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 a vital role in the food processing sector.
Figure 1.1 Illustration of radiofrequency heating [25].
As stated in studies, an excellent depletion in several microbes and pests achieved by radiofrequency heating in various food products such as eggs and its products, poultry, meat and its products, fish and shellfish, fruit juice and jam, canned fruit, starch, soy milk, molasses, pea protein concentrates, ready to cook meals, milk, and milk products, sweet desserts, cereals, and bakery products, spices, etc.
RF energy combining with other different thermal methods shows synergistic effects, notably making the RF pasteurization efficient particularly for agricultural materials having less moisture content.
RF uses a uniform and non-ionizing form of electromagnetic energy. RF system consists of two electrode plates made of metals in which the conducting materials are kept, generating the alternate electromagnetic field inside.
Electrodes are designed to provide an invariable electric field for different food shapes. Foods containing high moistures use the conventional layout of electrodes and rod-type electrodes used for dry products, which provides stray fields on the material on a conveyor belt.
RF is suitable for food materials in bulk with high ionic conductance [79].
According to the federal commission, the assigned RF frequencies mostly used are 27.12, 13.56, and 40.68 MHz for usage in industries, science, and medicine fields [25].
The heating systems used in the industry or R&D field are mainly free-running oscillators and the 50 ohms RF system.
Food placed between power generated oscillating circuit, consisting of a coil, condenser plates, a source of energy, and an amplifier [54]. An alternating electrical field generated between the electrodes causes the materials to reorient themselves towards the electrode poles of opposite charges.
While the food is heated, the given frequency is consistently monitored and maintained [54]. The generation of heat is dependent on various measures which are frequency, twice the value of the voltage applied, product proportions, and the dielectric loss factor of the material.
The 50 ohm RF system adjusts its impedance to 50 Ω, which should be the same as the generator impedance, thus delivering a stable heating process [25].
The high-frequency heating principle described as:
The equation yielded to;
In the above equation, the terms are as follows: dT/dt (°C/s), P (Watt/m3), c gives heat capacity of the dielectric material (J/kg), ρ is density in kg/m3, f is the frequency in Hz, dielectric loss factor E (V/m), and ε″ is the unreal value of the complex relative permittivity, ε* = ε″- jε″.
Higher the value of ε″, higher energy absorbance would be there at a particular voltage and frequency. RF heating rate observed to be in direct proportion with the value of ε″ and twice the electrical strength, but found to have an inverse relation with the heat capacity
