A comparison of the kinetics of mango drying in open-air, solar, and forced-air dryers

Abstract

Mangoes are under-utilized fruits that grow naturally in many sub-Saharan African countries. At the present time most mangoes are sold fresh in local markets. There is little done to preserve them for use during the off-season. Drying is one way in which the economic potential of mangoes could be exploited. This study was undertaken to investigate and compare the kinetics of mango drying using three basic drying methods: open-air drying on wire mesh racks; solar drying in a prototype dryer equipped with solar-powered exhaust fans; and forced-air drying in an Armfield Model UOP8 laboratory-scale tray dryer. Results could then be used to determine appropriate drying techniques for mango processing in sub-Saharan Africa on both local and commercial scales. Of these methods, forced air drying was found to provide the best overall results, based on water removal rates and general control over the drying process. Solar drying, while viewed as a promising technology for application in developing countries, was considerably slower than forced- air drying and is severely restricted by climatic conditions. A similar situation was observed for open-air drying, which was the slowest drying method of the three. Based upon mathematical models developed for each drying method, 11.6 hours was predicted as being required for mangoes in the forced-air dryer to a final moisture content of 10% (wet basis). Sixteen (16) hours and 24 hours of exposure to appropriate drying conditions were predicted as being required for solar drying and open-air drying, respectively. This could take three or four days to achieve under actual operating conditions. These times were supported by experimentally determined values. The impact of air temperature and linear air velocity on the drying kinetics of sliced mangoes were also investigated using the forced-air dryer. A linear velocity of 0.5 m/s was found to be sufficient for satisfactory drying of the mango slices when combined with an air temperature in the range of 50ΕC to 60ΕC. It is recommended that forced-air drying be utilized whenever possible for the drying of mango slices for both food safety and food quality reasons.