Thermal Food Processing

Radio Frequency Dielectric Heating 471
TABLE 15.1
Electromagnetic-Based Heating Methods
Heating Method Key Parameter Frequency Range
Ohmic
(Lower f)
(Electric field)
Capacitive dielectric
(Medium f )
(Electric field)
Radiative dielectric
(Higher f )
(Electric field)
Inductive–ohmic
combination
(Lower f )
(Magnetic field)
Inductive
(Medium f )
(Magnetic field)
Radiative magnetic
(Higher f )
(Magnetic field)
Common
Frequencies
s, E > sample
size
f, e′′, E 100 MHz < f < 100
GHz or wavelength <
sample size
Hysteresis losses
ƒ(B-H curves, f)
and eddy current
losses ƒ(H, s)
f, m′′, H 1 MHz < f < 100 MHz
or wavelength >>
sample size
f, m′′, H 100 MHz < f < 100
GHz or wavelength <
sample size
10 MHz, 27 MHz,
39 MHz, and others
915 MHz, 2.45 GHz,
5.8 GHz, 24.124
GHz, and others
50 Hz < f < 1 MHz 50 Hz, 60 Hz,
1–50 kHz, 450 kHz,
and others
50 MHz and others
915 MHz, 2.45 GHz,
5.8 GHz, 24.124
GHz, and others
Note: s = Electrical conductivity (S/m); E = RMS electric field intensity (V/m); e′′ = electric
permittivity (F/m); H = RMS magnetic field intensity (A/m); m′′ = magnetic permeability (H/m);
B = magnetic flux density (W/m ; f = electric field frequency (Hz).
2 )
Source: from Zhao, Y. et al., J. Food Process. Eng., 23, 25–55, 2000. With permission.
The frequency at which lattice limitations occur is called the Debye resonance. It
is the frequency at which maximum energy can be imparted to a medium for a
given electric field strength. Other higher-frequency limitations include the reduction
of field penetration into a medium as frequency is increased in the presence
of high electrical conductivity and high dielectric loss components. A typical existing
design for a dielectric heating system is shown in Figure 15.2.
RF dielectric heating differs from higher-frequency electromagnetic radiative
dielectric heating (e.g., microwave ovens). With RF heating, the wavelength of
the chosen frequency is large compared to the dimensions of the sample being
heated. With electromagnetic radiative heating, the wavelength is comparable to