Mathematical Simulation Of Ohmic Heating Of Food Systems

Thermal processing is still the predominant food processing and food safety tool available. To make food safer, it is usually needed to cook food a little longer, however, if over cooked food quality may suffer. The industry and academia have long been struggling to achieve a balance between food quality and food safety in thermal processing of high viscous, liquid food containing solid blocks, and solid foods. Non thermal food processing techniques such as ultra-high pressure, high voltage pulsing electric field, ultrasonic, light radiation have been developed to solving that problem but limit success has been reached so far in food industry as non-thermal processing cannot offer the same degree of guarantee for food safety as that of traditional thermal process. What is more, the deactivation of enzyme has always been an unresolved challenge to the non thermal food processing. Ohmic heating, which heats the food internally, has emerged as one of the most promising food thermal processes. However, as a relatively new technology, Ohmic heating is yet to be investigated further to realize its full potential. A lot of efforts have been devoted to study Ohmic heating experimentally, but progress is limited by the complexity of equipment and accurate online temperature determination. As a result, some researchers resorted to mathematical method to investigate Ohmic heating further. Great progresses have been made in the past two decades and even more work is still needed to illuminate the factors such as material physical properties, material and electrode geometry, and the interaction of different physics involved in food Ohmic heating on electric field and, hence, on temperature field.To further reveal the mathematical analysis of previous researches, a three-dimensional finite element method, based on food physics, electromagnetic, and fluid dynamics, was employed to study both the transient and steady state of ohmic with different shape of electrode and vairious degree of inhomogeneity of food system.1. Studied the effect of shape of electrode on Ohmic heating. It was found that the shape of electrode affected the distribution of current density and temperature. When the electric conductivity of whole food system was homogenous, rectangular and circular planar electrode resulted in even temperature and current density distribution, while bracket-shaped electrode, parallel arc-shaped electrode, and enatiomorphous arc shaped electrode resulted in uneven temperature and current density distribution with extremum of current density appeared at the corners of electrodes. For food system of inhomogeneous electric conductivity, the current density and temperature fields of all electrodes were not even. To heat a big volume solid material, choosing arc shape electrode might lead to evener temperature distribution and lower requirement of liquid filler between electrode and solid components, and hence increase heating rate and energy efficiency.2. Studied three dimensional models containing one insulator. The results showed that the cold spot of food system related not only to the electric conductivity of material, but also to the heat transfer in Ohmic heating. The position of insulator had great impact on the position of cold spot of food system, but little effect on the boundary heat loss of material. The heat loss and temperature of boundary depended on the heat transfer coefficient. The cold spot might move to the boundary if the heat insulation of food system was poor.3. Simulated the effect of relative electric conductivity ratio and orientation of solids in the multiphase inhomogeneous food systems. It was revealed that for a fluid system contain solid block, the shape, the size, and the ratio of electric conductivity of solid and fluid all had great effect on the heating rate of each component and the evenness of the temperature distribution. No matter the electric conductivity of solid was higher or lower than that of the surrounding fluid, insufficient heating of solid and over-heating of food system might occur. Adjust the electric conductivity ratio could benefit the heating process.4. Simulated food system containing multi-blocks of different electric conductivities.It was indicated that for multi-components system, the heating rate of each component was not total controlled by electric conductivity. Higher electric conductivity might lead to lower heating rate and vice versa. The component was heated mainly by heat transfer if its electric conductivity was lower to certain degree. The greater difference in electric conductivity among different components, the higher difference in temperature between hot and cold spots of the food system under heating. The interaction of components of different electric conductivity determined the distribution of current field and voltage field, and hence the heating rate of each component. No simple relationship existed between electric conductivity and temperature profile.5. Investigated the continuous Ohmic heating of fluid food system by coupling electric field, heat field and fluid field. It was demonstrated that during the continuous heating of liquid food system, the inlet velocity of liquid had profound influence on the temperature distribution of heating chamber. Lower inlet velocity led to over-heating while high inlet velocity resulted in lower temperature difference and overall temperature.