Microwave-Assisted Pulse-Spouted Bed Freeze-Drying of Stem Lettuce Slices—Effect on Product Quality

Abstract

Drying uniformity, microstructure, apparent density, rehydration, and texture properties were measured to evaluate the quality of stem lettuce slices dried in a pulse-spouted bed microwave freeze dryer. Drying was carried out in a 5-cm (o.d.) vacuum chamber at 80 ± 5 Pa, mean microwave power level of 3.2 W g−1 and pulse-spouting time interval of 10 min. Results show that microwave freeze-dried products in the pulse-spouted mode dried more uniformly as compared to those dried in steady spouted bed mode. Pulse-spouted bed mode also resulted in dried stem lettuce slices with lower discoloration, more uniform and compact microstructure, higher rehydration capacity (RC) as well as greater hardness after rehydration over shorter drying time relative to those obtained in a steady spouting condition.