Developing Ohmic Heating Pre-cooking For Eggs Based On Finite Element Method

China's egg production ranks first in the world,but its deep processing capacity is backward.It is an effective way to increase the added value of fresh eggs by pre-boiling,solidifying,peeling and dipping.Precooking is the key process of marinated eggs.The solidifying degree of egg white and yolk after pre-cooking is a significant factor affecting the quality of marinated eggs.At present,boiling water or steam is usually used to pre-boil eggs.However,due to uncontrollable heating conditions,it is easy to produce egg white and yolk with uneven solidification,which seriously affects the subsequent processing quality of marinated eggs.As a new food processing technology,ohmic heating technology has potential advantages and broad application prospects in the application of egg pre-cooking.In conclusion,based on the original ohmic heating device,a frequency conversion circuit was designed,and the conductivity and influencing factors of different concentrations of brine and eggshell were studied experimentally.The finite element model of ohmic heating pre boiled egg process was established and verified by using the measured results,and the electric field intensity,electric field density,electric field density and electric field density were determined.The effects of laying direction and pre-cooking time on the heating rate and temperature distribution of eggs were studied.The solidification state of egg white and yolk was predicted.Finally,the energy consumption and quality of eggs were compared with traditional heating methods.The main research contents and results are as follows:(1)Frequency conversion circuit design of egg pre-boiling device based on ohmic heating.Through the improvement of the existing ohmic heating system,the frequency conversion circuit was designed,mainly including rectifier circuit,intermediate circuit and inverter circuit.Through the design and selection of each link,a 50?5k Hz adjustable square wave AC frequency conversion circuit is finally designed to meet the frequency requirements of ohmic heating pre-boiled eggs,which can be used for pre-cooking and experiment of ohmic heating eggs.(2)Determination of heating medium concentration and analysis of eggshell conductivity.By analyzing the effects of temperature and frequency on the conductivity of egg white,yolk and salt water of different concentrations,the results showed that the conductivity of egg white,yolk and salt water had a linear relationship with temperature.The higher the temperature was,the greater the conductivity was.The conductivity of 0.2%salt water was suitable for pre-boiled eggs as ohmic heating medium;When the frequency is in the range of 50?1k Hz,the conductivity of 0.2%brine increases with the increase of frequency.When the frequency is greater than 1k Hz,the conductivity changes little with frequency.On the premise of meeting the equipment requirements,it is more appropriate to use 1k Hz to pre-cooked eggs;The research on the conductivity of eggshell shows that the conductivity of eggshell is very poor,which can be regarded as insulation material.When ohmic heating pre-boiled eggs,the heat inside the eggs is completely from the heat conduction of the external salt water.(3)The effect of ohmic heating conditions on egg pre-cooking process was studied based on finite element method.The results showed that the root mean square error(RMSE)between the measured value and the simulated value of the temperature at salt water,egg white and yolk was less than 10%,which proves the validity of the mathematical model.Based on the model,it was found that the temperature distribution in the heating tank is relatively uniform during the whole heating process;When the egg is placed vertically on the electrode plate,the temperature inside the egg rises faster;During the pre-cooking process,the egg white solidifies first,then the yolk solidifies until the yolk solidifies completely;Finally,by analyzing the influence of electric field intensity on the different solidification process of egg white and yolk,it was found that the pre-boiling efficiency of eggs can be effectively improved when the electric field intensity is 12.5 V/cm for 850 s.(4)Effect of ohmic heating on egg quality and energy consumption.The results showed that the temperature distribution in the heating tank is more uniform by ohmic heating;Under the same conditions,the energy consumption was 1/4 of that of the cooking pot;The quality of eggs in different positions was more uniform.It is proved that the ohmic heating technology has a broad development prospect and application value.