Microwave Reactors
In 1945, the heating effect of microwave energy was accidentally discovered by Percy Spencer when he noticed the melted candy bar while he was objected to the active radar. He decided to test it with popcorn and egg and the food was cooked using microwave. In 1947, the first commercial microwave oven so-called “Radarange” was developed by Raytheon with weight of 340 kg and height of 1.8 meter (Osepchuk, 1984). In 1967, the first domestic (home model) microwave oven was commercialized byAmana (a division of Raytheon).
Microwave heating is characterized by conventional heating being penetrative heating, non-contact which reduces of overheating of material surfaces, short thermal gradients, volumetric heating, energy saving, environmentally friendly, and fast which increases the production rate (Bogdal, 2005; Groisman and Gedanken, 2008).
Recently, the use of microwave reactor is growing rapidly in the process of heating in food stuff factories, such as food dehydration/frozen drying and food pasteurization/sterilization for storage control. The reason is due to the tendency of water to absorb microwave energy and generate the heat within the food. When the raw food is exposed to the microwave, the water molecules in the food will be induced to rotate and produce heat as shown in Figure 1 (You, 2017). Thus, the rate of water removal and the effective used energy are higher than hot-air drying method. Besides, the microwave heating is capable of maintaining original texture structure, nutrition, flavor and color of the food at specify heat temperature compared to conventional oven drying techniques.
The comparison of the efficiency of used energy between various heating techniques is listed in Table 1 (Constellation, 2016).
Table 1. Comparison of energy efficiency for various heating techniques (Constellation, 2016)
| Heating Techniques | Temperature (°C) | Heating Time | Used Energy (kWh) |
| Electric Oven | 350 | 1 Hour | 2.0 |
| Electric Convection Oven | 325 | 45 Minutes | 1.39 |
| Gas Oven | 350 | 1 Hour | 0.90 |
| Electric Frying Pan | 420 | 1 Hour | 0.90 |
| Toaster Oven | 425 | 50 Minutes | 0.95 |
| Electric Crockpot | 200 | 7 Hours | 0.70 |
| Microwave Oven | High | 15 Minutes | 0.36 |
There are two microwave frequencies allocated by the US Federal Communications Commission (FCC) for industrial, scientific and medical (ISM) use, which are 915 MHz and 2.45 GHz. Normally, most of the microwave heating applications are devoted to 2.45 GHz, since it provides a suitable compromise between power deposition and penetration depth as well as it is an unlicensed operating frequency. Villamiel et al. (1996; 1998) have demonstrated the effectiveness of microwave heating over conventional heating proved that volumetric heating at 2.45GHz produces lower denaturation levels of whey proteins and β-lactoglobulin as compared with conventional pasteurization. In addition, Coronel et al. (2003) used 915 MHz microwave applicator based on continuous-flow and it is shown that milk is heated rapid than water. While, three parameters were used by Clare et al. (2005) to evaluate the microwave heating as compared with UHT sensory, microbiological, and biochemical; the experiments exhibit similar biological and biochemical effects, while the sensory changes exhibited in UTH. The studies indicate the effectiveness of using dielectric heating to produce pasteurized milk that satisfying the biological, biochemical and sensory requirements avoiding an extensive heat damage.
Figure 1. Microwave heating mechanisms: Water molecules are oriented when exposed to microwave
Although, microwave reactors have been widely used, however, the efficiency of use and precision control of heating process are less concerned by users and reactor builders, especially applications in the laboratories. Microwave reactors can be categories in two types, namely multi-mode and mono-mode microwave reactor. The multi-mode reactor, such as domestic microwave oven, is capable of accommodating a high volume of food heating. However, energy efficiency in heating is relatively low and food temperature monitoring is less accurate due to the existence of multi-reflection of microwave energy in the heating cavity and the non-uniform heating area. On the other hand, efficient coupling between microwave energy and food is relatively high in mono-mode reactor cavity and it is ability to precisely control food heating (heating time and power level control), as well as the thermal properties of the food can be accurately characterized (specify heating temperature for certain application). In fact, the increase in temperature in food during heating is depended on the dielectric properties of the food (to be discussed in Section 2).
In this chapter, simple and precise mono-mode microwave reactor is designed, fabricated, and tested, especially for food thermal characterization in laboratories. Several scientific analysis of heating process is described. This chapter presents two proposed systems for milk pasteurization application, the first is conventional designed mono-mode reactor and the second is by proposing slotted coaxial antenna as pasteurizer.