Frequency- and temperature-dependent dielectric properties of fruit juices associated with pasteurization by dielectric heating

Dielectric properties of apple, pear, orange, grape and pineapple juices were studied from 20 to 4500 MHz at 15-95 °C. The dielectric constants ε^′ of fruit juices decreased as frequency increased. The loss factors ε^″ decreased with increasing frequency to a minimum between about 1000 and 3000 MHz, depending on temperature, and then increased as frequency increased. The frequency of that turning point increased, whereas the minimum ε^″ decreased with increasing temperature. The ε^″ values increased with increasing temperature below about 1000 MHz, and decreased with increasing temperature above 3000 MHz. Dielectric constants decreased consistently and linearly with increasing temperature. Linear or polynomial correlations for the dependence of dielectric properties on temperature were developed. The power penetration depth of all fruit juices decreased with increasing frequency. The depth decreased as temperature increased at lower frequencies and increased with temperature above 3000 MHz. The dielectric properties obtained in the study provide important parameters for juice pasteurization by dielectric heating.