Dielectric properties of ground almond shells in the development of radio frequency and microwave pasteurization

To develop pasteurization treatments based on radio frequency (RF) or microwave energy, dielectric properties of almond shells were determined using an open-ended coaxial-probe with an impedance analyzer over a frequency range of 10–1800 MHz. Both the dielectric constant and loss factor of almond shells decreased with increasing frequency, but increased with increasing temperature and moisture content. The absolute value of the slopes of log–log plots between loss factor and frequency increased with increasing temperature at low frequencies, especially at high temperatures and moisture contents. The effective electrical conductivity of shell samples was close to zero at the lowest moisture content (6% w.b.) and 3–9 times larger at 90 °C than 20 °C for the highest moisture content (36% w.b.). A good linear relationship was observed between permittivity and density at 1800 MHz. The power penetration depths at RF range (27 and 40 MHz) were about 6–24 times as deep as those for microwave frequencies (915 and 1800 MHz) at each corresponding temperature and moisture content. It is likely that RF energy may provide uniform heating and high throughput treatments for controlling Salmonella in in-shell almonds after washing.

► Permittivity of almond shells increased with increasing temperature and moisture.
► Slopes between frequency and loss factor increased with increasing temperature.
► Penetration depths at RF range were larger than those at microwave frequencies.