Finite element modelling and analysis of the solidification process of eggs using ohmic treatment

Egg pre-cooking can usually use boiling water or steam to heat the eggs at present. However, it is difficult to accurately control the temperature of the heating medium. There is also a great variation in the solidification state of egg whites and yolks, even leading to the product quality in the subsequent egg processing. Therefore, it is very necessary for an egg pre-cooked with a controllable heating temperature and an easily adjustable solidification state in egg production. Fortunately, Ohmic heating has been widely used to directly apply the current to the food during heating, particularly in food sterilization, defrosting, and steaming. The shorter heating times can also be obtained to reduce the temperature gradients, indicating better temperature control, compared with the conventional. In this study, ohmic heating was used to simulate the process of pre-cooked eggs using the finite element method. The test system was established for the egg ohmic heating, including the power supply, temperature acquisition module, thermocouple, heating vessel, electrode plate, and PC. A comparison was implemented on the temperature rise history for the internal solution of eggshells with different conductivity during ohmic heating. It was found that the electrical conductivity of eggshells was basically zero. The finite element model was then developed and validated in the ohmic heating of pre-cooked eggs using COMSOL Multiphysics software. The electric field intensity was the main reason for the different temperatures around the eggs during ohmic heating. There were uniform distributions of internal temperature and electric field intensity in eggs under different heating conditions. A systematic analysis was then conducted to clarify the effects of the electric field and placement direction on the internal heating rate and temperature distribution of the egg during solidification. The results showed that the eggshell was considered an insulator under ohmic heating. The ohmic heating of pre-cooked eggs was achieved to increase the temperature of the solution outside the shell under heat conduction. RMSE between the simulated and measured values was less than 5%, indicating the correct and valid finite element model. The long axis of the egg was placed along the direction of the vertical electrode plate, in order to improve the uniformity of internal heating. The egg solidification was divided into four stages (namely egg white not fully solidified, egg white fully solidified egg yolk not solidified, egg yolk not fully solidified, and egg yolk fully solidified). The solidification state of eggs was determined to control the solidification time and the state of egg whites and yolks during ohmic heating. The finding can provide a strong reference for the process and pre-cooking device of egg solidification by ohmic heating.