Читать книгу Thermal Food Engineering Operations - NITIN KUMAR - Страница 25

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 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].