Effects of ultrasound-assisted methods on the drying processes and quality of apple slices in microwave drying

Abstract

The strong physical effects of ultrasound can change the state of water in food. In order to study the effect of ultrasound on the microwave drying of apple slices, an ultrasonic online-assisted microwave vacuum drying (UOAMVD) device was designed. The curves of moisture content versus drying time, the change in dielectric constant and dielectric loss factor, and the flow characteristics of water molecules were obtained during microwave vacuum drying (MVD), microwave vacuum drying with the apple slices pretreated by ultrasound (MVDPU), and UOAMVD, was measured by a vector network analyzer and a low-field nuclear magnetic resonance and imaging analyzer. The differences in color and microporous structure of the different dehydrated products were determined. Compared with MVD and MVDPU, apple slices that underwent UOAMVD had a higher drying rate and improved drying uniformity, and the dielectric constant was significantly reduced. In addition, the color of the UOAMVD dehydrated samples was the closest to that of fresh apple slices, whereas the microporous structure was not obvious and there was granular starch overflow. UOAMVD significantly improved the microwave drying efficiency and quality of apple slices. This work provides a reference for enhancing the application of ultrasonic microwave combination drying.