AbstractDielectric properties of pineapple juice and its concentrates between 15 and 50°Brix were measured with open‐ended coaxial‐line probe technique at 10–90°C from 300 to 3,000 MHz. The dielectric constants decreased with increasing concentration, temperature, and frequency. The loss factors decreased to the minimum value and then increased, depending on temperature, as frequency increased. The loss factors increased as concentration increased from 10.9 to 35°Brix for all frequencies and then either increased or decreased depending on frequency with increase in concentration. As temperature increased, the loss factor increased for frequency lower than 1,000 MHz and decreased for higher frequency range. Ionic conductivity influenced loss factor at lower frequency. Electrical conductivity increased with increasing temperature, whereas it increased and then decreased as concentration increased. The effects of temperature and concentration on power penetration depth were opposite to their effects on loss factor. Nonlinear predictive equations were developed for electrical conductivity, dielectric constant, dielectric loss factor, and penetration depth as functions of temperature and concentration at 915 and 2,450 MHz which can be used for selecting process parameters during microwave concentration of juice.Practical applicationsVacuum microwave evaporation is a recent development in the concentration process of fruit juices. The process assures the efficient energy use and better product quality because of the volumetric heating of juice by microwave energy and its evaporation under vacuum. The knowledge of dielectric properties of fruit juice as functions of temperature, concentration, and frequency is essential for understanding vacuum microwave evaporation process, designing vacuum microwave evaporator, and evaluating the suitability of fruit juice for vacuum microwave evaporation.