Food can be heated by either direct or indirect methods.
Direct heating: In this method of heating, heat is generated within the product. Example; Dielectric heating, Ohmic heating, Pulse Electric Field (PEF), High Pressure Technology (HPT).
Indirect heating: This method of heating relies on heat that is generated externally being applied to the surface of food mostly by radiation, convection and conduction. Example; Infrared heating.
Non thermal preservation of food includes food preservation by following methods.
1. Ohmic heating
2. Dielectric heating
3. Pulse electric field (PEF)
4. High pressure treatment (HPT)
5. Oscillating magnetic field (OMF)
operates by direct passage of electric current through food product with heat generated as a result of electric resistance. In conventional heating, heat travels from heated surface to product by conduction and convection paths. Ohmic heating is also called “resistance heating” or “electro heating”. Here alternating electric current is passed through a food and electrical resistance of the food causes the power to be translated directly into heat. As the food is an electrical component of heater, it is essential that its electrical properties (resistance) are matched to the capacity of heater.
Principle of Ohmic heating: food that contain water and ionic salts are capable of conducting electricity but they also have a resistance which generates heat when an electric current is passed through them. The electrical resistance of food is the most important factor in determining how quickly it will heat. Electrical resistance of food is measured by using multimeter connected to a conductivity cell. The most important feature of Ohmic heating is the rate of heat generation which in addition to the electric resistance of the product, depends on the specific heat capacities of each component, the way that food flows the equipment and its resistance time in the heater.
If two components have similar resistances, the lower moisture (solid portion) heats faster than the carrier liquid. The resistance in an Ohmic heater depends on specific resistance of product and geometry of heater. The available 3 phase power sources in most countries have 220 – 240 volts per phase at frequency of 50 Hz. To make the best use of power, the geometry of heart and resistance of product have to be carefully matched.
If the resistance is too high, the current will be too low at maximum voltage. Conversely, if resistance is too low, the maximum limiting current will be reached at low voltage and again heating power will be too low.
Application of Ohmic heating in food industries:
1. Pasteurization of milk: It is done by pumping fluid between parallel plates having different voltage.
2. Aseptic processing: It is used in aseptic high temperature, short time processing of high value added ready to eat meals.
3. Preheating of products before canning
4. Blanching and enhanced diffusion process
5. Rapid heating and coagulation of sausages
6. Fast thawing of fish: the thawing time can be reduced to 2 to 3 times to air or water immersion thawing. Ohmic heating has lower operational cost and thawed products have similar physical and sensory properties as compared to conventional thawing.
7. Ohmic heating can be used for UHT sterilization of food especially those that contain large particle (up to 2.5 cm).
Advantages of Ohmic heating:
1. Suitable for continuous processing
2. Temperature sufficient for UHT processing can be achieved.
3. Heat transfer coefficient do not limit rate of heating
4. Food is heated rapidly 1°CS-1.
5. Food is heated at same rate throughout 1°CS-1 and absence of temperature gradient result in even heating of solid and liquid if their resistance are the same.
6. There is no hot surface for heat transfer as in conventional heating and therefore no risk of surface fouling or burning of the product which results in reduced frequency of cleaning.
7. Heat sensitive food or food components are not damaged by localized over heating.
8. Suitable for viscous liquid because heating is uniform and doesn’t have problem associated with poor convection in these materials.
9. Liquid containing particles can be processed and are not subjected to shearing force like in scraped surface heat exchangers.
10. Energy conversion efficiencies are very high (> 90 %)
11. Lower capital cost than microwave heating
Ohmic heating has been used to process various combinations of meat, vegetables, pasta and fruits when accompanied by a suitable carrier liquid. A variety of shapes including cubes, discs, spheres, rods and twists have been processed. The almost complete absence of fouling in Ohmic heating means that, after one product has been processed, the plant is flushed and the next product is introduced.
At the end of processing, plant is flushed with cleaning solution. The product is suitable for particulate food that contain up to about 60 % solids. In contrast to conventional UHT processing of particulate foods, where liquid component is important medium for heat transfer into particles. In Ohmic heating, high solid content is desirable for 2 reasons.
1. Faster heating of low conductivity particles than carrier liquid.
2. Plug flow in heater tubes.
Effect of Ohmic heating on food and food component:
Effect on microorganism and enzymes: In Ohmic heating condition, neither voltage are high enough to cause electroporation nor enough residence time to result in low voltage effect. Only thermal heating and rapid Ohmic heating cause microbial death. Complete peroxidase inactivation by Ohmic heating is less than 3 minutes as compared to 17 minutes in case of boiling water blanching.
Electro osmosis: electric food are known to enhance diffusion across membrane. The enhanced diffusion in electro osmosis may be due to the result of increased transport through cell membrane and increased permeability. The phenomenon is called electroporation. Ohmic heating is capable of heating large product very quickly and uniformly regardless of its size and shape. Thus enabling a considerable shortening of blanching time with no need of size reduction.
Effect on functional properties: protease could be inactivated by rapid heating method to minimize proteolytic activity. Degeneration of actin and myosin is minimized by Ohmic heating resulting in continuous network. Ohmic heating with rapid heating rate is an effective method for maximizing gel functionality without addition of enzyme inhibitors.
Effect on sensory quality: Ohmic heating is used for sterilization and pasteurization of number of products. Wide variety of low and high acid products as well as various extended shelf life products have been developed. By application of Ohmic heating, it is found to have retention of texture, color, flavor and nutrients comparable to or better than those of conventional processing methods such as freezing, retorting and aseptic processing.
Name : Pratiksha Shrestha
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.