Effects of temperature, moisture, and metal salt content on dielectric properties of rice bran associated with radio frequency heating.

Dielectric heating including microwave (MW) and radio frequency (RF) energy has been regarded as alternative thermal treatments for food processing. To develop effective rice bran (RB) stabilization treatments based on RF and MW heating, dielectric properties (DPs) with dielectric constant (ε') and loss factor (ε″) of RB samples at frequencies (10-3000 MHz), temperatures (25-100 °C), moisture content (MC, 10.36-24.69% w.b.) and three metal salt levels (0.05-2.00%) were determined by an open-ended coaxial probe and impedance analyzer. Results indicated that both ε' and ε″ of RB samples increased with increasing temperature and MC. The increase rate was greater at higher temperature and moisture levels than at lower levels, especially at frequencies lower than 300 MHz. Cubic order models were developed to best fit the relationship between DPs of RB samples and temperature/MC at five frequencies with R greater than 0.994. Both ε″ and RF heating rate of RB samples increased significantly with added NaCl (2%), KCl (1%) and NaOP (2%). The obtained data are useful in developing computer models and simulating dielectric heating for RB stabilization and may also provide theoretical basis for synergistic stabilization of RB under combined dielectric heating with metal salts.