Abstract
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 R2 greater than 0.994. Both ε″ and RF heating rate of RB samples increased significantly with added NaCl (2%), KCl (1%) and Na6O18P6 (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.
Highlights
Rice bran (RB), a by-product of rice milling, is obtained from outer layer of the brown rice kernel accounting for 6–8% of the paddy rice[1]
When the ε′′ of rice bran (RB) with two moisture contents (MCs) were plotted against frequency in the log-log plot (Figs 2C and 2D), it could be clearly seen that the negative linear relationships were obtained between ε′′ and frequency over 10 to 300 MHz, especially for sample with MC of 24.69% w.b
With NaCl increased from 0 to 2%, the penetration depth decreased from 322 to 82 cm at 27.12 MHz, while the corresponding decrease was from 15 to 5.6 cm at 915 MHz. These results indicate that the metal salt content instead of MC should be carefully monitored during RB stabilization using combined radio frequency (RF) heating with metal salt solutions
Summary
Rice bran (RB), a by-product of rice milling, is obtained from outer layer of the brown rice kernel accounting for 6–8% of the paddy rice[1]. Several methods have been adopted to stabilize RB by reducing the activity of lipase Among these methods, physical treatments using thermal energy with or without moistening are most commonly employed. Dielectric heating, including microwave (MW) and radio frequency (RF) energy, could rapidly raise the temperature of foodstuff volumetrically and significantly reduce the heating time[6,7]. To develop an effective dielectric heating treatment for food processing, it is important to understand dielectric properties (DPs), the major factors describing the interaction between electric fields and the food material. Knowledge of DPs of food materials is essential to the design, optimization and control of the thermal process with dielectric heating. In the past 5 years, DPs have been studied over different frequency, temperature and moisture ranges of food and agricultural products for various purposes. DPs of RB as influenced by temperature and MC and their heating behavior during RF treatments influenced by MC are still not available in the literature
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