**5. Microwave energy and dielectric properties of different materials**

According to what had been mentioned previously, the needed enthalpy for sublimation is provided by electromagnetic radiation at microwave frequency. Microwave heating is the result of the interaction between microwave fields and dielectric properties of material. In this heating, the required power for sublimation is carried by electromagnetic fields to the frozen region, independent of thermal characteristic of dried region, and then it is dissipated in frozen region and converted to heat. This means that any frozen point is a source of heat [3]. Compared to conventional freeze-drying process, penetrating electromagnetic fields and transferring the needed energy, independent of thermal characteristics of dried region, are the main factors in accelerating dehydrating process in microwave freeze-drying process [3]. Volumetric heating of frozen material accelerates drying process as fast as 75% [7].

For dielectric heating, some specific frequencies are released by the Federal Communications Commission (FCC) in microwave region: 913, 2450, and 5800 MHz [10, 11]. The 2450MHz is more common and used in home microwave oven [10].

Microwave radiation has unique advantages that make it suitable for food industry. The following are two of them:

(a) Microwave radiation is nondestructive and non-ionization radiation; hence, it is safe and does not contaminate and deteriorate the material.

(b) The electromagnetic waves are passed from dielectric materials in microwave frequencies, while these media are not transparent to light and infrared radiation (opaque). It is a useful tool for probing the dielectric material thoroughly [12]. This property is used in freeze-drying method to transfer energy inside the dielectric.

Also, microwave radiation is widely used in agriculture: remote sensing, short-range radars, and Doppler radar. Short-range radars are helpful for recognizing and tracking the seasonal migration of birds and insects. Also, Doppler radar is used to monitor the mass flow rate of crop [12], detecting and controlling the insects in stored grains and heating seeds with impermeable coat by microwave radiation to improve their germination and determination of moisture content of agricultural products [12].

**Figure 7** shows a setup used in [9] for freeze-drying process by injecting microwave power. A microwave oven supplies sufficient adjustable energy. A magnetron with the capability of delivering 1.2 KW adjustable power at 2450MHz is used in the oven (A). The proposed material (D) is located in the middle of microwave cavity (B), and they are surrounded by a vacuum chamber (C). This cavity has dimensions 39 × 39 × 51 cm, all of its walls are made of perforated aluminum sheets to supply the needed electromagnetic boundary conditions and provide a path to free flow of water vapor simultaneously. To avoid harmful reflection from the cavity, a circulator is used just after the generator. To absorb the reflected power, it is necessary to match other ports of the circulator. A twist (J) is used to change the polarization of transmitted wave. The electric field, at the output of twist, is vertical to slab of beef meat. A bidirectional coupler (H) is used to measure the forward and reflected waves. These data are used to deter-

mine lost power, including all components in addition to proposed material (D) [2].

**5. Microwave energy and dielectric properties of different materials**

Volumetric heating of frozen material accelerates drying process as fast as 75% [7].

more common and used in home microwave oven [10].

does not contaminate and deteriorate the material.

method to transfer energy inside the dielectric.

lowing are two of them:

For dielectric heating, some specific frequencies are released by the Federal Communications Commission (FCC) in microwave region: 913, 2450, and 5800 MHz [10, 11]. The 2450MHz is

Microwave radiation has unique advantages that make it suitable for food industry. The fol-

(a) Microwave radiation is nondestructive and non-ionization radiation; hence, it is safe and

(b) The electromagnetic waves are passed from dielectric materials in microwave frequencies, while these media are not transparent to light and infrared radiation (opaque). It is a useful tool for probing the dielectric material thoroughly [12]. This property is used in freeze-drying

Also, microwave radiation is widely used in agriculture: remote sensing, short-range radars, and Doppler radar. Short-range radars are helpful for recognizing and tracking the seasonal

According to what had been mentioned previously, the needed enthalpy for sublimation is provided by electromagnetic radiation at microwave frequency. Microwave heating is the result of the interaction between microwave fields and dielectric properties of material. In this heating, the required power for sublimation is carried by electromagnetic fields to the frozen region, independent of thermal characteristic of dried region, and then it is dissipated in frozen region and converted to heat. This means that any frozen point is a source of heat [3]. Compared to conventional freeze-drying process, penetrating electromagnetic fields and transferring the needed energy, independent of thermal characteristics of dried region, are the main factors in accelerating dehydrating process in microwave freeze-drying process [3].

Also, [3] presents a microwave freeze-drying setup in a laboratory scale.

150 Emerging Microwave Technologies in Industrial, Agricultural, Medical and Food Processing

Microwave energy is used to defrosting meat. It reduces the required time from hours to a few minutes. Also, it is used in sterilizing some heat-sensitive foods and cacao bean roasting [12].

In microscopic scale, when a dielectric is subjected to the electric field, their molecules are arranged to reduce the overall electric field in the bulk of dielectric (**Figure 8**). This arrangement depends on constitutive molecules and their polarizations. In other words, the molecules reacted to the applied field.

The molecules start to oscillate by applying the electric field with sinusoidal variation. Friction between molecules in the oscillation produces heat (as a thermal source) and increases the temperature of dielectric [7, 10]. Since the needed energy for oscillating molecules is provided by the electric field, the generated heat is as a result of energy conversion (from electromagnetic to thermal). Another phenomenon, which is effective in the loss factor of a dielectric, is ionic conduction. It relates to movement of dissolved ions, and the generation of heat when these ions collide with other molecules and atoms [10]. In a macroscopic view, this phenomenon is characterized by imaginary part of permittivity. Also, real part of permittivity is known as an ability of structure to be polarized.

Generally, the electromagnetic properties of each material are specified by relative electric permittivity (*ε<sup>r</sup>* ) and magnetic permeability (*μr* ), both of them are complex quantities. The real and imaginary parts of *ε<sup>r</sup>* , which are known as dielectric constant and loss factor, are related to stored and dissipated electrical energy in the material, respectively [12]:

$$
\varepsilon\_r = \varepsilon\_r' - j\varepsilon\_{r'}' \cdot \tan(\delta) = \frac{\varepsilon\_r'}{\varepsilon\_r'}, \ p\_d = \alpha \varepsilon\_0 \varepsilon\_r' \mid E/2 \tag{4}
$$

**Figure 8.** The arrangement of molecules when dielectric is subjected in external electric field.

where *pd* is the volume density of dissipated power (W/m<sup>3</sup> ) and *E* is root-mean-square value of electric field. This is the rate of absorbed microwave energy which is converted to heat. In reality, these parameters must be replaced in the right-hand side of Eqs. (1) and (2) [10].

The dissipated power is proportional to *ε"* and square of applied electric field. The more *ε"*, the more the dissipated power. In freeze-drying process, the dissipation factor of frozen meat is much more than a dried meat, so the dissipated power in the frozen region is much more than a dried region. The dissipated power can be maximized if the material is located in where the electric field is maximum. The more *ε'*, the more the ability in polarization for a dielectric.

To obtain the electromagnetic fields formed in foodstuff, Maxwell's equations must be solved. They depend on the cavity, dielectric property, and geometry of material [10]. As known, Maxwell's equations are a set of partial differential equations that are coupled to each other:

$$
\stackrel{\rightharpoonup}{\nabla} \times \vec{E} = -\frac{\partial \vec{B}}{\partial t}, \stackrel{\rightharpoonup}{\nabla} \times \vec{H} = \vec{J} + \frac{\partial \vec{D}}{\partial t'}, \stackrel{\rightharpoonup}{\nabla} \cdot \vec{D} = \rho\_r \stackrel{\rightharpoonup}{\nabla} \cdot \vec{B} = 0,\\
\vec{D} = \varepsilon\_0 \varepsilon\_r \vec{E}, \vec{B} = \mu\_0 \mu\_r \vec{E} \tag{5}
$$

The last two equations determine the interaction of electromagnetic fields with matter. By considering the boundary conditions, solving these equations determines electromagnetic fields.

As mentioned previously, microwave energy is needed to heat the frozen material in microwave freeze-drying method. Accordingly, a microwave system must be designed to generate and deliver microwave energy to an applicator in the chamber. Therefore, each microwave system consists of three parts, source, applicator, and transmission media, for transferring power from the source to the applicator [13]. The applicator plays the role of a load in this system. Usually, waveguides and their components are used to provide transmission media.

**Figure 9.** The position of magnetron among other microwave sources (reproduced from [14]).

Microwave Technology in Freeze-Drying Process http://dx.doi.org/10.5772/intechopen.74064 153

**Figure 10.** Top view of a magnetron (reproduced from [13]).

Typically, vacuum tubes are used for generating high power at microwave frequencies. They include klystron, magnetron, traveling-wave tube (TWT), and so on. The magnetron is a more common tube which is used in the industry and home applications [13]. The following figure shows the position of magnetron among other microwave sources.

The output power of magnetron is usually ranged between 500 and 1500 W for home microwave oven [14]. For industry application, this power is up to 10 KW and more [7, 13, 15] (**Figure 9**).

Generally, the mechanism of a vacuum tube is due to the interaction between electromagnetic field and electron beam inside a vacuum envelope (**Figure 10**) [14]. For magnetron, the envelope consists of multi-cavity resonators along the peripheral of a cylinder. An electron gun, coincided with the cylindrical axis of the tube, produces an electron beam in the magnetron. In practice, energy is transferred to the electromagnetic field through electron beam, after electron bunching occurs. The way for output power can be provided through a probe, loop, or window [14].

The efficiency of microwave oven is less than 50%, which is more than conventional heating [14]. For modern magnetrons, this parameter is 70% [13, 16] (**Figure 11**).

**Figure 9.** The position of magnetron among other microwave sources (reproduced from [14]).

**Figure 10.** Top view of a magnetron (reproduced from [13]).

where *pd*

∇

<sup>→</sup> <sup>×</sup> *<sup>E</sup>*

transmission media.

<sup>→</sup> = − <sup>∂</sup>*<sup>B</sup>* → \_\_\_ <sup>∂</sup>*<sup>t</sup>* , <sup>∇</sup> <sup>→</sup> <sup>×</sup> *<sup>H</sup>* <sup>→</sup> = *J*

is the volume density of dissipated power (W/m<sup>3</sup>

152 Emerging Microwave Technologies in Industrial, Agricultural, Medical and Food Processing

<sup>→</sup> + <sup>∂</sup>*<sup>D</sup>* → \_\_\_ <sup>∂</sup>*<sup>t</sup>* , <sup>∇</sup> <sup>→</sup> . *<sup>D</sup>*

shows the position of magnetron among other microwave sources.

[14]. For modern magnetrons, this parameter is 70% [13, 16] (**Figure 11**).

of electric field. This is the rate of absorbed microwave energy which is converted to heat. In reality, these parameters must be replaced in the right-hand side of Eqs. (1) and (2) [10].

The dissipated power is proportional to *ε"* and square of applied electric field. The more *ε"*, the more the dissipated power. In freeze-drying process, the dissipation factor of frozen meat is much more than a dried meat, so the dissipated power in the frozen region is much more than a dried region. The dissipated power can be maximized if the material is located in where the electric field is maximum. The more *ε'*, the more the ability in polarization for a dielectric.

To obtain the electromagnetic fields formed in foodstuff, Maxwell's equations must be solved. They depend on the cavity, dielectric property, and geometry of material [10]. As known, Maxwell's equations are a set of partial differential equations that are coupled to each other:

<sup>→</sup> = *ρ*, ∇

The last two equations determine the interaction of electromagnetic fields with matter. By considering the boundary conditions, solving these equations determines electromagnetic fields.

As mentioned previously, microwave energy is needed to heat the frozen material in microwave freeze-drying method. Accordingly, a microwave system must be designed to generate and deliver microwave energy to an applicator in the chamber. Therefore, each microwave system consists of three parts, source, applicator, and transmission media, for transferring power from the source to the applicator [13]. The applicator plays the role of a load in this system. Usually, waveguides and their components are used to provide

Typically, vacuum tubes are used for generating high power at microwave frequencies. They include klystron, magnetron, traveling-wave tube (TWT), and so on. The magnetron is a more common tube which is used in the industry and home applications [13]. The following figure

The output power of magnetron is usually ranged between 500 and 1500 W for home microwave oven [14]. For industry application, this power is up to 10 KW and more [7, 13, 15] (**Figure 9**).

Generally, the mechanism of a vacuum tube is due to the interaction between electromagnetic field and electron beam inside a vacuum envelope (**Figure 10**) [14]. For magnetron, the envelope consists of multi-cavity resonators along the peripheral of a cylinder. An electron gun, coincided with the cylindrical axis of the tube, produces an electron beam in the magnetron. In practice, energy is transferred to the electromagnetic field through electron beam, after electron bunching occurs. The way for output power can be provided through a probe, loop, or window [14].

The efficiency of microwave oven is less than 50%, which is more than conventional heating

<sup>→</sup> . *<sup>B</sup>*

<sup>→</sup> = 0, *D*

<sup>→</sup> <sup>=</sup> *<sup>ε</sup>*<sup>0</sup> *<sup>ε</sup><sup>r</sup>*

*E* <sup>→</sup>, *B*

<sup>→</sup> <sup>=</sup> *<sup>μ</sup>*<sup>0</sup> *μr*

*E* <sup>→</sup> (5)

) and *E* is root-mean-square value

waveguide. This structure is capable to store energy at specific frequencies. These frequencies are determined by characteristic equation. For dissipating the stored energy, it is enough to fill the cavity with lossy material. The cavity is excited by a probe, a loop, or an aperture in its sidewall. The excited fields are achieved by solving Maxwell's equations with respect to boundary conditions. The boundary conditions are zero tangential electric fields in all sidewalls of the cavity.

Dependent on the electrical length of different dimensions of the cavity, the cavities are divided into two categories: mono-mode and multimode cavities. The mono-mode cavity has small dimensions, in the range of wavelength, so that the single mode is excited inside it [13]. Therefore, this cavity is not suitable for big-volume material [13]. The excited mode is created at certain frequency because of resonant nature of mono-mode cavity. For maximizing absorbed power, the foodstuff must be located in the position(s) of the maximum electric field. Since maximum electric field occurs in one or a few positions, then the single-mode cavity is suitable for material with high loss. Increasing the dimensions of the cavity causes the excitation of different modes simultaneously. The excitation of different modes is dependent on cavity dimensions, the location of material and its electrical characterizations, and so on. The excitation of multimodes results on nonuniform field distribution in the cavity and non-

In freeze-drying method, frozen material is placed inside a cavity. The cavity is excited, and microwave energy is dissipated by material to provide needed energy for sublimation. In other words, the material absorbs microwave energy (a sink for microwave energy), and it is similar to a load for microwave system. Some stubs are used to match the load and prevent

Different methods are developed for measuring the permittivity of material. One of them uses resonant cavity to measure the permittivity. It is used in microwave frequencies and is based on perturbation theory. In this method, a small volume of material (relative to volume of the cavity) is placed in the point where the electric field is maximum. The existence of material in

Subscript 0 indicates the parameters of the cavity in the absence of material. Another method uses open-ended transmission line. This structure behaves as a resonator at microwave frequency. Different parameters affect the resonant frequency of this structure such as the length of line and fringing fields in the open end of line. The existence of different materials in the open end of line changes the fringing fields. In equivalent circuit, these fields are modeled by a capacitor (**Figure 13**). Its capacitance is affected by the medium at the open end of line. Changes in resonant frequency are due to permittivity of the proposed material, provided

For low frequency, it can be possible to use a capacitor for measuring dielectric permittivity of material. In this method, the proposed material is used as a dielectric for the capacitor. It is sufficient that this capacitor is connected to a sinusoidal signal generator with a resistor in a series of combination. Now, the voltage dropped through the capacitor is measured in two

(Δ*ε* |*E*0|<sup>2</sup> + Δ*μ* |*H*0|<sup>2</sup> )*dv* \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ <sup>∫</sup>*<sup>V</sup>*<sup>0</sup>

(*ε* |*E*0|<sup>2</sup> + *μ* |*H*0|<sup>2</sup>

)*dv* (6)

Microwave Technology in Freeze-Drying Process http://dx.doi.org/10.5772/intechopen.74064 155

uniform heating absorption in turn [13].

\_\_\_\_\_

that other key factors do not change [12].

the reflecting waves coming back to the source.

the cavity is equal to change in resonant frequency of the cavity [14]:

≈ − ∫*V*0

*ω* − *ω*<sup>0</sup> *ω*0

**Figure 11.** A microwave oven (reproduced from [14]).

The waveguide is a hollow conductor with uniform cross section, usually rectangular and circular, that is a mediator between source and load for transferring energy. **Figure 12** shows the rectangular and circular waveguides.

The energy transfer in waveguides is carried out in a form of propagating modes. To determine different modes in a waveguide, Maxwell's equations must be solved with respect to necessary boundary conditions (zero tangential electric fields in the sidewalls of waveguide). In reality, each mode determines the distribution of electric and magnetic fields in the transverse plane. Each mode is characterized by its cutoff frequency (*f c* ). For frequencies higher than *f c* , the mode propagates inside the waveguide, and the mode is evanescent for frequencies lower than *f c* [14]. Hence, the waveguides are known as high-pass structures. The mode with the lowest *f c* is known as dominant mode. From the lowest to highest frequencies, this mode is the first mode that is propagated in the waveguide. With increasing frequency, other modes are excited in the waveguide. This state is called overmode [14]. Usually, each waveguide is used in a dominant mode. The dominant mode is TE10 for rectangular waveguide and is TE11 for circular waveguide.

Among other guiding-wave structures, waveguide has low loss property in high frequency, which enables it to carry very high power.

According to the above discussions, a wave can be propagated along the waveguide. When the open end of waveguide is shorted, the power is reflected back (to the source), and standing wave is formed in the waveguide. A cavity can be made by blocking both ends of a section of

**Figure 12.** Rectangular and circular waveguides.

waveguide. This structure is capable to store energy at specific frequencies. These frequencies are determined by characteristic equation. For dissipating the stored energy, it is enough to fill the cavity with lossy material. The cavity is excited by a probe, a loop, or an aperture in its sidewall. The excited fields are achieved by solving Maxwell's equations with respect to boundary conditions. The boundary conditions are zero tangential electric fields in all sidewalls of the cavity.

Dependent on the electrical length of different dimensions of the cavity, the cavities are divided into two categories: mono-mode and multimode cavities. The mono-mode cavity has small dimensions, in the range of wavelength, so that the single mode is excited inside it [13]. Therefore, this cavity is not suitable for big-volume material [13]. The excited mode is created at certain frequency because of resonant nature of mono-mode cavity. For maximizing absorbed power, the foodstuff must be located in the position(s) of the maximum electric field. Since maximum electric field occurs in one or a few positions, then the single-mode cavity is suitable for material with high loss. Increasing the dimensions of the cavity causes the excitation of different modes simultaneously. The excitation of different modes is dependent on cavity dimensions, the location of material and its electrical characterizations, and so on. The excitation of multimodes results on nonuniform field distribution in the cavity and nonuniform heating absorption in turn [13].

The waveguide is a hollow conductor with uniform cross section, usually rectangular and circular, that is a mediator between source and load for transferring energy. **Figure 12** shows

The energy transfer in waveguides is carried out in a form of propagating modes. To determine different modes in a waveguide, Maxwell's equations must be solved with respect to necessary boundary conditions (zero tangential electric fields in the sidewalls of waveguide). In reality, each mode determines the distribution of electric and magnetic fields in the transverse plane.

propagates inside the waveguide, and the mode is evanescent for frequencies lower than *f*

known as dominant mode. From the lowest to highest frequencies, this mode is the first mode that is propagated in the waveguide. With increasing frequency, other modes are excited in the waveguide. This state is called overmode [14]. Usually, each waveguide is used in a dominant mode. The dominant mode is TE10 for rectangular waveguide and is TE11 for circular waveguide. Among other guiding-wave structures, waveguide has low loss property in high frequency,

According to the above discussions, a wave can be propagated along the waveguide. When the open end of waveguide is shorted, the power is reflected back (to the source), and standing wave is formed in the waveguide. A cavity can be made by blocking both ends of a section of

[14]. Hence, the waveguides are known as high-pass structures. The mode with the lowest *f*

*c*

). For frequencies higher than *f*

*c*

, the mode

*c*

*c* is

the rectangular and circular waveguides.

**Figure 11.** A microwave oven (reproduced from [14]).

154 Emerging Microwave Technologies in Industrial, Agricultural, Medical and Food Processing

which enables it to carry very high power.

**Figure 12.** Rectangular and circular waveguides.

Each mode is characterized by its cutoff frequency (*f*

In freeze-drying method, frozen material is placed inside a cavity. The cavity is excited, and microwave energy is dissipated by material to provide needed energy for sublimation. In other words, the material absorbs microwave energy (a sink for microwave energy), and it is similar to a load for microwave system. Some stubs are used to match the load and prevent the reflecting waves coming back to the source.

Different methods are developed for measuring the permittivity of material. One of them uses resonant cavity to measure the permittivity. It is used in microwave frequencies and is based on perturbation theory. In this method, a small volume of material (relative to volume of the cavity) is placed in the point where the electric field is maximum. The existence of material in the cavity is equal to change in resonant frequency of the cavity [14]:

the cavity is equal to change in resonant frequency of the cavity [14]:

$$
\frac{\omega - \omega\_o}{\omega\_o} \approx -\frac{\int\_{V\_\circ} (\Delta \varepsilon \parallel E\_0)^2 + \Delta \mu \parallel H\_0 \parallel^2) dv}{\int\_{V\_\circ} (\varepsilon \parallel E\_0)^2 + \mu \parallel H\_0 \parallel^2) dv} \tag{6}
$$

Subscript 0 indicates the parameters of the cavity in the absence of material. Another method uses open-ended transmission line. This structure behaves as a resonator at microwave frequency. Different parameters affect the resonant frequency of this structure such as the length of line and fringing fields in the open end of line. The existence of different materials in the open end of line changes the fringing fields. In equivalent circuit, these fields are modeled by a capacitor (**Figure 13**). Its capacitance is affected by the medium at the open end of line. Changes in resonant frequency are due to permittivity of the proposed material, provided that other key factors do not change [12].

For low frequency, it can be possible to use a capacitor for measuring dielectric permittivity of material. In this method, the proposed material is used as a dielectric for the capacitor. It is sufficient that this capacitor is connected to a sinusoidal signal generator with a resistor in a series of combination. Now, the voltage dropped through the capacitor is measured in two

**Figure 13.** Open-ended transmission line and its equivalent circuit (reproduced from [12]).

cases: a capacitor with and without the proposed material as the insulator of capacitor. The change in dropped voltage depends on the change in capacitance. Accordingly, the change in capacitance of capacitor is related to the permittivity of material.

Ref. [10] is a valuable and comprehensive paper that reviews many references and collects (and integrates) diverse information about dielectric property of different agriculture and food materials (flour, dough, milk, cheese, nuts, fish, seafood, meat). More discussions are

2450 MHz 54 60 56 65 61 69 57 71

Microwave Technology in Freeze-Drying Process http://dx.doi.org/10.5772/intechopen.74064 157

2450 MHz 10 18 15 17 14 16 17 14

**Fruit/vegetable Apple Banana Carrot Grape Mango Orange Potato Strawberry**

Moisture content (%) (wet basis) 88 78 87 82 86 87 79 92 Tissue density (g.cm−3) 0.76 0.94 0.99 1.1 0.96 0.92 1.03 0.76 *ε'* 915 MHz 57 64 59 69 64 73 62 73

*ε"* 915 MHz 8 19 18 15 13 14 22 14

The freeze-drying method is described in this chapter. Different relations, related to heat and mass transfer in the system, are mentioned. Time variation of temperature in different layers of system is shown. Then, the interaction between electromagnetic field and molecules of material is expressed. Dielectric properties of different foodstuffs are presented from different

The final stage of freeze-drying method is time-consuming when the frozen region is disap-

[1] Ratti C. Hot air and freeze-drying of high-value foods: A review. Journal of Food Engi-

available on how to change the dielectric property for different materials [10].

**Table 2.** The dielectric properties of some fruits and vegetables in two frequencies [10, 15].

references. Different parts of a microwave system are fairly explained, too.

pearing. This method is costly and affordable for high-value food like coffee.

Department of Electrical Engineering, Shahid Beheshti University, Tehran, Iran

Address all correspondence to: mohsen\_kalantari@ymail.com

**6. Conclusion**

**Author details**

Mohsen Kalantari

**References**

neering. 2001;**49**(4):311-319

While for most of agricultural products, *μr* is assumed to be the unity, since most of the foodstuffs are nonmagnetic products [10], the permittivity for each material depends on its moisture content, operating frequency, temperature, etc. [12]. Different models have been proposed to follow the frequency variation of a dielectric such as Drude, Debye, Lorentz and Cole-Cole models [10].

For food products, the chemical compositions such as salt play a decisive role in dielectric properties of products [10]. Also, dielectric properties can be affected from physical properties, such as bulk density, particle size, and homogeneity [10]. For hygroscopic material, the water content is the most important factor in the dielectric constant [10].

The dielectric constant for water is 78 at room temperature. For the moist food, this parameter ranges between 50 and 70, and their loss factor is less than 25 [10]. For organic constituents in food material, the dielectric constant and loss factor are less than 3 and less than 0.1, respectively [10, 17].

Researches and studies have shown that water is the most important part which absorbs microwave energy in foodstuff [10, 17–19]. The dielectric property of water is listed in **Table 1** for different frequencies.


**Table 2** lists the dielectric properties of some fruit and vegetable in two frequencies.

**Table 1.** The dielectric property of water [10, 20].


**Table 2.** The dielectric properties of some fruits and vegetables in two frequencies [10, 15].

Ref. [10] is a valuable and comprehensive paper that reviews many references and collects (and integrates) diverse information about dielectric property of different agriculture and food materials (flour, dough, milk, cheese, nuts, fish, seafood, meat). More discussions are available on how to change the dielectric property for different materials [10].
