Radio frequency disinfestation treatments for dried fruit: Dielectric properties

Phytosanitary/quarantine regulations for many countries require that certain dried fruit be disinfected prior to export; however, current technologies involve the use of toxic chemicals and conventional thermal methods are either undesirable or cause loss of volatile components, browning and texture change. Newer physical methods including dielectric heating have been considered, but information on dielectric properties of dried fruits is lacking. Because the loss factor of insect pests, Indian meal moth (Plodia interpunctella) and navel orangeworm (Amyelois transitella), is several times (26-36) greater than that of dried fruits, RF treatment in particular has great potential for insect disinfection. In this study, the dielectric properties of raisins, dates, apricots, figs, and prunes with water contents of 15-30.2 g/100 g, were determined between 10 and 1800 MHz over a range of 20-60 °C. The dielectric constant and loss factor of all samples decreased with increasing frequency, but increased with increasing temperature at each frequency. The loss factor of all samples increased with increasing water contents/water activity (0.5-0.7). The penetration depths (dps) of RF energy in all samples decreased with increasing frequency and temperature. The deep dp (28.4-103.7 cm) at 27 MHz indicates the potential for developing continuous and large-scale RF treatments for postharvest insect control in dried fruits.

► Permittivity of dried fruits decreased with increasing frequency. ► Permittivity of dried fruits increased with increasing moisture and temperature. ► Moisture affects the dielectric properties of samples more than other components. ► Differential heating between insects and samples is possible at RF range. ► Penetration depths of samples decreased with increasing frequency or temperature.