Microwave- and ultrasound-assisted convective drying of raspberries: Drying kinetics and microstructural changes

The drying kinetics, microstructural alteration, and rehydration properties of raspberry samples were studied experimentally in this work. Five different drying programs with the application of microwaves (MW) and ultrasound (US) were used, including convective drying (CV) as a reference test. The drying experiments were performed using a hybrid chamber dryer equipped with airborne ultrasound and microwave generators. The modified Page model was successfully used to describe the drying kinetics of raspberry fruits. Next, microstructural properties (porosity and total pore volume) of the dried samples were determined from the postprocessing of the images which were acquired using a lab-scale X-ray microtomograph. The results show a significantly shorter drying time by 54-64% for CVUS, 69% for CVMW, and 79% for CVMWUS; and a lower energy consumption resulting in energy saving of 14-23% for CVUS, 54% for CVMW, and 59% for CVMWUS as compared to CV. It is also shown that the average drying rate of raspberry samples increases by maximum fourfold with the application of both ultrasound and microwave radiations in CV. In addition, a higher porosity, total pore volume, and a better rehydration property were found for the raspberry samples dried with US than with MW assistance.