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 dielectric properties of selected low-moisture products (corn starch, curry, paprika, 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, to determine the change in dielectric properties at higher temperatures. 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 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. It was possible to obtain a comparison between the two techniques and to have a general idea of the range of values for dielectric properties of foods with low moisture. The OECP technique requires good contact between product and probe, which was possible with the compression system developed in this study. The cavity perturbation technique has been proven to be reliable for many foods, especially at low moisture contents, requiring a specific microwave applicator to be designed for each frequency range. Graphical abstract