which perceived the tougher texture by microwaving than the convection oven [32]. [52] reported a remarkable increase in cooking loss with augmenting core temperature and time of bovine muscle during microwave heating.
Microwave-assisted Infrared Heating In the food sector infrared (IR) radiation offers a wide range of advantages such as an express regulation response, rapid heating with minimum changes in product quality. However, as discussed earlier, its weak penetration power makes this technique only used for surface heating. Besides, there are also chances of unwanted fraction and swelling of the material due to prolonged exposure to IR radiation. Nevertheless, merging microwave with IR heating becomes profitable and could help to tackle all these drawbacks which occur during the process on the surface and inside of food [60].
Microwave-assisted Infrared Baking Microwave heating combined with IR increases the manufacture of confectioneries which was restricted before due to two major aspects, i.e., uneven dispersal of moisture inside the food and poor penetration power. Merging this energy becomes a possible approach to overcome the challenge as the microwave helps to reclaim the time of processing and administrating an effective dispersal of temperature within the material, whereas IR heating remarkably subscribes to crust formation and browning [20]. [55] observed that the legume cakes hold a better characteristic texture with increased volume and desirable surface color when treated in a combination of infrared and microwave as compared to the only conventional oven. Similarly, a gluten-free bread was prepared in an MW-IR oven using different concentration of flaxseed and gums which was kept at a frozen temperature at 20°C for 10 days before baking [56]. The results turned out to be much better as compared to the conventional oven with a darker color, softer texture, and higher volume.
Microwave-assisted Infrared Roasting Roasting is considered to be an appropriate method for flavor, texture, and color enhancement which can be done with limited investment giving to a high production quantity. However, there are not many reports with the explored application of MW-IR grouped for roasting. [76] studied the roasting characteristics of hazelnut using and compared with conventionally roasted ones. In the MW-IR oven, the optimum roasting time was 2.5 min at the power level of 90% with lower and upper halogen lamps powered at 20% and 60%, respectively, whereas conventionally it took 20 min at 150°C. When it comes to quality, the hazelnuts roasted in both techniques showed similar characteristics attributes concerning moisture content, color, and fatty acid composition. However, the above results confirm the reduction of the roasting time significantly which is therefore also recommended for other food materials.
Microwave Cooking The foremost usage of microwave is cooking. This section reports various studies of microwave and effect on the various cooking parameters such as color retention, quality, and taste for different food materials. [70] studied the chemical changes associated with skipjack tuna (Katsuwonus pelamis) during the process of boiling at 100°C, frying with sunflower oil at 180°C, then put through canning and at the end microwave heating for 10, 15 and 20 s. It was found that the health beneficial PUFA loss was minimum with boiling, 70–85% during frying, 100% with the canning, and 20–55% during microwave heating. Cholesterol content slightly increased in microwaving with no increase while cooking whereas the highest content was observed with canning and it got lowered during the frying process which could be leaching cholesterol from tuna while frying into the oil. Thus, taking into account all the methods more fatty acids can be preserved with microwave heating [70].
Blanching is usually utilized for the inactivation of enzyme and color retention, for varied fruits and vegetables by immersing it in hot boiling water or steaming with acids or salts solution. Microwave blanching offered the extreme retention of color, chlorophyll, and ascorbic acid contents with better preservation of quality parameters. [17] reported microwave blanching as a better technique than the traditional blanching for peanuts in terms of time and energy saving. However, processing at a high temperature in microwave blanching results in ashy off-flavor.
Microwave-assisted Ultrasonication Ultrasound plays a major role in the food industry and has been applied to vary processing techniques like extraction, drying, sterilization, and freezing with various advantages like maintaining the food quality parameters, augmented food preservation, and also assists in thermal treatments. On the other hand, it also reduces the cost of production by eliminating some of the purification steps [73]. However, ultrasonication does affect the physiochemical parameters, degraded the quality, exhibiting off-flavor in the food material [12]. Therefore, the fusion of microwave and ultrasound making it the microwave-assisted ultra-sonification technique renders a collaborative effect eliminating the drawbacks attached to the individual techniques [13], and therefore, the collective skill has been extensively premeditated for the food sector. The ultrasonication technique with microwave assistance has been verified as an innovative process for rapid and effectual extraction. The most unique feature which it has is the exceptional achievement of weakening the hydrogen bonds and subsequently augmenting the penetration rate of solvent into the matrix by amplifying the dipole rotation hence enabling systematic solvation [42].
Microwave Sterilization The intention of sterilization or pasteurization is usually done to kill or make inactive all the microorganisms present in the food, ultimately strengthening the safety and encompassing the serviceable life of the food. A study was conducted by [17] where the different effect of sterilization was observed without fluctuation of the microwave conditions (temperature and power). The log cycle 5.12 reduction of Salmonella typhimurium on jalapeno pepper was observed using a microwave with water-assistance at 950 W to reach a temperature of 63°C for 25 s. 4.45 log reduction was also studied on coriander foliage at 3 × 108 CFU/g at 63°C for 10 s. A similar study was conducted for Salmonella enteritidis in potato omelette which was treated under varied microwaving conditions to test the inactivation rate of 6.30 log CFU/g. It is noted that the inactivation of microorganisms quicker with the increase in power level during microwave sterilization [78].
Microwave-powered Cold Plasma As the name suggests it is a non-thermal technique, especially engaged in the food sector for microbial decontamination. DNA in the chromosomes inactivate its microbes which are later destroyed through plasma [74]. [83] examined the effectiveness of the microwave treatment combining with cold plasma to prevent the growth of Penicillium italicumandim and observe the storage stability of the mandarin at 4 and 25°C. The outcome presented the highest inhibition of Penicillium italicumandim i.e., 84% reduction in disease incidence for 10 mins at a power level of 900 W, combined with nitrogen (N2). Besides, no visible differences in titratable acidity, soluble solids, or weight loss were marked. Likewise, [67] verified that the population of Salmonella typhimurium and Escherichia coli O157:H7 reduced drastically up to 2.8 log CFU/g on lettuce using microwave-cold plasma treatment with nitrogen gas at 400 W. it showed the bactericidal effect with no damage to the quality and sensory parameter of lettuce.
1.3.3 Radiofrequency (RF) Heating
1.3.3.1 Principal and Mechanism
Radiofrequency (RF) ranges from 300 kHz to 300 MHz in the electromagnetic spectrum [54]. It comes under dielectric heating in which direct, in-depth penetration happens inside the food in the range of 1–100MHz EM waves [31]. Figure 1.1 Illustrates the schematic diagram for radio-frequency heating [25]. The lower frequency is applied and is more appropriate for processing the large volume materials; hence it is observed as a fast and volumetric heating method [14]. The frequencies used in industries for heating applications are 13.56 and 27.12 MHz. The food placed between electrodes is heated using transmitted electromagnetic energy. The RF energy is transferred over the free space and through the not resistant packaged materials. High field strengths generated provide sufficiently higher heating rates in foods. Dielectric heating is useful in colder temperature range or even less than the freezing point of foods [54]. The product mainly targeted RF heating but not in the surrounding environment. The heat is generated inside the food material through ionic conductance and dipole rotation. In the treatment, the moisture got equalized within the product without any over-drying or heating of the material. Following the environment-friendly perspective, the more efficient use of dielectric techniques plays