Microwave Freeze Drying of Apple Slices Based on the Dielectric Properties

Freeze drying (FD) yields the best quality of dried apple slices but requires a long drying time and is not cost-effective. To mitigate these problems, a microwave freeze drying (MFD) technique was developed to dry apple slices. The relationship between corona discharge and microwave power at various pressures and initial moisture content conditions was studied to avoid the possibility of corona discharge during MFD. It was found that with change of moisture content and temperature of samples during MFD, the dielectric property also changed, which resulted in dynamic microwave dissipation. Based on the dielectric property of samples, a changed microwave loading scheme can lead to perfect product quality and greatly reduce the drying time; thus, MFD can be used to replace the traditional FD technique.