**1. Introduction**

Population growth of human societies results in increasing the demand for requirements such as food, clothing, housing, etc. Meeting them requires new industrial methods instead of traditional ones. The production of foodstuff is included in this principle. Today, different processes are being done on the mineral, vegetable, and animal products. Some of them are pasteurization, sterilization, conservation, etc. Each of them is used for a specific purpose. During these processes, physical, chemical, and biological changes occur. They affect the quality of foodstuff (color, flavor, volume) [1].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Drying is the most common way to increase the life of food products to make them easier to maintain [2]. Meanwhile, microwave technology has achieved a significant position among other methods in food industry. Not only is this method used in food industry but also in pharmaceutical industry and medical sciences, for removing water from aqueous solutions and preserving the blood, bone, and skin [3–5].

In conventional method for drying foodstuff, it is heated, usually by flowing hot air, to evaporate its moisture. Also, the heating can be done by other methods from direct solar radiation to using microwave energy [1, 6]. In freeze-drying method, removing the moisture content of material is done by sublimation of water molecules with internal heating after freezing the material and creating a vacuum [7]. Compared with conventional methods, it causes small irreversible changes in food and thus keeps the quality of product at an excellent level [1, 2, 7]. Rehydration, color (browning), and volume (volume reduction and consequently shrinkage) are key parameters in determining the quality of foodstuff and are considered in [1]. Low temperature in this method helps to stop most bivological reactions, and hence it is suitable for dehydrating heatsensitive material like biological products [1–3]. However, this method is expensive [1, 7]. It is suitable for valuable foodstuffs like coffee [1]. Accordingly, researchers are trying to find the optimal method by a combination of different methods.

Ref. [1] is a valuable review on the studies, which are done about the quality of foodstuff from different drying methods, and collects and presents different graphs about these parameters. Author in [1] presents a chart which determines the contribution of energy consumed in different operations of freeze-drying process. Also, the cost breakdown for drying two samples (high- and low-value foods) is determined in [1].

What happens during these stages? The heating of the frozen food by microwave energy causes the frozen bulk temperature to increase. With increasing bulk temperature, frozen molecules of water receive enough energy and transit from solid phase to gas phase (sublimation of frozen molecule of water). These molecules migrate (from frozen bulk) into vacuum region of chamber. In other words, the moisture is removed from frozen region, and food material is dried. This sublimation starts from the outer layer. Now, a new region forms in frozen bulk from interface of food material and vacuum and dried region (**Figure 2**). As the time proceeds, the interface between dried and frozen regions will retreat. Therefore, the frozen bulk of material is thinned, and the volume of dried sec-

**Figure 2.** Profile of different layers with typical variations of temperature in them for an infinitely long slab of material

Microwave Technology in Freeze-Drying Process http://dx.doi.org/10.5772/intechopen.74064 145

**Figure 1.** Steps must be passed in microwave-assisted freeze-drying process.

According to the above discussion, we are faced with three physical phenomena: producing thermal energy by dissipation of microwave energy (in frozen bulk), heat transfer (in frozen and dried regions of food material), and mass transfer which is related to the movement (flow) of water vapor in the system (in the dried region). The heat transfer in the freeze zone is done by conduction, while the heat is transferred in the dried region in conduction and

As mentioned previously, with increasing the temperature of bulk, two heat transfers are carried out in the bulk. The heat transfer in the frozen region is conductive, while the heat transfer in dried region is a combination of conductive and convective. The heat transfer in

tion increases [2].

with thickness L (reproduced from [2]).

convention.

**3. Heat and mass transfer equations**

frozen region obeys the following relation:

Drying (or dehydrating) is removing moisture content from a material. This phenomenon, which required phase change in water content of material, requires a lot of energy [6].

In the traditional method, the needed energy must be transferred from dried layer (into frozen bulk), which has the low thermal conductivity. This means that it takes long time [6]. Microwave technology helps to transfer the needed energy in a form of electromagnetic wave into the frozen region, independent of thermal characteristics of dried layer. Then, the electromagnetic field is dissipated in frozen region and increases its temperature. Since the field is distributed in the frozen region, the dissipation occurs throughout frozen bulk. In fact, it creates internally volumetrically heating [2, 6, 7].

In [8], it is mentioned that the volume reductions (shrinkage) for strawberry dried by freezedrying and air-drying methods are around 6.6 and 80%, respectively [1, 8].

Ref. [3] considered the conventional and microwave-assisted freeze-drying methods. It showed that the drying time is less than 20% for microwave-assisted freeze-drying method because of volumetrically heating in this method.
