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 and density of the product.
In the circuit, the triode valve assists the oscillations produced, and the potentiometer adjusts the output power. The galvanometer shows the supply of incident power as well as reflected power in the generator [79].
1.3.3.2 Advantages and Disadvantages
Advantages:
1 Suitable for large and thick foods
2 The investment cost is less or Low maintenance costs
3 Easy to control and understand [59]
4 Improves the level of the moisture
5 RF is energy efficient
6 Over dehydration and heating is prevented on the surfaces
7 The shortcomings faced in energy or heat transfer controlled using
8 shorter process lines [79]
9 Increased throughput
10 Simpler construction [54]
Disadvantages:
1 Risk of arching
2 The frequency band is narrow
3 Occupy large floor space [59]
4 The high initial capital cost of equipment and operating cost
5 Fluctuations in electrical costs
6 Different designs and applicators required to meet product-specific requirements [79]
7 Reduced power density [54]
Using a good quality of applicator’s design with specific fine-tuning, technologically advanced tools and highly skilled technicians sums up to be an efficient RF system.
An exemplary RF system would have greater penetration depth, the volumetric heating advantage of dielectric heating than other techniques, along the hybrid systems produce effectual, swift, and graded results [79].
1.3.3.3 Applications
RF dielectric has various applications in lumber, textile, and food industries, etc.
RF treatment is giving after the biscuits baked in cereals, to dry foods (herbs, spices, vegetables, potato products, pasta products), thawing, etc. The efficient role of RF is seen in moisture removal during the baking process.
It is also used for the thawing of meats and fish in large amounts, and its volumetric nature enhances the acceleration in the process without affecting the distribution of temperature throughout the product [54].
1 i) RF application in Fresh food processing:In Fresh fruits: A significant reduction in brown rot was seen when dielectric treatment (27.12MHz, 15kW) was given with hot water. Infection in peaches being reduced using RF heating and brown rots controlled in nectarines. In Fresh Vegetables: Vacuum-packaged Caixin (a green leafy vegetable) with an adequate microbial count resulted in a significant decrease in the number using ohmic heating with a 20 mm gap in electrode without affecting any other property of the material [26].
2 ii) RF heating on fresh meats showed results when treated while meat processing, as reduced E. coli count, shelf life extension wherein it has higher heating uniformity and had a shorter cooking time [31, 75].
3 iii) Significant reduction of A. parasiticus in corns seen using a combination of RF heating with hot air [25, 26].
4 iv) RF heating system in food industries used to inactivate bacteria in wheat flour, caixin, peach, pepper spice, apple juice, stone fruit, almonds, ham, etc., using free-running or 50Ω type system at a frequency of 27.12MHz [31].
5 v) It also used for disinfestation of rice, lentils, legumes, walnuts, etc., using a free-running oscillator at a frequency of 27.12MHz [31].
6 vi) Also used in cooking purposes in beef, egg white, meat emulsion, pork ham, turkey, meat, etc., using free running or 50 ohms at a frequency of 27.12MHz.
7 vii) Used in the thawing of pork, tuna fish, beef using 50 ohms at a frequency of 13.56-27.12MHz [31].
8 viii) For enzyme inactivation in miso paste, apple, myrosinase using 50 ohms at 27.12MHz frequency.
9 ix) Application in roasting and drying of nuts as well for improving energy efficiency and product quality [31].
10 x) Dry-blanching with RF was observed to be an efficient method to treat products in