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Ohmic Heating
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Tuesday, 01 March, 2011, 08 : 00 AM [IST]
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B V Venkateshaiah and Manuka Krishna
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Its applications in food industry
Ohmic heating is a novel technology for heating fluids in the food for the purpose of sterilization and pasteurization. It is an advanced thermal processing method wherein the food material, is heated by passing electricity through it.
It provides fast heating rates, more uniform heating and improved product safety and quality. This system enables destruction of micro organisms and viruses before damaging the biochemical functionality of the product. The technology can be widely applied to dairy industry for processing of milk and milk products and for processing of fruit juices etc, in the food industry.
Ohmic is an advanced thermal processing method where in the food material, which serves as an electrical resistor, is heated by passing electricity through it. Electrical energy is dissipated into heat, which results in rapid and uniform heating. Ohmic heating is also called electrical resistance heating, joule heating or electro heating.
Ohmic heating is defined as a "process where in (primary alternating) electrical currents are passed through foods or other materials with the primary purpose of heating them." Ohmic heating is distinguished from other Electrical heating methods either by the presence of electrodes contacting the food (as opposed to microwave and inductive heating, where electrodes are absent), or by frequency (unrestricted, except for the specially assigned radio or microwave frequency range.
Principle of Ohmic heating
The Ohmic heating effect occurs when an electric is passed through an electricity conducting product. In practice, low frequency alternating current (50 or 60 Hz) from the public mains supply is used.
Mechanism of heat production In Ohmic heating.
● In Ohmic heating alternating electrical current passes through food sample resulting in internal energy generated in the food. This produces an inside out heating pattern.
● Excited cells vibrate causing friction and energy dissipates in the form of heat.
Mechanism of microbial inactivation:
The principal mechanism of microbial inactivation on ohmic heating is thermal in nature and non thermal mechanism i.e. mild electroporation mechanism. Electroporation is the formation of holes in a cell membrane, due to varying electric field. The low frequency used in ohmic heating (50 or 60Hz) allows cell wall to build up charges and form pores which is not the case with high frequency where electric field is reversed before sufficient charge buildup occurs at the cell walls.
Mechanical design of Ohmic heating column
The Ohmic heating column typically consists of four or more electrode housings machined from solid blocks of PTFE (Poly Tetra Fluro Ethylene) and encased in stain less steel, each containing a single cantilever electrode. The electrode housings are connected using stainless steel spacer tubes lined with an electrically insulating liner. Suitable lining materials include Poly Vinylidene Fluoride (PVDF), polyether ether ketone (PEEK) or glass.
The column is mounted in a vertical or near vertical position with the flow of product in an upward * direction. A vent positioned at the top of the heater ensures the column is always full. The column is configured such that each heating section has the same electrical impedance and hence the inter connecting tubes generally increase in length towards the outlet. This is because the electrical conductivity of food products usually increases with increase in temperature.
APPLICATIONS Dairy Industry
● Processing of UHT milk/ Pasteurized milk
● Processing of UHT con-centrated milk/Evaporated milk
● Processing of Ice cream mixes etc.
Food Industry
● Processing of fruit juices
● Processing of fruits
● Processing of fruit salads
● Processing of particulate products up to 1 inch
● Processing of tomato ketchup
● Processing of chocolate puddings
● Processing of soups and gravies
● Processing of jelly and coffee puddings
● Processing of whole liquid egg, scrambled eggs
● Processing of meat and poultry products
● Processing of wine, beer etc.
● Processing of blood and other proteina-ceous bio-logical products.
Other Applications in food industry
● Blanching
● Extraction
● Drying
Processing of UHT milk Plant sterilization
Pre-sterilization of the ohmic heater assembly, holding tube and coolers is carried out by recirculation of a solution of sodium sulphate or sodium carbonate or an alternative food compatible mineral salt solution, at a concentration that approximates to the electrical conductivity of the food material, to be processed.
The use of sterilizing solution of similar product electrical conductivity minimizes adjustment of electrical power during subsequent change to product, thus ensuring a smooth and efficient change over period with little temperature fluctuation.
Processing
Once the plant has been sterilized the reveled solution is cooled using a plate heat exchanger in the recirculation line. When steady-state conditions are reached, sterilizing
solution is collected or run to drain, and product is introduced into the hopper of a positive displacement feed pump. Typically, this would be a marlen twin piston metering pump. Back pressure during the change over period is controlled by regulating of top pressure in an interface catch tank using sterile compressed air or nitrogen. This tank serves to collect the sodium sulphate/product Once the interface has been collected, product is diverted to the main aseptic storage vessel where top pressure is similarly used to control back pressure in the system. Back pressure is maintained at a nominal 2 bar when heating high acid product at temperature of 90-95 C. A 4 bar pressure is used for low acid products where sterilizations temperature of 120-140 C are applied. The higher back pressure is necessary to prevent boiling of the product in the ohmic heater system.
Product is progressively heated to the required sterilizations temperature as it rises through the ohmic heater assembly. It then enters an insulated holding tube before being cooled in a series of tubular heat exchangers. After cooling product enters the main storage reservoir prior to aseptic fillings.
Cleaning
When food processing is complete, power is turned off and the plant rinsed with water. This is followed by cleaning using a 2% (w/v) solution of caustic soda, which is recirculated at 80 C for 30min.
Owning to the high electrical conductivity of the cleaning solution, it is heated using a PHE in the recirculation line, and not using ohmic heating principle.
Processing of orange juice
In a case study scientists (Leizerson and Shimon et al, 2005) compared pasteurized orange juice, which had been heated at 90°C for 50 sec, with orange juice that was treated at 90,120, 150"C for 1.13, 0.85, 0.68 sec in an ohmic heating system with early findings indicating that the sensory shelf life of orange juice could be extended to more than 100 days i.e. doub
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