ABSTRACT
The radio-frequency (RF) band of the electromagnetic (EM) spectrum covers a broad range of frequencies, typically either in the kHz range (3 kHz < f ≤ 1 MHz) or the MHz range (1 MHz < f ≤ 300 MHz). Microwaves (MW) are similar to RF waves in heating behavior but have a higher frequency range, between 300 MHz and 300 GHz (Ryynanen, 1995). This part of the EM spectrum, with its low-energy content (less than 10 eV), is nonionizing, because the frequencies of EM spectrum do not have sufcient energy to ionize biological atoms. Due to the fact that several frequencies in this band are reserved for communication and other transceiving purposes, only selected frequencies, namely, the ISM frequencies, are permitted for domestic, industrial, scientic, and medical applications. These frequencies are 13.56 MHz, 27.12 MHz, and 40.68 MHz (for RF) and 915 MHz, 2450 MHz, 5.8 GHz, and 24.124 GHz (for MW). RF heating, or capacitive dielectric heating, is an innovative technique among several that are based on electrotechnologies, including
12.1 Introduction .......................................................................................................................... 215 12.2 Principles of RF Heating and Drying ................................................................................... 216 12.3 Dielectric Properties and Measuring Techniques ................................................................. 217
12.3.1 Dielectric Constant ................................................................................................... 218 12.3.2 Dielectric Loss Factor ............................................................................................... 218 12.3.3 Relationship between Electrical Conductance and Capacitance .............................. 218 12.3.4 Reaction to Electromagnetic Radiation .................................................................... 219
12.4 Advantages of RF Drying .....................................................................................................220 12.4.1 Increased Throughput and/or Shorter Process Lines ...............................................220 12.4.2 Improved Energy Efciency .....................................................................................220 12.4.3 Improved Control ......................................................................................................220 12.4.4 Contactless Heating ..................................................................................................220 12.4.5 Simpler Construction and Space Saving ................................................................... 221 12.4.6 Increased Power Penetration ..................................................................................... 221 12.4.7 Improved Moisture Leveling and Uniform Moisture Distribution ........................... 221 12.4.8 Improved Food Quality through Selective Heating .................................................. 221
12.5 Disadvantages of RF Drying ................................................................................................ 221 12.5.1 Equipment and Operating Cost ................................................................................ 221 12.5.2 Reduced Power Density ............................................................................................ 222
12.6 RF-Assisted Hot-Air Drying ................................................................................................ 222 12.7 Research and Industrial Applications of RF Drying ............................................................ 223 12.8 RF Postbaking Drying ..........................................................................................................226 12.9 Conclusions ........................................................................................................................... 227 References ......................................................................................................................................228
