activity (POD) was seen in 3-7 min. time with a gap of 8-8.6 cm; rehydration enhancement, retention of the textural properties were also recorded [24].
11 xi) Application in wood drying (10-30MHz), agricultural product drying (27MHz), food drying, etc. [79].
12 xii) Production of Biodiesel from moist microalgae: Cell disruption of Algae done using RF heater at a frequency of 27.12MHz and maximum power output of 6kW and the esterification/transesterification reactions were also promoted [48].
1.3.4 Ohmic Heating
1.3.4.1 Principal and Mechanism
Heating technology has observed some recent advances with the following technology development, viz Dielectric heating (Radiofrequency and microwaves), induction and Ohmic heating. Ohmic Heating or joule heating is electro-heating, or electroconductive heating or direct electric resistance heating in which heat generated within the food material is due to the generation of alternating electric current passed through the food material. In 1900, the Ohmic heating technique was utilized for the Pasteurization of infant milk. In the early twentieth century, the electro-pure process was relevant commercially, but later with time, it became prevalent during the 1980s.
The food itself acts conductor in electric resistance heating as the voltages are applied, the current between the electrode and ground helps in determining the resistivity of the food as per Ohm’s law. Increasing the current or voltage, thereby increase in temperature increases the conductivity of foods [54].
In Ohmic heating, unlike other thermal methods, the electrode is in contact with food; less frequency is applied compared to the frequency of radio or microwaves, and the waveform is usually a sine wave. Resistance heating systems help with the production of products with high storage stability through proper maintenance of food in terms of color and nutritional value [34]. Figure 1.2 depicts the circuit diagram of static (batch type) resistance heating process [46].
Ohmic heating is defined as the amount of heat generated in which electrical current passes through the food and current resisting the flow of electricity. Its principle is based on the direct application of Ohm’s Law, wherein, the current through the conductor between the two points is directly proportional to the voltage.
Figure 1.2 Circuit diagram of static (batch type) resistance heating process [46].
Heat is generated internally due to the alternating current applied to the food, and the OH ratio is directly dependent on the electrical conductivity of food.
Voltage by the alternating current applied at both ends of the electrode, which interacts directly with the supplied food. Food treated in the ohmic heating chamber should come in contact with the electrodes. Since food contains the right amount of ionic salts and water molecules, electrical energy is converted into thermal energy to generate heat internally, and almost all energy inputs convert to heat without a loss factor. The heat generated can be used for other food processes such as Pasteurization, sterilization, and blanching related to OH applications. Resistance heating devices instrumentation includes an electrode (electrodes may be of stainless steel, aluminum, titanium, or platinum-coated titanium), power supply, an insulating tube or container or a heating chamber trapping the food sample inside it, data logger system, a current sensor, a thermocouple, and a Personal computer. The significant parameters involved are electrode configuration (flow of current across or parallel to the product flow direction), the gap between the two electrodes, heater shape, AC frequency, the voltage applied, the speed of the product, a supply of electricity, amount of charge per unit time. Some other factors considered are the type of product and its properties in particular specific conductance and rate of heating; total solids in food material, the heat capacity, viscousness of the material, density, the size and shape of the particles, and inclination of an electric field. The electric field intensity, E and σ, the electrical conductance, and Q are the heating rate that is directly proportional to the field strength and the square of the electrical conductivity. The increased temperature, conductance determine the efficiency of OH and the values of temperature, voltage, current, the time displayed in the data acquisition system [61].
The electric field in the homogeneous medium determined by:
E0 = ∇U, in which U is the electrostatic potential
So, generation of heat per unit volume would be;
where σL defined as the electrical conductivity of dispersed or liquid materials
Changes in the conductance with the temperature for solid and liquid foods mostly taken in a linear form, described as follows:
wherein, σin described as the electrical conductivity at the start point of temperature Tin and m is described as a constant of proportionality (°C-1)
The resistance in the ohmic heating defined as:
where R is total heat resistance in ohms, Rs (ohms per meter) is specific product resistance, x(m) is the gap between the electrodes, and A(m2) is the electrode area.
1.3.4.2 Advantages and Disadvantages
Advantages
1 Particulate foods less than one cubic inch considered appropriate in the resistance heating process; solids content range of 20-70% is deemed to be significant in the liquid-particle mixture flow reaching the plug flow.
2 Mixtures of particles and liquids are evenly heated in some situations because the properties of fluids and the particles are similar such as viscosity, conductance, heat capacity, flow rate, etc.
3 The required temperatures can be achieved rapidly in the case of ultra-high temperature processes as well.
4 There is no hot surface for heat transfer, so the risk from flaming or overprocessing of the product is low.
5 High-energy conversion efficiency.
6 Capital cost relatively low.
7 Colour and nutritional value of food retained.
8 System is environment-friendly.
9 Process control is better than the conventional heating method and more straightforward as well.
10 Reduced fouling compared to that seen in conventional heating.
11 It is an exemplary process for shear-sensitive products since the flow rate is low.
12 Fast and uniform processing of liquid and solid phases which reduces any harm due to heat and retention of nutrients is there [34]. Products treated in ohmic heating have better textural properties than traditional heating procedures.
13 Rapidly heated and heating is volumetric in nature.
14 Easily controllable.
15 Rapid inactivation of microbes and enzymes [46].
16 Storage and distribution in ambient temperature combined with the aseptic filling system.
Disadvantages
1 Installation cost and operation costs are high for ohmic heating systems compared to conventional processing methods.
2 Fat globules present in food are not