Ohmic heating
- 1. Presented by : ANJALI SUDHAKAR (01 PFE/15) OHMIC HEATING Assignment under the course : ADVANCED FOOD PROCESS ENGINEERING PFE (503)
- 2. OUTLINES o Introduction o Principle o Parts of ohmic heating o Mechanism o Advantage o Application o Disadvantage o Suggestion for improvements o Case study
- 3. INTRODUCTION Ohmic heating, a thermal electrical heating method, is also termed as resistance heating. Ohmic heating is direct heating method where food is in contact with the electrodes. The concept of ohmic heating is quite simple. The passage of electric current through an electrically conductive food material obeys Ohm’s law (V = IR); and heat is generated due to the electrical resistance of the food.
- 4. Almost all electric power is transformed into heat. It is possible to heat the product containing large particles upto 2.5 cm in size which would be damaged in conventional equipment, to sterilization temperature of upto 140°C in less than 90 sec. It is regarded as Green process.
- 5. PRINCIPLE Ohmic heating works on the principle of Ohm’s law of electricity. Where V is the voltage (volts) I is the amperage (amperes) R is the resistance (ohms) V = I * R
- 6. MAIN PARTS OF OHMIC HEATING SYSTEM Contains mainly 3 parts: 1. Power supply 2. Heater assembly 3. Control panel
- 7. OHMIC HEATING PROCESS DIAGRAM
- 8. MECHANISM Food product (k) Conducts electricity Collision of molecules Momentum transfer to these molecules Increase in kinetic energy thereby heating product
- 9. The interaction between the local field strength and local electrical conductivity will govern the local heat generation according to Where Q is heat generation rate per unit volume (W/m³) E is the electric field strength (V/cm) k is the electrical conductivity (S/m) λ is the resistivity (ohm-meter) J is the current density (A/m²) Q = E²k = λJ²
- 10. The actual heating rate for the substance can then be calculated from the equation: Where T is temperature in degree Celsius t is the time in second ρ is the density (kg/m³) C is the specific heat capacity(kJ/kg- C) ρC is the volumetric heat capacity dT /dt = Q/ ρC
- 11. FACTORS INFLUENCING HEAT GENERATION RATE Electrical field strength - can be adjusted by changing voltage. Electrical conductivity - practically possible only between the range of (0.01 -10 S/m) and it works optimally in the range (0.1 - 5 S/m). Temperature - depend on the electrical conductivity.
- 12. ADVANTAGES Temperature required for UHT processing can be achieved. No problem of surface fouling or over heating of the product. Useful in pre-heating products before canning. Energy conversion efficiencies are very high. Suitable for continuous processing. Lower capital investment as compared to microwave heating and conventional heating.
- 13. Large-scale process can be carried out in heavy-duty ohmic cookers or batch ohmic heaters. It has a high solid-loading capacity. Causes less nutrient loss. It provides rapid, uniform treatment of liquid and solid phases with minimal heat damage. Less maintenance cost. Eco-friendly.
- 15. The ohmic heating system allows for the production of new, high-added-value, shelf-stable products with a quality previously unattainable with alternative sterilization techniques, especially for particulate foods. Ohmic methods offer a way of processing particulate food at the rate of HTST processes, but without the limitation of conventional HTST on heat transfer to particulates.
- 16. MICROBIAL INACTIVATION DURING OHMIC HEATING Microbial inactivation Non-thermal Mechanical (disruption of cell membrane) Chemical (Formation of free radicals and metals ions ) Thermal Kill bacillus subtilis spores
- 17. PHYSICAL AND CHEMICAL CHANGES Nutritional effect Larger particle (smaller surface to volume ratio) Shorter time Reduction in solute loss
- 18. Protein Coagulation / Denaturation - High molecular weight proteins are more susceptible to heating. - Coagulate protein and partially purify proteolytic enzymes in fish. - Mild ohmic heating (55°C for 3 min at 90 volt) is the efficient step for concentrating the proteinase.
- 19. SPECIFIC FOOD PRODUCTS Meat Fish Milk •Complex food •Muscle fiber •Uniform heating •No change in water soluble protein •Effective pasteurization with no additional protein destruction
- 21. ECONOMICS OF OHMIC PROCESSING Economically attractive & sustainable Practical benefits > 90% energy efficiency
- 22. DISADVANTAGES o Lack of generalized information. o Requested adjustment according to the conductivity of dairy products. o Narrow frequency band. o Difficult to monitor and control. o Complex coupling between temperature and electrical field distribution. o Limited to DC current.
- 23. SUGGESTIONS FOR IMPROVEMENT Develope predictive, determinable and reliable models of ohmic heating. Reliable feedback control to adjust the supply power according to the conductivity change of the dairy liquid. Developing real time-temperature monitoring techniques for locating cold-spots and overheated regions during ohmic heating. Developing adequate safety and quality-assurance protocols in order to commercialization of ohmic heating technology.
- 24. CASE STUDY Tomato peeling by ohmic heating with lye-salt combinations: Effects of operational parameters on peeling time and skin diffusivity The Ohio State University, Department of Food, Agricultural and Biological Engineering, 590 Woody Hayes Drive, Columbus, OH 43210, USA
- 25. CONTENTS Introduction Objective Materials and methods Results and discussion Conclusion
- 26. INTRODUCTION Base (NaOH/KOH). Provides smooth surface of peeled tomato. Generate environmental problem. Difficult and costly to treat. LYE
- 27. OBJECTIVE 1. To determine the effects of field strength and salt-lye composition on the time required for peeling, and 2. To determine the effect of electric field and temperature on the diffusivity of sodium hydroxide through tomato peel.
- 28. MATERIALS AND METHOD 1). Experimental setup Power supply- 60 Hz , 0-1000 volt Ohmic heater- L= 0.201 m, D= 0.051 m Titanium electrode- gap= 6.2 cm Thermocouple Data logger
- 29. 2. Experimental procedure Receive local tomato of same variety Heating starts at 25±1°C/ record parameters Power supply off when peel cracked Tomato was peeled by washing in water
- 30. 2.1. Effect of electric field strength and type of fluid medium NaCl / NaOH Mixture(%w/v ) 0.01/0.01 _ _ _ _ _ 3230 4840 6450 0.01/0.05 _ _ _ _ _ 3230 4840 5650 0.01/0.1 _ _ _ 1610 2420 3230 _ _ 0.01/0.5 _ _ 1210 1610 _ _ _ _ 0.01/1.0 645 806 1130 1450 _ _ _ _ 0.03/0.01 _ _ _ _ _ 3230 4840 _ NaCl / KOH mixture 0.01/0.5 _ 806 1210 1610 2020 _ _ _ 0.01/1.0 _ 806 1130 1290 _ _ _ _ Field strength (V/m) Table: Experimental treatments for studying effects of electric field strength, and concentrations of NaCl/NaOH and NaCl/KOH mixtures on tomato peeling.
- 31. 2.2. Diffusion analysis during tomato lye and ohmic peeling Peel preparation (t= 0.02 cm, d=1.8cm) Check cell leakage/ Kept in water bath Kept in diffusivity cell (2 reservior : 1250cm³) Pour 950 cm³ pre- heated 7% NaOH and 0.01% NaCl . Pre-assigned temp. (50º & 65ºC) is recorded Continuously stir solution Withdraw 5 ml of NaCl at every 1 minute Continue it until diffusivity rate gets steady Diffusivity is measured by a software FLUENT
- 32. RESULTS AND DISCUSSION Effects of electric field strength and concentrations of media NaCl/NaOH mixture
- 33. Relationships between the total amounts of NaOH diffusing through the tomato skin (Q) and time (t) during ohmic and control treatments (At 50ºC) (At 65ºC)
- 34. Diffusivity values of NaOH diffusing through the tomato skin over time during ohmic and control treatments (At 50ºC) (At 65ºC)
- 35. CONCLUSION Ohmic tomato peeling treatments of 0.01/0.5% NaCl/NaOH at 1610 V/m and 0.01/1.0% NaCl/NaOH at 1450 V/m were the conditions that required the shortest time for cracking. For NaCl/KOH mixtures, 0.01/0.5% NaCl/KOH at 2020 V/m and 0.01/1.0% NaCl/KOH at 1450 V/m required the shortest time for cracking. Following an initial period, diffusivities for lye peeling with ohmic heating were greater than those without ohmic heating at both 50 and 65 C. The electric field enhances the diffusion of NaOH through the tomato skin during the peeling process.
- 36. REFERENCE Nagasri, Pisit Wongas & Sastry, Sudhir.K., (2016). “Tomato peeling by ohmic heating with lye-salt combinations: Effects of operational parameters on peeling time and skin diffusivity.” Journal of Food Engineering. 186: 10-16. Andrew Proctor, 2011. “Alternatives to Conventional Food Processing”. Royal Society of Chemistry. 307-334.
- 37. Thank you