**6. Conclusion**

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

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

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

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, respec-

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**

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

**Frequency (MHz) 0.6 1.7 3 4.6 7.7 9.1 12.5 17.4 26.8 36.4** *ε' T* = 20°C 80.3 79.2 77.4 74 67.4 63 53.6 42 26.5 17.6 *T* = 50 °C 69.9 69.7 68.4 68.5 67.2 65.5 61.5 56.3 44.2 34.3 *ε" T* = 20°C 2.75 7.9 13 18.8 28.2 31.5 35.5 37.1 33.9 28.8 *T* = 50 °C 12.5 3.6 5.8 9.4 14.5 16.5 21.4 27.2 32 32.6

is assumed to be the unity, since most of the

capacitance of capacitor is related to the permittivity of material.

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

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

water content is the most important factor in the dielectric constant [10].

While for most of agricultural products, *μr*

Cole-Cole models [10].

tively [10, 17].

for different frequencies.

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

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

The final stage of freeze-drying method is time-consuming when the frozen region is disappearing. This method is costly and affordable for high-value food like coffee.
