Thermal performance enhancement in hot air and solar drying of pineapple slices with ultrasonic vibration

Abstract

Hot air and solar drying are among the most popular preservation methods for fresh fruits. Ultrasonic technology could enable a faster dehydration rate at low temperatures, hence, preserving vitamins and nutrients in fruits. In this work, the application of ultrasonic vibration to pineapple drying was investigated. Experiments were conducted in hot air as well as direct solar dryers. The temperature in the hot air dryer was controlled at 60 °C, with an applied vibration frequency of 40 kHz. Three cycles of 10, 20, and 30 min with 1 min ultrasonic operation per interval were tested. The results showed that for the cycle of 10 min with 1 min vibration applied, the moisture content of pineapple slices was decreased from 87 to <13.7 %wb at a maximum evaporation rate of 49 g/h. Similar tests in the solar dryer proved to reduce the drying time from 3 to 2 days, with an increased evaporation rate of 10.6 g/h. Minimum specific energy consumption was realized by applying 1 min of ultrasonic vibration in a 10 min cycle with the solar dryer. Maximum thermal efficiency of over 63% was achieved with ultrasonic assisted solar drying.