Revolutionizing Food Processing: A Comprehensive Review of Microwave Applications

: The review explores the extensive applications of microwaves, covering the non-ionizing radiation spectrum from 300 MHz to 300 GHz. It emphasizes the growing role of microwaves in modern food technology, delving into their evolution, principles, advantages, and drawbacks, the analysis delves into the unique dynamics of microwave heating, considering factors like frequency, permittivity, and thermal properties. Microwaves offer benefits such as fast and even cooking, nutrient preservation, and improved thermodynamic output, the use of radiation in sterilization, drying, pasteurization, and blanching, showcasing their efficiency in achieving product sterility while minimizing nutrient damage. Microwaves are important for modern food processing to meet dietary needs.


Introduction
Ionizing and non-ionizing waves make up the electromagnetic spectrum (Thukral et al., 2020).In the case of ionizing radiation, gamma rays and x-rays emitted are the main sources, whereas ultra-violet rays, visible light, IR radiation, and microwaves are non-ionizing sources of radiation that can be traced to a number of sources.High-frequency ionizing radiation (higher than 1014 Hz), which is able to ionize atomic bonds in molecules, may cause very severe situations that involve cell damage and damage to DNA.On the contrary, non-ionizing electromagnetic radiation does not have enough energy to strip molecules from their bonds, and it varies from 1 Hz to 1014 Hz.However, these radiations exert thermal effects on the human body depending on distance, frequency, power, and duration.Electromagnetic radiation is used in many fields, like energy transmission lines

Suggested Citation
Kakar, U.M., Sarwari, A., Rahime, M., Hassand, M.H. & Niazi, P. (2024).Revolutionizing Food Processing: A Comprehensive Review of Microwave Applications.European Journal of Theoretical and Applied Sciences, 2 (2), 38-47.DOI: 10.59324/ejtas.2024.2(2).04 (ETL) and transformer stations, electric trains, cathode ray tube (CRT) displays used in TVs and computers, induction furnaces and welding machines used in the industry, and all kinds of electrical household appliances (iron, microwave ovens, cordless phones, electric blankets, refrigerators, etc.) used in our homes (Özkaya et al., 2018).Over the past few years, such frequencies of electromagnetic waves as those used in food processing and product preservation have been observed.The microwave wavelength of the electromagnetic spectrum is the latest it employs in food processing and preservation, which, by the way, leads to important microbial activity inactivation.In this work, we will look into this technology and its impact on food.

Microwave Energy
Microwaves are a form of non-ionizing energy that heats molecules by using the process of either ion migration or dipole rotation, which doesn't alter the chemical structure (Li et al., 2019).The energy of microwaves exists in the electromagnetic spectrum, which is defined both by the wavelength from 1 mm to 1 meter and by the frequency range from 300 MHz to 300 GHz.These are used mainly for industrial food production and home purposes and generally range between 915-2450 MHz and 2450 MHz for domestic use.

An overview of Microwave History
Microwaves were the result of the desired transferring rate enhancement of energy, which resulted in an increase in frequency.In 1946, Dr. Percy Spencer, an engineer who was selfstudying, was experimenting with magnetrons (a new vacuum tube).He observed that the chocolate in his pocket was melted without knowing that it was related to the magnetron.This event contributed to the adoption of microwave energy in the food industry (Sumrall et al., 2019).Nevertheless, it was not until 1952 (Spencer, 1952) that the first patent was issued, which described an industrial conveyor belt microwave system, and 10 years later it was applied in industry.Microwave technology was brought up in the Second World War, mostly for communication purposes and with the development of radar.This resulted in the design of high-power microwave generators.According to Yi et al., (2023) the major applications were the finishing of potato chips drying, the pre-cooking of poultry and bacon, the tempering of frozen food, and pasta drying.

Principles of Microwave
This microwave energy perfectly absorbs in and then the polar molecules in food (such as water molecules) will follow the rotation of the alternating electromagnetic field.The water molecule is a dipole since one of its ends is positive opposite and the other one is negative.Due to that fact, a water molecule acts like a magnet, and the dipoles are oriented in the same direction that is determined by the electromagnetic field.Here, the rotation of these water molecules shall be the source of heat for cooking the food (Meda et al., 2017).In this view, the composition of a food affects its heating inside a microwave oven.The dipoledipole interaction makes food with a higher moisture content heat up faster.In nonconductors, microwave heating refers to dielectric heating at frequency bands that are capable of enhancing polarization.Nevertheless, electrical energy gets coupled from an electromagnetic field in a microwave cavity with the food, and the heating, in addition to its dissipation within the food product, takes place.This consequently leads to a sudden rise in temperature within the product (Niazi, P., et al., 2023).Energy is transferred at a molecular level by the interaction of molecules with the electromagnetic field (especially the molecular friction, which is caused by the dipole rotation of polar solvents and by the conductive migration of dissolved ions).Another mechanism of heating with microwaves is ionic polarization, which causes ions to move back and forth while trying to align themselves with the oscillating electric field (Sun et al., 2016).

Working Process of Microwave Oven
The process begins by converting low-voltage alternating current and high-voltage directcurrent electrical energy into direct current.The magnetron (the heart of the microwave), after using direct current, produces microwaves with a frequency of 2450 megacycles per second (2.45 GHz).The microwaves are driven by the antenna on top of the magnetron into the waveguide.The waveguide channel directs microwaves into a fan-like apparatus called a stirrer, which scatters them inside the oven body (Campañone et al., 2023).Afterwards, the microwaves are reflected by the metal walls of the oven's interior and are absorbed by the molecules in the food.As each wave is composed of both positive and negative parts, the molecules in the food are pushed back and forth at twice the microwave frequency (4.9 billion times a second).

Magnetron's Operating Principle
The magnetron is also known as the heart of a microwave oven, and it changes electric power into microwave radiation.Magnetron runs on an alternating low voltage current and a high voltage direct current.Next, the transformer changes the incoming voltage to the desired levels, and a capacitor, together with a diode, removes the high voltage and then converts it into DC.Electrons are emitted from a central terminal, called the cathode, inside the magnetron and are attracted by the positively charged anode surrounding the cathode (State, 2018).Rather than a straight-line motion, the permanent magnets make the electrons take a circular path.After passing by resonant cavities, there is the creation of a continuous pulsating magnetic field, or electromagnetic waves.

Variables Influencing the Microwave Heating Frequency
The microwave heating has two frequencies allocated (915 and 2450 MHz) for food, with frequencies of 0.328 and 0.122 m, respectively (Tang, 2015).These wavelengths are characterized by the fact that energy-material interactions mostly occur in this region, and consequently, heating occurs due to molecular friction.

Dielectric Properties
The electrical properties of microwave heating belong to the category of dielectric properties, which imply the specific behavior of food materials with respect to electromagnetic energy.At first, when we preheat, the microwave it reaches a temperature higher than the present value.To balance the influence of the higher initial temperature, the power of the MW oven should be decreased, the sample mass should be increased, or the product should be heated for a shorter duration (Bogale Teseme & Weldemichael Weldeselassie, 2020).

Food Shape and Placement
The shape of the sample has an effect on the penetration depth of the microwave, heating rate, and uniformity of heating.Heat is unevenly distributed in irregular products, which vary in size due to the difference in product thickness.As the size and wavelength ratio becomes smaller, the center temperature will rise.The size of the particulates determines how much heat energy is needed.Apart from that, the evenness of the shape also improves the consistency of the heat distribution within the product.The heat of the food is uniform in a case of spherical or cylindrical form, while it is not in a case of a square.Increasing the surfaceto-volume ratio is responsible for increasing the heating rate.

Thermal Properties and Temperature of Foods
The thermal conductivity of food is the governing factor in microwave heating.In the course of microwave heating, food items with high thermal conductivity dissipate heat more quickly than those with low conductivity.Therefore, foods that have high thermal conductivity will also take less time to attain a uniform temperature during holding (Kour et al., 2023).The dielectric properties may depend on the material and may also depend on temperature.Both moisture content and temperature can vary during the heating process and henceforth exhibit the combined effect on the dielectric constant, dielectric loss factor, loss tangent and thus on heating behavior (Chen et al., 2022).

Mass and Moisture Content
There is a direct link between the mass and the amount of the microwave power absorbed, which should be used for the attainment of the necessary heating.For a smaller mass the batch oven is used often, but for a large capacity equipment the conveyors are used.This equipment has great heating uniformity because of product movement through the microwave field.The critical (minimum) sample mass for efficient operation of each microwave oven is usually around 250 ml of water load in a 1 kW oven.No, the product does not absorb significant amount of microwave power below this level, and they can damage the magnetron at very low loads.Moisture content has impact on dielectric properties of the food product as well as on penetration depth of the microwaves.Foods with a high moisture content show poor microwave penetration depth, which leads to uneven heating rate unlike, foods with a low moisture content that will present a more uniform rate of heating due to deeper microwave penetration.During microwave heating, the moisture content of the product at the start and the rate of moisture evaporation are significant (Zhang et al., 2019).

Advantages and Disadvantages of Microwave
The first of the advantages is that heat is produced in the product itself.Microwaves permeate the foodstuffs from inside, giving rise to the internal cooking of the full volume of food quickly and equally by decreasing the processing time and energy.The rapid heat transfer causes the maintenance of nutrients, vitamins, flavor, sensory properties, and the color of food as well.As a result of the removal of the hot heat transfer surfaces, minimum scale depositions are found.Microwaves have high conversion efficiency to heat; over 80% of the energy delivered by the microwaves in these devices can be converted to heat (Asomaning et al., 2018).It has a perfect geometry for Cleaning in Place (CIP) systems.Microwaves are useful for heat-sensitive, high-viscous, and multiphase fluids, as well as low-cost system maintenance.Primarily, the area of effect of small heat conductivities, or heat transfer coefficients, is no longer a significant problem.Consequently, it means that big pieces can be heated within a shorter time, and the temperature distribution will be more even.These benefits usually produce more output.Microwaves demand a lot of understanding or experience just to estimate the moderate effects like uneven heating or thermal runaway, along with the great power of engineering intelligence (Muthukumarappan & Swamy, 2019).On the other hand, the energy required for microwave heating is electricity in its expensive form.

Application of Microwave Microwave Drying
The application of microwaves, along with air drying with the help of convective air, is defined by Asomaning et al., (2018).Initially, the heating process is faster, which results in heating to the level of evaporation temperature and consequently moving large volumes of vapor inside the product to the surface, where it is removed by the air surrounding the product.This induces a quicker drying process in this step and will cause the pores to puff, making the surface much more porous to help with the water flow to the surface.Secondly, moisture from the outer surface of the product is lost while the inner core of the particle remains moist.It follows that in this case, the application of microwave energy will be carried out when the drying rate is low.This means that the MW energy releases the moisture that is trapped at the moisture evaporation temperature and forces the moisture to move to the outer layer.In addition to that, the microwave energy can be used for the final stages of drying, in which the coming-off-bound water can be managed and the effect of product shrinkage, which can reduce the drying rate as time goes by, can be controlled.Given Maftoonazad et al., (2022), the microwave-dried products are expected to have better rehydration characteristics due to the quick energy absorption by water molecules that leads to an outward vapor flux (Maftoonazad et al., 2022).This causes water to escape rapidly, which causes its evaporation.In addition, rapid drying can act to prevent the shrinkage caused by tissue structure breakdown, which often happens when air-drying at a conventional rate.In recent years, microwave drying has been developing as an alternative drying method for many kinds of ready-to-eat food products, including fruits, vegetables, snack foods, and dairy products.Food products like plain yogurt, cranberries, carrot slices, model fruit gels, skimmed milk, milk, casein powders, butter, and fresh pasta and potato slices (Huang et al., 2023) and (Guzik et al, 2022) have been successfully dried by the microwave-vacuum application and/or by a combined microwave-assisted convection process.While several others suggested a two-step drying method that involves a primary forced air convective drying in the first step, followed by a microwave drying at the second stage (Radojčin, 2021).Nevertheless, studied results tell us that microwaving makes it possible to enhance the product quality in ways like a better smell, quicker and better rehydration, remarkable energy savings, and time savings in comparison with hot air drying alone.After all the processes (from finishing to air stream grading to stem separation to drying and roasting), the finished green tea has to run through.In this stage, the microwave oven is the instrument the team uses to decrease the water content of the tea from 8% to 2%.In the long run, the tea leaves go to a revolving drum in which there is a bottom longwave infrared heater and a microwave installed above them, where they roast into delicious, pleasant dry tea.

Microwave Blanching
Proctor and Goldblith, first in 1948, published blanching microwave experiments with 3000 MHz for some green vegetables.Also, it was revealed that maximum vitamin C retention by vegetables was possible (Proctor & Goldblith, 1948).Restaurants discovered that microwave blanching was more efficient in terms of watersoluble vitamins than conventional blanching methods.Xiao et al., (2017) found that microwave branching of green beans resulted in higher retention rates of vitamin C as compared with hot water-blanched samples.A benefit of microwave blanching over traditional blanching methods is the speed of microwave cooking, conserving energy, precise process control, and faster start-up and shut-down times noted by Paciulli et al., (2022).So, microwave heating was suggested by several researchers as an alternative blanching method for the retention of food products nutritional quality; that was done by Ranjan et al., (2017).Microwave heating was suggested by several researchers as an alternative blanching method for the retention of food products nutritional quality; that was done by Kutlu et al., (2022).A high-temperature microwave blanching process of peanuts, on the other hand, was found by the researcher to be the reason for the occurrence of stale, floral, or ashy-off flavors.Ferreira et al., (2020) statistically proved a positive impact of microwave blanching on the quality of nutrients in broccoli.Her data showed that the microwave technique preserves the nutrients better than hot water blanching while approaching the nutritional profile of fresh broccoli.Tanaka and his team compared the flavor and sensory quality of fresh dill with that of dill, which was blanched for 30 seconds in hot water at 90°C, then blasted in a microwave at 900 W for 30 seconds.Overall, the synergistic effect of hot water blanching and microwave treatment led to good-quality products.

Microwave Baking
Microwave baking is a way of baking items such as cakes and breads, along with cookies and other bakery products, in the microwave, using the oven.Conventional baking techniques involve convection and radiation as the main sources of heat.Microwave heating mainly takes advantage of the microwave's capability to create heat by causing movement in the water molecules in food items.The method possesses the advantage of being rather quick and effective, unlike traditional ovens, where baking takes longer.Nevertheless, perfect puffiness and crunchiness take more time and require baking in the microwave oven along with the conventional oven with some tables.

Microwave Steaming
Steaming in a microwave oven is a way to cook any food by using the energy of the microwaves that creates steam and cooks the food.Application of this method is prone to vegetables, seafood, and a particular grain in general.Generally, water is placed together with food when utilizing a microwave oven to prepare food.Microwaves use electromagnetic energy to activate water molecules into steam, which then cooks your food.The short cooking time, which hardly affects the taste, is arguably the outstanding quality of microwave steaming.It ensures you receive a home-cooked, non-fatty meal you don't have to cook from the same person who did it.

Microwave Grilling
Microwave grilling is the process of combining microwave technology with grilling elements to add a touch of smoke and shine to food.Such a method most often tends to involve meat, poultry, and vegetables.Unlike the conventional grilling method that uses a conventional heat source (flame, charcoal, or electric), microwave grilling first cooks and heats ingredients in the microwave and also adds flavor to the featured grilled element.This option is more timesaving in comparison with the old default grilling, as it cuts the cooking period, making it more convenient for individuals who want to have the taste of grilling but without the extended cooking.In such appliances, microwave grilling is commonly featured as one of the items that allow for a versatile cooking experience.

Pasteurization and Sterilization
During his research, Stanley & Petersen, (2017) acknowledged that microwave sterilization at 128 °C for 3 minutes provided the best products when compared to canning (120°C for 45 min) or retorting in a foil pouch (125°C for 13 min).Based on the work of Principato & Spigno, (2024) no significant alterations were observed in texture or flavor loss when applications of microwave at 915 MHz to sterilize cooked macaroni and cheese in pouches were used.If the observation of Sruthy et al., (2022) is led to believe, then the HTST-type process, also known as microwave sterilization, is not only is used to inactivate spoilage microorganisms but also to preserve the food quality Continuousflow microwave pasteurizer with a Teflon tube (12 cm; 0.635 cm) wrapped around 2450 M.
The effectiveness of pasteurization was determined by having phosphatase enzyme, standard plate, and coliform count as the cause.Munir et al. (2019) reported a study on microwave pasteurization of milk, which results in lower levels of denaturation of whey proteins in comparison to conventional thermal processes; however, the denaturation of lactoglobulin was almost equal in both processes (Munir et al., 2019).The study of microwave pasteurization and sterilization performed for closed systems of prepacked foods like yoghurt and pouch-packed meals, as well as continuous pasteurization of cooking fluids like milk (Singh et al., 2023).

Microwaving guideline & Safe materials
Guidelines for microwave heating operations should be provided for the safety of the individuals and also provide for the situation where the food is cooked effectively and uniformly.This method recommends the use of microwave-safe containers with a lid to prevent chemical leakage, need to pierce a couple of dishes in order to avoid explosions, stirring or rotating food within microwave to ensure even cooking, covering food with a microwave-safe lid or stretched plastic wrap (with a vent) to catch steam and heat food evenly, adding standing time after microwaving for heat diffusion In total, these recommendations are intended to increase the versatility and safety of microwaves through improvement of the entire microwave cooking process by considering different aspects of it.Some materials are recognized as safe to put into the microwave, but they may still cause material damage or injury by not heating food products properly.For a safe mealtime in your microwave, options include heatproof glasses, such as microwave-safe ceramics; unprinted paper towels/napkins; special microwaveable plastics; and silicone material labeled for microwave use.It stresses that one should use valid labels on vessels and stay away from materials that carry metallic features as well as those that are warped or damaged.The general message is to be mindful and use the microwave with a guide to remain safe and avoid unforeseen damage.

Health Concerns
The issue of health is quite substantial for microwave cooking.The microwave is a multipurpose appliance.It is generally regarded as being safe.However, there may be certain issues with microwaving.One factor is nutrient loss, and such a process is much less destructive than other cooking practices.Also, with nonmicrowavable plastic containers, exceptions can be made in case they discharge some dangerous chemicals when heated.The liquid goes into the supercritical region under the microwave, and it can erupt by disturbing the liquids in the process.
Another injustice is temperature inequality, where the area near the microwave feels very hot while the other area is cold.To overcome these issues, it is recommended that people use microwave-safe containers and be careful with some types of plastic.Also, superheated liquids should be handled with care, and cooking times and instructions must be followed exactly.

Challenges and Limitations
Now most of the kitchens have microwaves and typical problems of their use are just forgotten by most of their users.

Uneven Heating
One huge challenge that nuking tends to have been the possibility of uneven temperatures.Ovens using microwaves employ electromagnetic waves with the ability of exciting water molecules in a material, which, in the process, generates heat (Mello et al., 2014).Nevertheless, the distribution of such heat could be uneven, causing the observation of warmer parts and cold regions due to the thermal deposits.For this, turntables, such as those available on platform rotators, or microwavesafe stirrers are commonly advised.To prevent some foods with varying thickness or density from being unevenly heated, for example, to make the cooking time more even, is another major consideration.

Overcoming Taste Differences
A common issue with microwaves is the alteration of taste which often differs significantly from the traditional cooking devices (Guo et al., 2017).This can be experienced when heating foods in the microwave and they tend not to have that browning or caramelization of foods that you might in a normal oven with baking or grilling.Even though microwaving is a convenient regime, at the similar time, it does not prepare same taste of difference cooking techniques.Uniting evaporating during the cooking by means microwave of and in other methods, for example finishing in a conventional oven or on top of a stove, may assist in solving the taste differences and thus improve the cooking experience.

Environmental Impact
Microwaving also has appareled environmental impacts, particularly in relation to the amount of energy it uses.The microwave ovens run on electricity and the power consumption by various brands vary the same energy.Even though microwaving is in some measure more energy-efficient than other cooking methods, a thorough appraisal of its general environmental impact should be conducted (Chizoba Ekezie et al., 2017).Furthermore, it is possible that issues might arise during microwave production and when the appliances are disposed of.They contain electronic parts and other materials that require responsible handling.

Conclusion
Microwaves have revolutionized modern culinary and preservation practices, transforming how we prepare and store food.This thorough examination has highlighted the many roles of microwaves, from their historical development and basic principles to their strengths and weaknesses.Microwaves are now essential in cooking, providing advantages like fast and even cooking, preserving nutrients, and enhancing energy efficiency.The integration of microwaves in food preservation has significantly contributed to extending shelf life, reducing the need for preservatives, and ensuring the retention of nutritional integrity.The meticulous analysis of microwave dynamics, considering factors like frequency, permittivity, and thermal properties, provides a deeper understanding of their unique heating mechanisms.Microwaves are versatile and effective in sterilizing, drying, pasteurizing, and blanching food, maintaining high product sterility and minimizing nutrient loss.As people focus more on the quality and health of their diets, microwaves play a crucial role in meeting modern lifestyle and culinary needs.

Figure 2 .
Figure 2. Structure of Microwave Heating Device