Continuous Sterilization of Particulate Foods by Ohmic Heating: Critical Process Design Considerations

Abstract

Research at this laboratory for the past several years has involved determination of electrical conductivities of foods, microbial death kinetics, process modeling and experimental verification. Finite element models developed for heating of solid-liquid mixtures in a continuous flow ohmic heater indicate that if all particles and liquid are of equal electrical conductivities, the particle cold spots heat slightly faster than the liquid. For high concentration mixtures, if all particles are of low electrical conductivity, the mixture heats slowly due to high effective resistance, but the particles still heat faster than the fluid. However, if a single particle of unusually low electrical conductivity enters the heater, it will thermally lag the fluid since the current has alternate low-resistance pathways around it. Under these conditions, the potential for underprocessing exists. Particle concentration has been found to be important in determining whether or not particles heat faster than fluids. Under low concentrations, particles will typically lag fluids, while high concentrations favor faster particle heating. These findings have been verified experimentally in a static ohmic heater. Conditions involving a radial velocity profile are discussed.