Recent work is experimental in nature and performed in Japan, Taiwan, and several other countries. Applications are mostly from areas such as dehydration, drying of vegetables, fish, rice, roasting of coffee, cocoa, and cereals, heating of floor, frying of meat, baking of pizza, biscuits, and bread, enzymes, and pathogens inactivation. Also, for thawing, blanching, sterilization, pasteurization of packing materials, and surface pasteurization, these techniques have been used.
The major effects on food involve the quick heating of food surfaces sealed in moisture and aroma compounds. Variation to components of food surfaces is equivalent to those that happen during baking.
1.3.2 Microwave Heating
1.3.2.1 Principal and Mechanism
With the increasing demand for healthy foods, there is a repeated effort given to enhance and optimize different processing techniques in food, to meet the expectations of consumers. With the advancement of emerging technologies, microwave energy has become an indispensable part of every household system. The use of microwave has expanded from heating and defrosting to thawing, blanching, sterilization, drying, etc., in food industries [20, 69]. Microwave is electromagnetic waves with a frequency which ranges from 300 Mhz to 300 GHz. Frequency of microwave used for domestic purposes is 2.45 GHz, whereas the frequency for industrial purposes is 915 MHz [8].
Table 1.1 Applications of Infrared heating: [1].
| Industry | Methods |
|---|---|
| Agriculture | Incubation and warming |
| Bottling | Drying |
| Glass | Drying, curing the varnish or paint on back—mirrors and tempering layers |
| Medical-applications | Incubation and warming |
| Environmental chambers | Heating |
| Food | Toasting, cooking, food warming, drying, broiling, and melting |
| Pharmaceutical | Drying water from powder—tablets |
| Metal treatment | Preheating—aluminum; steel |
| Paper | Laminating Calendaring—rolls Adhesive—labelsDrying water from—towels |
| Paint | Primer, topcoat alkyd, acrylic—steel panels, Drying—bicycles, vehicles bodies, aluminiumbodies |
| Textiles | Moisture elimination—carpets Latex and PVC backingMoisture elimination from dyes |
| Plastics | Laminating Annealing FormingEmbossing |
Microwave is a varying magnetic field that generates heat on interaction with, and absorption by, certain dielectric materials, and with the positioning of the direction of the electric field, the native thermal motion of the polarity molecule changes [26]. Water, the dominant polar molecule, consists of separated molecules of an oxygen atom with a negative charge and hydrogen atom with a positive charge which combinedly structure into an electric dipole. When these dipoles fluctuate swiftly back and forth from positive to negative in the direction of the electric field numerous times per second, these express reversals produce frictional heat. This implies that the polar molecules in food play a vital role in the heating performance of food in the microwave system. Due to this frictional rise in the temperature of the water, food components get heated up by convection and conduction. The loss factor dielectric constant of the food determines the depth of penetration of both microwaves and RF energy [49]. This also relies on the varying temperature and moisture concentration of the sample material plus the frequency of the electric field. Overall, with lesser frequency and loss factors, we get more depth of penetration. Energy distribution varies with food samples which also governs the depth of penetration of the microwave inside the food. As the food material to be heated in the microwave matches with the wavelength of the material, it becomes difficult to manage the heat uniformity of microwave heating which can be taken as a crucial constraint for industrial application of microwave heating. Thus, a central obligation for microwave energy application and microwave equipment in the food industry is the potential to accurately regulate heating uniformity. Microorganisms are not affected as a result of microwave radiation but are susceptible to the heat generated because of the radiation. Microwaves are likely to be a channel through ceramics, thermoplastics, and glass whereas they are absorbed by carbon and water; reflected by metals but conceivably transmitted using metal hollow tubes and on transiting amongst diverse materials get refracted like the visible light. Microwaves can also be focused on a beam [77].
The microwave energy is transferred to food through the contactless transmission of the wave. This system ensures the uniform heating of food samples during the operation. The equipment comprises a magnetron which is the generator, guide waves which are the aluminium tubes, and for a continuous operation, it has a tunnel attached with a conveyor or a metal compartment for batch operation. These chambers and tunnels are sealed by absorbers or traps to prevent the microwave from escaping and causing injury to the operator [11].
In the microwave system, the two oscillating perpendicular fields, i.e., electric and magnetic, act directly on the heating material, converting the part of absorbed energy to thermal energy. The interaction of the microwave radiation with chemically bound water present in the food material generates high pressure and temperature due to the absorption of the characteristic photonic energy of electromagnetic waves. This process causes moisture evaporation, resulting in pressure exertion on the plant material to cellular and subcellular level leading to swell up and rupture eventually [44].
Microwaves are defined by two mechanisms:
(a) Ionic polarisation: ions present in the solution, when suspended to the electric field, orient themselves, experiencing acceleration and an upsurged kinetic energy. When ions collide with each other it gets converted into heat. This frequent collision increases the density or the concentration of the solution which is also known as the ionic polarization effect [3], whereas in gases the collision becomes difficult due to the spacing between the molecules. In food material, cations are generated by the presence of salts of sodium, potassium, or calcium whilst chlorine produces anions.
(b) Dipole rotation: When the polar molecule strives to situate itself into the fluctuating electric field caused by the microwave, the dipole rotation is created, where the oscillation of the dipolar species leads to the collision with the surrounding producing heat [80]. With the increase in temperature, the dipole movement decreases, whereas ionic conduction increases hence, food samples with both the compounds when heated by the microwave, first governed by the dipole rotation and then with the increase in temperature governed by ionic conduction. The comparative involvement of these methods of heating hinges on the concentration and flexibility of sample ions, plus on the sample’s relaxation time [36].
1.3.2.2 Advantages of Microwave in Food Industry
Microwaves have the capacity of penetrating deep into the food materials which offers a remarkable advantage of the reduction of the processing time for varied different processes like sterilization, drying, etc. (Table 1.2). Microwave heating also provides unvarying temperature gradients, avoiding charring of surfaces of the