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Dielectric properties of food, agricultural products, biological tissues, resins, plastics, ceramics, soils, and wood at RF and microwave frequencies are considered. The influence of three main factors—frequency, temperature, and moisture content—on complex dielectric constant behavior of lossy materials is analyzed. The most popular measurement techniques—the open‐ended coaxial probe, transmission‐line, and perturbation methods—are described.

Microwave (MW) vacuum dehydration using temperature to control the level of MW power demonstrated potential in improving the performance of the process. Product surface temperature measured by an infrared temperature sensor was used to control MW power at any level between 0 and 3 kW. Multiple linear regression analysis indicated an r 2 = 0.942 for prediction of final moisture content and r 2 = 0.985 for prediction of puffed character of grapes based on product temperature, time, specific energy, fresh fruit sugar, and fresh fruit moisture content. Temperature was found to be the most significant predictor. The elemental and compound contents of grapes dried using MW vacuum was compared to sun-dried raisins. The grapes dried using MW vacuum exhibited better preservation. Vitamin A was found in the MW-vacuum-dried grapes but none was detected in the raisins, and Vitamin C, thiamine, and riboflavin were also higher in the MW-vacuum-dried grapes than in the raisins.

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