Dielectric Properties of Low Moisture Foods Measured by Open-ended Coaxial Probe and Cavity Perturbation Technique

The measurement of dielectric properties of foods is essential in the design and control of microwave drying systems as they describe the capability of a material to absorb, transmit and reflect electromagnetic energy. The relative complex permittivity, ɛ r * , explains the interactions that may take place between a product and microwave. ɛ r * is influenced by its composition (including the moisture content) and temperature. The real component of this complex permittivity is related to energy storage and is called dielectric constant, \({\text{ε}}_{\text{r}}^{{\prime }}\). The imaginary component, the loss factor or \({\text{ε}}_{\text{r}}^{\text{''}}\), is associated to energy dissipation. The dielectric properties of selected low-moisture products (corn starch, curry, paprika, dried chives, rice grain and wheat grain) were evaluated by open-ended coaxial probe (OECP) and cavity perturbation techniques. Semi-skimmed milk powder was heated at 50 and 60°C, and the change in dielectric properties with increasing temperature was measured. The increase in moisture content (from 7.19 to 13.08%, wet basis, w.b.) and its influence on the relative complex permittivity was verified for semi-skimmed milk powder. The results showed that the dielectric constant tends to increase with the increase in temperature and moisture content, and with the decrease in frequency from 2450 to 915 MHz. Values ranged from 1 for corn starch (OECP at 2450 MHz) to 4.36 for rice grain (cavity perturbation at 915 MHz). The loss factor ranged from 0.02 for curry to 0.48 for rice grain, both at 2450 MHz. Open-ended coaxial probe requires close contact between the sample and the probe; therefore, this system was not fit for measuring the dielectric properties of the tested powders (air gaps between the probe and the samples). The cavity perturbation technique has been proven to be reliable for many foods, especially at low moisture contents, but requires a specific microwave applicator to be designed for each evaluated frequency.