Thermal Food Processing

Radio Frequency Dielectric Heating 475
cavity method. 12 The probe method is based on a coaxial line ending abruptly at
the tip that is in contact with the material being tested. This method offers broadband
measurements while minimizing sample disturbance. The measured reflection
coefficient is related to the sample permittivity. 13 The probe method is the easiest
to use because it does not require a particular sample shape or special containers.
The transmission line method involves placing a sample inside an enclosed transmission
line. The cross section of the transmission line must be precisely filled with
sample. This method is usually more accurate and sensitive than the probe method,
but it is difficult to use and time consuming. The resonant cavity method uses a
single-mode cavity. Once a sample of known geometry is placed in the cavity, the
changes in reflected power of the cavity and the frequency of resonance are used
to compute the dielectric property of the sample. The cavity method can be accurate
and is especially suited for samples with a very low dielectric loss factor. However,
this method provides dielectric properties at only one fixed frequency. 14
Frequency, temperature, salt content, moisture content, and the state of moisture
(frozen, free, or bound) are the major factors that influence dielectric properties
of food materials. Many studies on dielectric properties of food in the
microwave frequency range have been reported for different frequency ranges,
temperatures, and moisture contents. 15–21 Several comprehensive reviews on
dielectric properties provide good sources of useful experimental data from many
foods and agricultural products. 22–27 However, the amount of information available
on the dielectric properties of foods in the frequency range most common to RF
dielectric heating (1 to 200 MHz) is very limited. Some of the reported studies
are summarized below.
Ede and Haddow 28 measured the dielectric constant, power factor, and specific
conductivity of foods, including beef, pork, eggs, flour, and vegetables, using a
Marconi Q-meter over the temperature range of –39 to 100°C and frequency
range of 0.1 to 40 MHz. Results indicated that for foods that are poor conductors,
the electrical conductivity varied very little with frequency. For foods that are
poor dielectrics, the variation in electrical conductivity with frequency was high.
They also found that for most foods, the electrical conductivity decreased with
reduced temperature and showed a sharp drop at the freezing point.
Bengtsson et al. 29 measured the dielectric constant and loss tangent for lean
beef and codfish over the temperature range of –25 to 10°C and frequency range
of 10 to 200 MHz using a Boonton RX-meter. They found that the values of
dielectric constant and loss tangent decreased with increased frequency and also
showed a sharp increase at defrosting temperatures (~1 to 2°C). Variation in
dielectric properties caused by variability in raw material and by frozen storage
was relatively small for meat and codfish. Dielectric properties were quite similar
for lean meat, lean fish, and herring, but of much lower value for fats.
Kent and Jason 30 elucidated the nature of the mechanisms that influenced the
dielectric properties of frozen fish muscle (cod and haddock) and measured the
dielectric properties over the frequency range of 0.1 Hz to 10 GHz. The results
revealed that the time dependence of the dielectric properties was attributable to
changes in the distribution of water between the solid and liquid states.