Complex permittivity of fully ripe palm fruit and its application for microwave heating

This paper presents the frequency and temperature-dependence complex permittivity (ε = ε\´ + jε") of three layers (the mesocarp, shell and kernel) in palm fruit var. Tenera. The complex permittivity was measured and reported at frequencies between 0.5-6 GHz from 10-70°C. Dominated by the frequency characteristic of free water in these three layers, the ε\´ and ε" decreased significantly with the increase in temperature at the measured frequencies. The temperature characteristic of ε at 2.45 GHz was investigated. It was found that ε\´, ε" and the loss tangent for the mesocarp layer were higher than those of the kernel and shell layer. This was especially so for the loss tangent case; this means that the mesocarp layer can absorb microwave energy and convert it to heat more readily than the other two layers. The penetration depth,Dp, for each layer at 2.45 GHz between 10-70°C was computed and confirms volumetric heating does occur in all three layers. Temperature-dependence models for εof all three layers at 2.45 GHz were formulated by applying regression analysis to the measured ε with a 95% confidence interval. The three models were applied to form a multilayer concentric model for palm fruit and can be applied for microwave heating. The temperature distribution profiles in the palm fruit predicted from the model and the experimental results showed good agreement.