The geometry of shadows: effects of inhomogeneities in electrical field processing

Ohmic heating uses the inherent electrical resistance of a material to generate heat. To produce safe, high quality, food, it is important to investigate any inhomogeneities which map occur in the process. The presence of any inclusions of significantly lower electrical conductivity in a solid-liquid mixture can cause a cold shadow within the liquid; internal heating differences can arise in a particle caught in this region. Experimental measurements and computational modelling of these problems have been carried out. When the particle was an electrical insulator, differences in heating rate due to thermal conduction gave significant temperature differences, convective mixing within the liquid had a marked effect on thermal response. For electrically conducting particles, heating rates are due to internal resistance heating as well as thermal conduction effects. In both cases, the size of the shadow is of the order of magnitude of the inclusion size. Simulations predicted the type and magnitude of the effects seen: it is difficult to model temperatures without accurate data on convective mixing. (C) 1999 Elsevier Science Ltd. All rights reserved.