Ohmic and moderate electric field processing: developments and new applications

Ohmic heating has been attempted at various times during the history of food processing, and finally appears to be finding its niche in a range of applications (both food and non-food). Recent developments in electrode materials and solid-state power supply technology to minimize electrolytic effects and reduce cost have made the technology commercially attractive. The number of commercial suppliers is also increasing. This presentation will review the developments in ohmic heating, starting with basic principles and applications in sterilization of solid-liquid mixtures. A key consideration is the identification and management of cold-spot temperatures in designing continuous flow processes for commercial sterility. In addition, it is also possible to sterilize packaged foods using ohmic heating within the package, as developed in a NASA project intended for providing high quality food supplies for the planned Mars mission. The package developed for this purpose has also been found useful in subsequent containment and sterilization of waste. Recent work on inactivation of bacterial spores has shown that ohmic heating accelerates inactivation of bacterial spores of Geobacillus stearothermophilus and Bacillus coagulans in comparison to conventional heating under the same temperature history, suggesting that the oscillating electric field of ohmic heating has a nonthermal effect. Further applications have been more recently discovered include the use of the electric field component (Moderate Electric Field Processing) to control permeabilization of bacterial as well as eukaryotic cells. This has led to a wide array of applications including extraction, drying, blanching, peeling and fermentation. The peeling operation, which is being implemented on a plant scale, involves a decrease of an order of magnitude in the use of caustic compounds without compromising peeled product quality or yield. Fermentation under moderate electric fields has been shown to reduce lag phase of bacterial growth when using pure waveforms.