**4. Electromagnetic waves dryers**

Electromagnetic spectrum ((EMS) is the range of all wave types of EM radiation represented in Radio, Microwave, Infrared, Visible light, Ultraviolet, X-ray, and Gamma-ray. It is known that the Sun is a source of energy across the full spectrum, and its electromagnetic radiation covers the atmosphere constantly. The EM waves already have several applications in the agriculture field such as imaging, remote sensing, quality sensing, and dielectric heating in both pre-harvest and post-harvest treatments as shown in **Figure 24**. Agricultural products are considered dielectric materials and thus can store electric energy and convert it into heat. The converted heat is different from one plant to other according to its permittivity (ε) in general. This value of permittivity (ε) is noticeably frequency-dependent. Therefore, the dielectric constant parameter for agricultural products varies with frequency. For instance, the permittivity (ε) of water has an absorption peak in 24 GHz frequency. As a result, the temperature of the water stored in the agricultural products rises and evaporates, its moisture content decreases, and the drying process takes place. As a result, the drying processes of agricultural products using electromagnetic waves took a great place compared with traditional methods of drying, especially for agricultural products that are highly sensitive to heat, as one of the best modern technological solutions to maintain the quality of agricultural products and the production of dried products. Therefore, in the following, the most famous electromagnetic spectrum bands used in the drying process will be presented, which are both infrared and microwaves.

## **4.1 Infrared dryers**

Conserving energy and achieving the best quality of dried products have become the most important factors that determine the usefulness and success of operating any drying unit. Where heat is transmitted through the drying unit in three forms

**Figure 24.** *Shows some applications of the EM spectrum.*

conduction, convection, and radiation. In this regard, when the temperature of the fresh material increases, the molecular motion gains more energy; as a result, it causes changes in the structure of the material as well as its chemical properties to increase the shelf life and improve the quality [2]. Drying agricultural products, one of the most important processes of food processing and preservation, is a high priority in achieving food security. Where, drying process aims to improve the stability of food products, by reducing water activity, which leads to a decrease in microbial activity and thus limits physical and chemical changes during the storage of dried products [109]. The common conventional approach to the drying process is using convection heat transfer by transferring heat from hot air to the target product by convection, and as a result, the water evaporates into the air also by convection. However, this conventional drying method was characterized by many related disadvantages such as time consumption, low efficiency, unfair exposure to high temperatures, quality variation, in addition to high energy consumption [110]. These disadvantages of traditional drying by convective led to the innovation of other drying technologies to overcome these drawbacks such as drying by microwave, infrared (IR), osmotic drying, fluidized bed, and hybrid drying methods (integrating two or more drying methods). IR drying technique is one of the most important modern drying technologies compared with traditional drying because it is characterized by short process time, uniform temperature, high heat transfer coefficient, good quality of final products, improved energy efficiency, and safe as mentioned [111–113]. Therefore, [114–119] concluded that drying by IR heating is a promising method to achieve highquality dried products and is suitable for fruits, vegetables, grains, and other highvalue products. IR heat derives from the IR radiation that lies between the visible light (Vis) spectrum and the microwave band along the electromagnetic spectrum as depicted in **Figure 25**. IR drying technology idea is based on making changes in the electronic, rotational, and vibrational states of the atoms and molecules of the fresh material when exposed to IR radiation within the wavelength range of 780–106 nm.

IR band is divided into three regions, near-IR (NIR) is the first region whose wavelength ranges from 750 to 2500 nm to 0.75–1.4 μm at temperatures between 400 and 1000°C, the second region called mid-IR (MIR) with wavelength ranges from 2500 to 25,000 nm to 1.4–3 μm at temperatures between 400 and 1000°C.

*Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

**Figure 25.** *Depict highlighting infrared (IR) radiation region.*

While the wavelength of the third region (far-IR, FIR) ranged from 25 103 to 106 nm ≃3–1000 μm at temperatures above 1000°C [120, 121]. After IR radiation penetrates the fresh material surface, IR rays move and vibrate the constituent molecules of the fresh material through the frequency of the IR band within a frequency of 60 103–150 103 MHz, which results in friction between the molecules and leads to rapid internal heating [114, 122–124]. There are many sources of IR radiation according to their operational power, whether they are electrically heated or gas-fired generators to generate IR energy. The most common electrical heated sources are reflector-type IR incandescent lamps (incandescent vacuum lamp, gas-filled lamp, and tungsten halogen lamp), IR emitter-type quartz tubes, ceramic, and radiant panels as shown in **Figure 26**. While the traditional IR sources used for heating are electric heaters that produce infrared radiation in a range of 1100–2200°C and gasfired generators, which consist of a perforated metal plate heated by gas flames until the temperature of the metal plate rises and infrared energy is radiated at a temperature range of 343–1100°C [110, 125, 126]. IR radiation sources can provide different wavelengths ranging from short to long wavelengths according to the voltage value applied to the IR emitters. The efficiency is another significant factor in the evaluation of both electric IR heaters and IR gas heaters, where the efficiency of electric IR heaters ranges from 40 to 70%, more than the efficiency of IR gas heaters, which ranges from 30 to 50%, and emit medium to long IR. In addition, Sheridan and Shilton, 1999, mentioned that the appropriate IR wavelength for industrial heating ranges from 1.17 to 5.4 μm, which corresponds to temperature values from 260 to 2200°C, and found that the best efficiency of heat transfer by IR radiation source occurs when the heater turns red. The idea of electric infrared emitters is based on passing an electric current to an electric heater through a high-resistance wire such as nichrome wire, iron chromium wire, and tungsten filament. When the metal wire is heated to the glowing stage and the temperature rises to 2200 K, it will emit NIR with a wavelength between 0.7 and 1.4 μm. Accordingly, the Incandescent lamp type for producing IR radiation is classified as a short-wave IR emitter, while the quartz tube type is classified as a medium-IR wave emitter [126].

**Figure 26.** *Shows the types of infrared sources.*

Many studies have reported that organic matter and water are the main constituents of fresh material or foodstuffs, which is based on the absorption of IR radiation significantly, especially at Mid and Far IR [113, 127, 128]. Where, the water absorption spectral coefficient was noted at 3 (MIR) and 6 (FIR) μm different wavelengths as shown in **Figure 27** and concluded that these wavelengths are considered suitable to be fixed in large-scale IR dryers for food products that generally contain 90% water. Confirming this, both [129, 130] mentioned that food efficiently absorbs IR radiation at wavelengths greater than 2.5 μm through a change in the state of vibration of the vibrating mechanism, which leads to a high temperature of the product being dried.

Several studies have proved that IR heating as a non-traditional drying method has many advantages and benefits such as uniform heating, short processing time, high heat transfer rate, high efficiency (80–90%), low energy consumption, low cost

**Figure 27.** *Depicts the water absorption coefficient at different IR wavelength.*

## *Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

characterized, and improving final product quality in addition to it could be used as an application to measure water content in food products. Nowak et al. [113, 131–133] observed that far-infrared drying helps retain sensory quality in products such as sweet potatoes, grapes, Cordyceps militaris, and mangoes. Also, [134] investigated the effects of both hot air temperature and the IR drying method on the kinetics of persimmon slices, and the results found that the logarithmic model was the best model fitted to the experimental IR drying. In this regard, [135] studied the combined hot air at temperature levels (of 55, 65, and 75°C) and the IR drying method at radiation lamp power levels of 150, 250, and 375 W, on the persimmon fruits' moisture loss kinetics. The study finds that the drying time was reduced by 36% when increasing the drying hot air temperature from 55 to 75°C, while the drying time was reduced by 68.4% with increasing IR radiation power from 150 to 375 W. Nowak and Lewicki [113] dried the apple slices with IR radiation and by convection under equivalent conditions and reported that the heat-irradiated apple slices evaporated much more water than that not heated by IR energy until 80% of water is removed. Sun et al. [136] mentioned that the IR drying method combined with hot air as a pre-drying method can save 20% of drying time as compared with the IR drying alone throughout the drying of a thin layer of apple pomace. Chen et al. [117] conducted a comparative study between traditional hot-air (HA) and innovative drying methods of short- and medium-wave infrared radiation (SMIR) for drying jujube slices. The results find that the jujube slices dried by SMIR were of better color, higher retentions of vitamin C, total flavonoids content (TFC), and cyclic adenosine monophosphate (cAMP) content than the HA drying method, in addition to shorter drying time and higher drying efficiency. Also, the effects of the IR drying method on carrots were studied by [137]; the results pointed out that increasing IR drying time caused dramatic changes in the water state in dried carrots. Moreover, [138] used a combined drying method of IR and freeze drying to produce high-quality dried bananas at reduced cost, and the results showed that the dried banana samples were of a better color, higher crispness, and higher shrinkage compared with those produced by using regular freeze drying. As well, [139] achieved a considerable moisture reduction and higher drying rates in drying bananas with IR drying compared with hot air drying in the early stage.

## **4.2 Microwaves dryers**

Microwaves (MW) are a form of electromagnetic beam, as are radio waves, ultraviolet radiation, X-rays, and gamma-rays. MV is located in the electromagnetic beam range between radio and infrared bands as shown in **Figure 28**. MW has wavelengths of about 30 cm to 1 mm, and frequencies ranging from about 1 GHz to nearly 300 GHz [140]. Microwaves have many applications such as communications, radar, astronomy, and remote sensing, and the most famous application for most people is cooking. Where the MV rays are absorbed by water at certain frequencies.

This property of MV is useful in cooking. Water in the food absorbs microwaves, which cause the water to heat up, then cook the food. Therefore, MV was used as a heating system in several industrial applications such as food, chemical, and materials processing, for example, cooking food and drying fruits and vegetables in both batch and continuous operations. MW radiation as a drying technique is based on the passage of microwaves through the material causing a molecule oscillation [114, 141], which leads to the volumetric heating of the material. MW volumetric heating (MWVH) is a way of using MW to penetrate uniformly throughout the volume of the product, thus delivering energy evenly into the body of the material. Hence, equally

**Figure 28.**

*Shows microwave band on electromagnetic beam.*

heated the entire volume of a flowing liquid, suspension, or semisolid. Conversely, the traditional thermal processing methods rely on conduction and convection from hot surfaces to deliver energy into the product. A comparison study between MW and convective drying methods is shown in **Figure 29**. Alibas and Yilmaz [142] studied the effects of both these two drying methods on the drying kinetics, thermal properties, and quality parameters of orange slices. The results show that the MW drying

**Figure 29.** *Graphical comparison steps of the MW and convective drying methods.*

## *Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

processes were completed between 16 and 136 min depending on eight different microwave output power levels between 90 and 1000 W. On the other hand, in convective drying processes completed within the range 460–3120 min, at four different drying temperatures of 50, 75, 100, and 125°C. As well, the energy consumption was measured, and it observed that the MW drying method's energy consumption was very low at high and low powers. Finally, it is concluded that the most suitable drying method is MW drying at medium powers of 350 and 500 W by considering both drying and quality parameters.

Microwave drying technology is characterized by low energy consumption and short processing time, more uniform and energy efficient, making it an attractive source of thermal energy. However, some results indicated that microwave radiation alone is not sufficient to complete a drying process with high quality. Therefore, it is recommended to combine techniques, such as forced air or vacuum, to further improve the efficiency of the MW process [114, 141, 143–148]. Accordingly, several studies on drying using MW of various fruits and vegetables attached with an auxiliary system such as convective and vacuum methods reported that is more efficient than both MW and conventional drying techniques individually. As well, both [141, 148–153] mentioned that the MW drying technique is widely used in incorporation with hot air-drying systems. Where, the hot air removes water in a free state from the product surface, while the MW radiation removes water from the inner of the product. Furthermore, it is concluded that the MW-hot air combination drying systems not only increase the drying rates but also better retain the quality of the final products. In this regard, Sharma and Prasad [154], modified and developed a 600 W, 2450 MHz MW oven into an MW-hot air drier as shown in **Figure 30**. In order to explore the possibility of using a combined microwave-convective drying technique for processing garlic and assessment of the quality of the finished product. The results showed that the combined MW-hot air drying resulted in a reduction in the drying time and an extent of 80–90% in comparison to conventional hot air drying and a superior-quality final product. Alibas [155] studied the chard leaves quality characteristics during drying by MW, convective, and combined MW-convective. The results showed that the drying periods lasted 5–9.5, 22–195, and 1.5–7.5 min for MW,

**Figure 30.** *Shows the microwave dryer attached to hot air.*

convective, and combined MW-convective drying methods, respectively. Furthermore, the optimum drying period, color, and energy consumption were obtained for the combined MW-convective drying method. The optimum combination level was 500 W MW applications at 75°C.

Also, [156] studied the influence of MW-convective drying on chlorophyll and the color of herbs. The findings proved that the MW with auxiliary convective is a promising technique permitting the obtainment of dried material of high quality, additionally processing short time that can be an economic factor and incentive for the application of that method of drying on an industrial scale. In this regard [157] used different drying methods for Pistacia Atlantica seeds to study the impact on drying kinetics and quality properties. The results indicated that the MW drying method has afforded higher moisture removal in a shorter period compared with the traditional drying methods. Moreover, it was found that the essential oil composition was not considerably influenced by the MW drying method, and the texture quality is appropriate. Furthermore, [153] studied the effect of convective and vacuum MW drying on the bioactive compounds, color, and antioxidant capacity of sour cherries. It is found that in case of an increase in air temperature during convective drying as well as the increase in material temperature during VMWD deteriorated all the quality parameters of dried product. However, VMWD turned out to be much better than convective drying and competitive with a freeze-drying method. As it turned out that the best quality of the dried product and its more attractive color were found at VMWD at 480 W, followed by drying at MW power reduced to 120 W, which corresponds to anthocyanins content. As well, [158] dried pumpkin slices using convective and vacuum-MW drying methods to determine the drying kinetics, drying shrinkage, and bulk density, as well as to measure the color and carotenoid content of pumpkin slices dehydrated. They find that the vacuum-MW method has approximately tenfold shortened the time of pumpkin slice drying as compared with the convective method. Considering the viewpoint of color and carotene content, the vacuum-MW drying method was more effective than the convective method. Moreover, when use was made of vacuum-MW method, the dried products had a more attractive color. Sutar and Prasad [159], used microwave vacuum drying as shown in **Figure 31** to study the effect of vacuum in microwave drying operation and track the kinetics and moisture diffusivity of carrot slices. The results find that with the

**Figure 31.** *Schematic diagram of the laboratory microwave vacuum dryer.*

*Drying Technology Evolution and Global Concerns Related to Food Security… DOI: http://dx.doi.org/10.5772/intechopen.109196*

#### **Figure 32.**

*Schematic diagram of the microwave vacuum drying system.*

increase in microwave power density, the drying rates were increased and proved that the optimum model to predict the drying behavior of carrot slices' overall process conditions was the Page model. The combination of convective and vacuummicrowave (VMW) methods for drying kinetics and quality of beetroots was investigated by Figiel [160]. Where, convective drying with 60°C hot air and integration between convective pre-drying (CPD) and VMW drying method at 240, 360, and 480 W were used to dehydrate the Beetroot cubes.

The results showed that the VMFD method significantly reduced the total time of drying and decreased drying shrinkage in comparison with the convective method. Furthermore, the VMM-treated samples exhibited lower compressive strength, better rehydration potential, and higher antioxidant activity than those dehydrated in convection. Also, it is found that increasing the MW wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the combined method. Additionally, [161] optimized the drying process of Polygonum cuspidatum by using MWvacuum drying and pretreatment methods as shown in **Figure 32**.

Where, a microwave vacuum drying system is designed and built that consists of a microwave drying unit, a power and temperature control unit, a moisture condenser, a vacuum pump, a vacuum manometer, and a PC-based data acquisition unit. The pretreatment methods were blanching for 30 s at 100°C, drying at 60°C, microwave pretreatment methods, and followed by microwave vacuum for 200 mbar at 50°C. Finally, it is concluded that it can be used to scale up the microwave vacuum drying system to a commercial scale.
