Passage time distributions of cubes and spherical particles in an ohmic heating pilot plant Original Research Article

Ohmic or electrical resistance heating involves the passage of electrical current, heating the food by internal generation. The time spent in the heating and holding parts is critical for product sterility and quality. The term passage time (rather than classical residence time) was defined as being more suited to the characteristics of this process. The objective of this study is to determine the passage time distribution (PTD) of real and model foods in the heating column and holding sections of a 10 kW ohmic heating pilot plant. Experiments have been conducted using spherical sodium alginate particles and potato cubes. Experimental variables include particle concentration and volumetric flow rate. PTDs observed in the holding section have small standard deviations. Mean normalised passage times decrease with increasing flow rate and the same pattern is observed for the minimum and maximum normalised passage times. The mean normalised passage times remain constant with increasing solid concentration. PTDs in the heating section were found to have large standard deviations, the ratio between maximum and minimum passage times being as high as 5 in some cases. For an equal thermal treatment all particles are required to spend the same amount of time in the heating and holding parts of the installation. In the holding section this appears to be approximately the case, however, the wide range of passage times measured in the heating section could cause problems of overcooking some particles to ensure commercial sterility.