between the bananas at the same overall degree of ripeness showed only small variations, but overripe bananas were found to have larger diffusivities compared with ripe and unripe bananas. (c) 2007 Elsevier Ltd. All rights reserved.