Ohmic Heating

 Ohmic Heating 

Also termed ‘resistance heating’ or ‘electro-heating’, this is a more recent development in which an alternating electric current is passed through food, and the electrical resistance of the food causes the power to be translated directly into heat. As the food is an electrical component of the heater, it is essential that its electrical properties (its resistance) are matched to the capacity of the heater.

The concept of direct heating in this way is not new, but it has been developed into a commercial process during the last 15 years by the APV Baker company, using a licensed design by EA Technology. The process can be used for UHT sterilization of foods, and especially those that contain large particles (up to 2.5 cm) that are difficult to sterilize by other means. It is now in commercial use in Europe, the USA, and Japan for: • aseptic processing of high added-value ready meals, stored at ambient temperature for

 • pasteurization of particulate foods for hot filling 

 • pre-heating products before canning 

 • high added-value prepared meals, distributed at chill temperatures (Fryer, 1995). 

Ohmic heating is more efficient than microwave heating because nearly all of the energy enters the food as heat. Another important difference is that microwave and radio frequency heating has a finite depth of penetration into a food whereas ohmic heating has no such limitation. However, microwave heating requires no contact with the food, whereas ohmic heating requires electrodes to be in good contact. In practice, the food should be liquid or have sufficient liquid with particulate foods to allow good contact and to pump the product through the heater. 

Principles

The principles of ohmic heating are very simple as illustrated in Figure. Ohmic heating is based on the passage of alternating electrical current (AC) through a body such as a liquid-particulate food system which serves as an electrical resistance in which heat is generated. AC voltage is applied to the electrodes at both ends of the product body. The rate of heating is directly proportional to the square of the electric field strength, E, and electrical conductivity. The electric field strength can be varied by adjusting the electrode gap or the applied voltage. However, the most important factor is the electrical conductivity of the product and its temperature dependence. If the product has more than one phase such as in the case of a mixture of liquid and particulates, the electrical conductivity of all the phases has to be considered. The electrical conductivity increases with rising temperature, suggesting that ohmic heating becomes more effective as temperature increases, which could theoretically result in runaway heating. A difference in the electrical resistance and its temperature dependence between the two phases can make the heating characteristics of the system very complicated. Since electrical conductivity is influenced by ionic content, it is possible to adjust the electrical conductivity of the product (both phases) with ion (e.g. salts) levels to achieve effective ohmic heating. 

In ohmic heating, microbes are thought to be thermally inactivated. Other contributions to the kill mechanism have also been suggested. A mild electroporation mechanism may occur during ohmic heating operating at low frequency (50–60 Hz) which allows electrical charges to build up and form pores across cell walls.

Advantages

The advantages of ohmic heating are as follows: 

• The food is heated rapidly (1ºC s-¹ ) at the same rate throughout and the absence of temperature gradients result in even heating of solids and liquids if their resistances are the same 
• Heat transfer coefficients do not limit the rate of heating 
• Temperatures sufficient for UHT processing can be achieved 
• There are no hot surfaces 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 
• Heat-sensitive foods or food components are not damaged by localized overheating 
• Liquids containing particles can be processed and are not subject to shearing forces that are found in,      for example, scraped surface heat exchangers  
• It is suitable for viscous liquids because heating is uniform and does not have the problems associated     with poor convection in these materials 
• Energy conversion efficiencies are very high (>90%) 
• Lower capital cost than microwave heating 
• Suitable for continuous processing. 
• Heating food materials by internal heat generation without the limitation of conventional heat transfer    and some of the non-uniformity commonly associated with microwave heating due to limited dielectric penetration. Heating takes place volumetrically and the product does not experience a large temperature gradient within itself as it heats. 
• The higher temperature in particulates than liquid can be achieved, which is impossible for conventional heating. 
• Reducing risks of fouling on heat transfer surface and burning of the food product, resulting in minimal mechanical damage and better nutrients and vitamin retention. 
• High energy efficiency because 90% of the electrical energy is converted into heat. 
•  Optimization of capital investment and product safety as a result of high solids loading capacity. 
• Ease of process control with instant switch-on and shut-down. 
• Reducing maintenance cost (no moving parts). 
• Ambient-temperature storage and distribution when combined with an aseptic filling system. 
• A quiet environmentally friendly system.

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