Principle Of Ohmic Heating

The ohmic heating effect occurs when an electric current is passed through a conducting product (Fig. 1). In practice, low-frequency alternating current (50 or 60 Hz) from the public main supply is used to eliminate the possibility of adverse electrochemical reactions and minimize power supply complexity and cost.

In common with microwave heating, electrical energy is transformed into thermal energy. However, unlike microwave heating, the depth of penetration is virtually unlimited and the extent of heating is governed by the spatial uniformity of electrical conductivity throughout the product and its residence time in the heater. For most practical purposes the product does not experience a large temperature gradient within itself as it heats, and liquid and particulate are heated virtually simultaneously. The requirement to overprocess the liquid to ensure sterility at the center of a large particulate as with scraped surface and tubular heat exchangers is therefore reduced. This results in less heat damage to the liquid phase and prevents overcooking of the outside of the particulates.

Another major advantage is that there are no heat transfer surfaces, which reduces the possibility of deposit formation and the transfer of burnt particles to the finished product. Nor is there a need for mechanical agitation of the product, which can cause loss of true particulate identity.

The applicability of ohmic heating depends on product electrical conductivity. Most food preparations contain a moderate percentage of free water with dissolved ionic salts and hence conduct sufficiently well for the ohmic effect to be applied. The system will not directly heat fats, oils, alcohol, bone, or crystalline structures such as ice.

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