Investigation of temperature dependent dielectric properties and drying kinetics of raw cucumber using cavity perturbation technique for microwave drying process

Microwave drying has many benefits, namely energy efficiency and process speed when drying high moisture foods such as vegetables, but process optimization is contingent on knowing the properties of material in dynamic environments. Here, we examined microwave drying kinetics of raw cucumber slices (Cucumis sativus L.) and characterized temperature-dependent dielectric properties in conjunction with a novel in-situ measuring system. A custom-designed experimental setup was created containing a Vector Network Analyzer (VNA), a cavity perturbation system at 2.45 GHz, and an ultra-precision load cell to simultaneously measure mass loss and dielectric properties at 80 W. Our results confirmed that dielectric properties were strongly tied to temperature. The dielectric constant (ε^′) decreased from 73.5 to 60.5 and the dielectric loss factor (ε^″) decreased from 14.2 to 9.8 as the system increased temperature from 20°C to 80°C. The decreasing trend, in contrast to materials with strong ionic conductivity, forms the theory that dipolar relaxation of free water is the predominant mechanism for cucumber. From the drying kinetics we gave evidence of a constant rate period followed by a falling rate period and achieved a good fit with the Page model to experimental data. The dynamic changes in dielectric properties were shown to be substantially involved in the kinetics of drying rate and energy absorption efficiency. These results will provide fundamental data to develop adaptive control algorithms to avoid overheating and dynamically regulate heating balance in commercial microwave drying applications.