Dielectric Properties of Ground Hazelnuts at Different Frequencies, Temperatures, and Moisture Contents

To develop advanced drying methods using microwave or radio frequency energy, the dielectric constant (ε′) and loss factor (ε″) of ground hazelnuts at moisture contents between 4.6% and 20.3% wet basis (w.b.) were determined from 10 to 4500 MHz at 20°C to 60°C using an open-ended coaxial probe and a network analyzer. The results showed that both ε′ and ε″ decreased with increasing frequency but increased with increasing moisture content and temperature. Both ε′ and ε″ decreased more at the low frequency end of the range than at the high frequency end, especially for high moisture levels. They increased almost linearly with increasing temperature. Third-order polynomial models could be used to describe ε′ and ε″ as functions of moisture content and temperature at selected frequencies. Each model provided a good fit to the experimental data at a significance level of 0.0001 and with a coefficient of determination greater than 0.990. The power penetration depth decreased with increasing frequency, moisture content, and temperature. Large penetration depths at frequencies below 100 MHz may provide large-scale drying.