Dielectric properties of kiwifruit associated with a combined radio frequency vacuum and osmotic drying

To develop a novel combination drying method combining radio frequency (RF) with vacuum and osmotic dehydration (OD), information on dielectric properties is essential so as to understand the interaction between the electromagnetic field and the matrix undergoing drying. In this paper, the dielectric properties of kiwifruit samples were determined across the frequency range of 10 and 3000 MHz using an open-ended coaxial-line probe and impedance analyzer at moisture contents ranging from 19.8% to 79.6% on a wet basis (w.b.) and temperatures between 20 and 80 °C. The effect of pre-treatments including OD and hot air drying (AD) on the dielectric properties, physicochemical properties and RF-vacuum drying characteristics of kiwifruit were also studied. The results showed that both the dielectric constant and loss factor decreased with decreasing moisture content and increasing frequency. At high moisture contents (above 65% w.b.), the dielectric constant decreased slightly with increasing temperature, whereas at lower moisture contents (below 50% w.b.), the dielectric constant increased sharply with increasing temperature. In addition, the OD resulted in less kiwifruit quality deterioration than AD in terms of titratable acidity, ascorbic acid, soluble solids and color. Although dielectric permittivities of OD treated kiwifruits were slightly lower than those of AD treated samples, the effect of the OD treatment on RF energy coupling was negligible compared to the AD. Therefore, OD may provide an alternative way for AD as a pre-treatment before RF drying for kiwifruits with high-quality characteristics.