Dielectric properties of dry fruits in powder form at microwave frequencies

To facilitate future pasteurization experiments utilizing microwave energy, the dielectric properties of five dried fruits - almond (Prunus dulcis), cashew nut (Anacardium occidentale), pistachio (Pistacia vera), peanut (Arachis hypogea L.) and walnut (Juglans regia) were investigated at room temperature (28 degrees C). Samples were taken in powder form to investigate the effect of increased surface-to-volume ratio on their dielectric properties, as applicable in the case of nanomaterials. The measurements were conducted at four different frequencies (3.30, 7.50, 9.30and 14.70GHz) in the microwave spectrum by using the two-point method and a MATLAB program was used for solving the transcendental equation to obtain values of dielectric constant and loss factor. The findings revealed a decreasing trend in the values of dielectric constant of the five dried fruits as the frequency increased. However, no consistent trend was observed for the dielectric loss factor of the fruits with increasing frequency. Further, the penetration depth of electromagnetic waves in all the five dry fruit samples was observed to decrease as the frequency increased. On comparing results for dielectric parameters of dry fruits in powder form with those of whole fruit kernels, no definite trends are observed for epsilon ' and epsilon '', but the penetration depth is found to decrease when the sample is taken in powder form.