Dielectric properties of pistachio kernels as influenced by frequency, temperature, moisture and salt content

Abstract. Conventional hot air drying for pistachio nuts results in high energy consumption and low product quality. To develop advanced drying methods based on microwave (MW) and radio frequency (RF) energy, dielectric properties of pistachio kernel samples at different frequencies (10-4500 MHz), temperatures (25-85 °C), moisture contents (3-27%, w.b.) and three salty levels (moisture content: 15% w.b.) were measured by an open-ended coaxial-line probe and network analyzer. The results showed that the permittivities of non-salted pistachio kernel samples were dependent on moisture content, temperature of samples, and frequency of the applied electric field. Both dielectric constant and loss factor increased with increasing temperature and moisture content. The rate of increase was greater at higher temperature and moisture levels than at lower levels, especially in low frequencies. Dielectric loss factor increased with increasing salty levels of pistachio kernel samples, but dielectric constants were not significantly affected. Quadratic polynomial equations were developed to relate dielectric properties of the non-salted samples to temperature and moisture at four specific frequencies with R2>0.978. Penetration depth decreased with increasing frequency, moisture content, temperature and salty levels. It is likely that low frequencies, such as RF, may provide potential large-scale industrial drying applications for pistachio nuts with acceptable uniformity and throughputs.