Temperature non-uniformities and risk of overheating at a direct ohmic heating of foods

This paper is concerned with temperature non-uniformities especially the estimation of the risk of runaway and overheating at direct ohmic heating of food. The runaway criterion Rw, reflecting the role of temperature dependent electric conductivity and thermal resistance, is suggested. The influence of cooling, singularities at electrodes and fouling layer are analyzed, assuming properties of typical food materials (meat, milk) and typical process parameters. Results indicate that the risk of runaway at a very fast heating can be reduced by cooling a part of surface perpendicular to the intensity of electric field (cooled electrodes). It is further shown, that even if a small irregularity of the electrode surface (e.g. natural roughness) has a strong effect on the local intensity of electric field, temperature effects are seen only if the height of irregularities is comparable with the distance of electrodes (>5%). On the other hand, even very thin fouling layer at an electrode causes significant overheating, as soon as its electric conductivity is low.