Thermal Food Engineering Operations. NITIN KUMAR
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Название: Thermal Food Engineering Operations
Автор: NITIN KUMAR
Издательство: John Wiley & Sons Limited
Жанр: Техническая литература
isbn: 9781119776413
isbn:
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 appropriately heated or heated slowly in ohmic systems because of no conductance (since no salt and water presence) [34].
3 Longer processing time may be required in heating systems to achieve appropriable alterations in food materials, e.g., process of gelatinization of starch.
4 Nutritional and physical attributes of the material bear alterations in the rapid ohmic heating process.
5 Ohmic electrodes may get corroded after the cooking process, and hence final product may also get affected.
6 In continuous ohmic textural softening in beans; changes in the uniformity of product (volume enlargement) were seen, low quality of duct plugging (because of the large size of soybeans) [22, 23].
7 Change in the cell membrane of raw produce due to heat transfer by electro-thermal effect resistance heating, particularly at a low frequency.
8 Loss in Nutritional values and increase in BOD of effluents, e.g., cell-based material coming out from the heating medium is inappropriate for some products or processes
9 Capital funding and safety issues [21–23].
10 Inert electrode material.
11 No proper control in the electrical conductance of all the food constituents because then different heating rates would be there for various components [54].
12 Unavailability of data on essential parameters affects the heating, such as dwell time, direction, level of loading.
13 Inappropriate temperature conformance technology to identify cold/hot locations spot [54].
1.3.4.3 Applications
Ohmic heating has several uses in the food processing industry. It has successful applications in various food products (solids or liquid/fluids or a mixture of both); like juices, fruits and vegetables, СКАЧАТЬ