Combined radio frequency-vacuum and hot air drying of kiwifruits: Effect on drying uniformity, energy efficiency and product quality

Two drying methods, both individually and in their combination [RF-vacuum drying (RFVD), hot air drying (HAD) and RFVD+HAD] were evaluated for three-layer drying of 6 mm thick kiwifruit slices in order to shorten drying time, improve energy efficiency and product quality. An electrode gap of 95 mm and vacuum of 20.1 kPa were selected for RFVD applications based on previous studies. Results showed that the total drying time was the shortest (480 min) when using RFVD, followed by RFVD+HAD (600 min; 20% longer) and HAD (900 min, almost double). However, non-uniform drying patterns were observed in both RFVD and HAD methods, but the RFVD+HAD process resulted in a more uniform moisture distribution both within and among the fruit slices. The average energy efficiency of HAD was improved remarkably from 9.92% to 22.93% by applying the RFVD+HAD method. Additionally, the RFVD+HAD process resulted in better product quality attributes like color, shrinkage ratio and rehydration capacity. Therefore, the RFVD+HAD method is recommended as a time and energy efficient drying technology for kiwifruits with a better moisture distribution and product quality.