Dielectric properties of dehydrated apples as affected by moisture and temperature

Dielectric properties directly influence microwave drying characteristics of food products. A knowledge of dielectric properties of foods as a function of moisture content and temperature is essential in the design and control of microwave drying systems. Dielectric constant ε′ and loss factor ε″ of Red Delicious apples (Malus domestica Borkh.) were measured over a moisture content range of 4% to 87.5% at 22°C and 60°C. At high moisture content (>70%), free water dispersion and ionic conduction accounted for the dielectric behavior. At medium moisture (∼23%), ionic conduction played a major role. At low moisture contents (∼4%), bound water accounted for the major dispersion mechanism. A decrease in moisture content resulted in a decrease in ε′ and ε″. Based on this study, we expect a strong moisture leveling effect when drying apples from 50% to 4% at elevated temperatures in 915 MHz or 2.45 GHz microwave drying systems.