Investigation of Effect of Ultrasound Pretreatment on Drying and Rehydration Characteristics and Microstructure of Apple Slices

In this study, the effects of ultrasound pretreatment on hot air drying and rehydration characteristics and microstructure changes of apple slices were examined. Slices of apple cv. Granny Smith having 5 mm thickness were used as the material. Pretreatments with ultrasound application were done by an ultrasonic generator with 20 kHz frequency. Effect of pretreatment time (10, 20, 30 min) and amplitude (55 and 100 %) were evaluated. Drying experiments were performed in a cabinet drier at 60°C with 0.3 m.s-1 constant air velocity. Application of ultrasound pretreatment decreased the drying time of apple slices in some conditions. Amongst the five thin layer drying models tested, the Page and Modified Page models found to be the best fitting models for the expression of the drying of apple slices. Microstructural changes caused by ultrasound pretreatment enhanced the rehydration ability of dried apples. As the pretreatment time and ultrasound amplitude increased, the rehydration ratio had an increasing trend. Peleg model represented the rehydration kinetics of the samples successfully. Changes in the microstructure of the apple samples caused by the ultrasound pretreatment were clearly observed with scanning electron microscopy images. Structural changes, caused by ultrasound pretreatment in apple tissues, resulted in low shrinkage values.