Thermal Food Engineering Operations. NITIN KUMAR

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СКАЧАТЬ 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 food; and when utilized for drying, the probability of giving case hardening is less during microwave heating [4]. It could operate both in the continuous and batch process because of the integration of the microwave energy with the convection process. Microwave aids in the general energy conservation which is created from the entire plant volume. Microwave heating is also the predominant pertinent to food processing since it has the aptitude to evade the charring of temperature-sensitive resources. A key shortcoming can be the equipment’s capital rate and consequently most expected to be utilized for top-quality products [47].

Table 1.2 List of some techniques combined with microwave technology.

Mode	Applications	Benefits	Drawbacks	Reference
Ultrasound	Enzyme activity Drying Extraction	Precise electronic control.Competent energy savings.	-	[38]
Cold Plasma	Sterilization of microorganisms	Maintained product quality.	Inappropriate for impenetrable peels.Probability of getting slenderize.	[35]
Infrared Heating	Tempering Baking Drying	Refining rehydration Properties and quality.Reduced processing time by 95%.	Probability of escalating compactness and slenderness standards.Manufacturing constrained by the equipment size and operating cost.	[55]
Freeze Drying	Dehydration	Quick energy dissipation.Energy savings up to 40%.Advanced volatiles retention level.	Tough to control quality at high MRP. Lengthier drying time at low power.	[30]
Convective Drying	Dehydration	Reduce overheating.	Induces slight discoloration reactions.	[68]

       1.3.2.3 Application of Microwave in Food Processing Technologies

      The food processing industry utilizes microwaving immensely for different purposes like cooking, preservation, drying, sterilization, and heating of foods [26]. These particular applications have several advantages, such as microwave drying offers lower bulk density and lower shrinkage along with overhead rehydration ratio and saves power when compared to customary drying [27]. Similarly, the antioxidant activity and bioactive compounds, as well as the striking colors of different fruits and vegetables cooked with or without water, could also be maintained through microwave cooking or heating. It can also minimize antinutritional aspects, temporarily upsurge in digestibility of in-vitro protein. And when it comes to microwave sterilization, it ensures not only food safety, but also reduces the potential risk of any microbes’ attack on the food, inactivating enzymes to preserve the nourishment of food. This section reviews various reports on different applications of the microwave, their advantages, and effects on the quality parameter of food materials.

      Microwave СКАЧАТЬ