The principles of ohmic heating

Ohmic heating is a thermal process in which heat is internally generated by the passage of alternating electrical current (AC) through a body such as a food

AC power supply

Food y

Electrode

Insulator tube

Electrode

Fig. 19.1 A schematic diagram of an ohmic heating device.

system that serves as an electrical resistance. Ohmic heating is alternatively called resistance heating or direct resistance heating. The principles of ohmic heating are very simple, and a schematic diagram of an ohmic heating device is shown in Fig. 19.1. During ohmic heating, 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, the electrical conductivity, and the type of food being heated. The electric field strength can be controlled by adjusting the electrode gap or the applied voltage, while the electrical conductivities of foods vary greatly, but can be adjusted by the addition of electrolytes.

Sufficient heat is generated to pasteurise or sterilise foods.3 Generally, pasteurisation involves heating high-acid (pH < 4.5) foods to 90-95°C for 30-90 seconds to inactivate spoilage enzymes and microorganisms (vegetative bacteria, yeasts, molds, and lactobacillus organisms). Low-acid (pH > 4.5) foods can support Clostridum botulinum growth, and depending on the actual pH and other properties of the food, require heating to 121°C for a minimum of 3 minutes (lethality Fo = 3min) to achieve sterility (12D colony reduction).

Within the past two decades, new and improved materials and designs for ohmic heating have become available. The Electricity Council of Great Britain has patented a continuous-flow ohmic heater and licensed the technology to APV Baker.4 The particular interest in this technology stems from the food industry's ongoing interest in aseptic processing of low-acid liquid-particulate foods. In the case of particulates suspended in viscous liquids, conventional heating transfers heat from the carrier medium to the particulates, and the time required to heat sufficiently the center of the largest particulate (the designated 'cold-spot') results in overprocessing.5 In contrast, ohmic heating is volumetric and heats both phases simultaneously. Ohmic heating is a high-temperature short-time method (HTST) that can heat an 80% solids food product from room temperature to 129°C in about 90 seconds,6 allowing the possibility to decrease the extent of high temperature overprocessing. A stark contrast between ohmic heating and conven tional heating is that ohmic can heat particulates faster than the carrier liquid, called the heating inversion,7 which is not possible by traditional, conductive heating.5

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