Ohmic and moderate electric field treatment of foods: Studies on heat transfer modelling, blanching, drying, rehydration and extraction

Ohmic heating has been studied since the early 1900s. Due to volumetric heating and the existence of an electric field, ohmic heating presents some advantages as well as challenges over conventional heating methods. It is necessary to investigate the differences of the process compared to conventional methods. Basic mechanisms of the electric field effects on food during processing should also be investigated.; Research objectives were (1) to determine the shrinkage, water consumption, and energy use for mushroom blanching under conventional and ohmic heating conditions; (2) to compare effects of pretreatments, ohmic and conventional blanching, on drying and rehydration rate of vegetable tissues; (3) to investigate the effect of Moderate Electric Field (MEF) processing, frequency and electric field strength on permeability; cellular images of raw, MEF and hot water treated mint leaves were also investigated by transmission electron microscope; (4) to compare two- and three-dimensional computational models for heating of a solid in a liquid and experimental data to see if qualitative and quantitative differences existed between the two approaches.; It was possible to heat mushroom caps above 82°C in an ohmic heater in less than 2–3 min compared to 5–7 min in conventional blanching. In addition, it was possible to heat up to 50% solid content during ohmic blanching compared to use of excess amounts of water (60kg/400 L) in conventional blanching. Drying or rehydration rate and final product structure were affected by thermal treatment and starch gelatinization for vegetable tissues. MEF processing significantly increased the extraction yield for fresh mint leaves due to additional electric field strength during heating. Dried mint leaves or fermented black tea leaves were not affected by the treatment. Low frequency resulted in higher extraction rates for fresh mint leaves. Studies on the influence of electric field strength showed that electrical permeation is achieved even at low electric field strengths. Computational and experimental results show that the differences in heating patterns between mixed and static 3D systems are not observed under similar conditions within 2D systems.